1

PHN 1: HA HC NC

Chng 1: MT S KIN THC C BN V HA NC

1. 1. Nc v tnh cht ca nc 1.1.1. Thnh phn, cu to v tnh cht ca nc a. Thnh phn, cu to ca nc

Nc l mt hp cht ho hc rt c bit, trong mi nguyn t hiro gp mt in t vo i in t dng chung vi nguyn t oxy to thnh lin kt cng ha tr. Trong mi phn t nc c hai nguyn t hiro v mt nguyn t oxy.

H2 O2 H2O3000C

Hai nguyn t hiro lin kt vi oxy to gc lin kt 105o.

Trong nguyn t oxy, ht nhn ca n thng c in tch rt mnh. Chnh v th

n c xu hng ko in t bt khi nguyn t hiro nh hn. Kt qu l chng c u th trong mi lin kt cng ha tr. Do , trong phn t nc c in tch dng gn vi nguyn t hiro v c in tch m gn vi nguyn t oxy.

Hyro c 3 ng v Proti (1H), Dtri (2H) v Triti (3H). Trong thin nhin 1H

chim t 99,985 99,986% tng s nguyn t; 2H chim t 0,0139 0,0151% tng s nguyn t; ng v 3H c tnh phng x, vi chu k bn hy l 12,4 nm.

Oxy cng c 6 ng v: 14O, 15O, 16O, 17O, 18O, 19O nhng ch c 3 ng v thin nhin l 16O (chim 99,759% tng s nguyn t), trong khi 17O (chim 0,037%) v 18O (chim 0,037%).

Hiro c pht hin vo th k th 16, do nh gi kim thut ngi thy s l paraxen (1493-1541). Oxy c pht hin ln u tin vo nm 1711 do Karl Wilhem Scheele do mt nh ha hc ngi thy in (1742-1786).

Nc c M = 18 l nc thng, chim 99,8% tng lng nc t nhin.

Nc c M 19 l nc nng, chim 0,2% tng lng nc t nhin. Hm lng cc loi nc nng trong t nhin phn b rt khc nhau. Nguyn

nhn l do hng lot cc qu trnh vt l, ha hc, sinh hc xy ra khc nhau to ra s phn b cc ng v (H v O) khc nhau.

Nc l mt phn t phn cc, nn cc phn t nc c tnh cht hp dn ln nhau nh lc ht tnh in. S hp dn ny to nn mi lin kt hiro, nh nhit thng chng trng thi lng. Gia cc nhm phn t nc tn ti xen k vi cc phn t nc n l: mH2O (H2O)m c H < 0.

Gi tr m thay i theo nhit ( th hi m = 1; th rn m = 5;...). trng thi rn, cu trc c bn gm mt phn t nc trung tm v bn phn

t xung quanh, tp hp thnh hnh t din (hnh 1).

2

S bn ha cu trc ca nc khng nhng do c mt cc ion l trng ca n, m cn do a thm vo nhng phn t to c lin kt hiro vi cc phn t nc. Mt khc, cu trc ca nc c lm bn, khi nc cn nm trng thi lng v khi trn ln n vi cht khc.

b. Mt s tnh cht ca nc

- Nc thng v nc nng c nhng tnh cht vt l khc nhau: Bng 1. Mt s tnh cht vt l ca nc

Tnh cht vt l Nc thng

)( 216

21

Nc nng

)( 216

22 D

T khi (d) 277oK 1 1,1056

To nng chy (oK) 273 276,8

To si (oK) 373 374,42

- p sut kh quyn l 1 atm, nc ng c OoC, si 100oC, rt cao so vi im si ca cc hp cht tng t cng nhm.

- Nhit si ca nc gim khi p sut bn ngoi gim (Bng 2)

Bng 2. Mi lin h gia nhit si v p sut ca nc thng

Nhit si t (oC) 0 10 20 30 40 50 100

p sut P (mmHg) 4,5 9,2 17,5 31,8 55,3 92,5 760

- Nc l mt loi dung mi rt tt, c kh nng ha tan mt s cht rn, khi nng cht tan trong nc cng ln th nhit si cng cao v nhit ng c ca dung dch cng thp.

- ho tan ca cc kh trong nc ph thuc vo nhit v p sut. - Sc cng b mt ca nc ln hn sc cng b mt ca cc cht lng khc. - Nc l cht lng khng c mu, trong sut, cho nh sng v sng di i qua (hp

th nh sng sng ngn mnh hn) gip cho qu trnh quang hp c th thc hin su trong nc.

- Nc c khi lng ring (t trng) cc i 3,98oC ( 4oC) khng phi l im ng bng, do vy m nc n ra khi ng bng. T trng ca nc thay i theo nhit .

i vi nc tinh khit s thay i ny nh sau:

3

Bng 3. S thay i t trng ca nc tinh khit i vi nhit

t(oC) T trng (kg/dm3) t(oC) T trng (kg/dm3)

0 0,99987 20 0,99828

4 1,00000 25 0,99707

10 0,99973 30 0,99567

15 0,99913 100 0,95838

- Nhit bay hi ca nc (540 cal/g) ln hn nhit bay hi ca cc cht khc, cho nn nc c s dng rng ri trong cc qu trnh truyn nhit.

- Nhit ha tan ca nc cao hn cc cht lng khc (tr NH3) v to iu kin gi nhit im kt tinh ca nc.

- Nhit dung ring ca nc (1cal/g.oC) cao hn ca cc cht lng khc (tr NH3) nn c th n nh nhit cc vng a l khc nhau.

- Nc c th ng vai tr l mt cht kh, cht oxy ho:

2H2O - 4e 4H+ + O2 E0 = - 1,230V

2H2O + 2e 2OH- + H2 E0 = - 0,828V - Tt c cc qu trnh trao i cht ca t bo phi thng qua nc, bao gm: Cc

qu trnh ng ho; cc qu trnh d ho v ln men. - Nc l trung tm cuc sng ca vi khun.

1.1.2. S ha tan cc cht trong nc Nc l mt dung mi c bit: nhit thng trn b mt tri t, nc l

cht lng c khi lng phn t nh nht li phn cc mnh, v vy c kh nng xm nhp ha tan rt nhiu cc cht v c, hu c thnh dung dch. Vi phn t nh v phn cc mnh, nc cng c kh nng thm t v phn r cc cht kh tan to thnh cc h phn tn nh keo, huyn ph. a. S ha tan cht rn trong nc

Nc c th to lin kt hiro vi cc cht ha tan. Cc cht ha tan c th dng ion cng c th khng dng ion. C hai loi ny u c th ha tan trong nc. S ha tan bao ch yu gm 2 qu trnh: l hc v ho hc (hirat ho).

- Qu trnh l hc thng thu nhit: a > 0.

- Qu trnh hirt ho thng to nhit: b < 0.

Nhit ca qu trnh ho tan (l lng nhit to ra hay thu vo khi ha tan mt mol cht tan vo nc):

= a + b (1-1)

Nu > 0 : th qu trnh ha tan l thu nhit.

Nu < 0 : th qu trnh ha tan l to nhit. V d: Ha tan NaCl vo nc, qu trnh c th chia lm 2 bc: - Bc 1: Ph v cu trc tinh th ca cht tan to thnh tiu phn ring bit. Tip l ph v lc lin kt lin phn t gia cc phn t dung mi to ra cc khong trng cho cc tiu phn ca cht tan xm nhp. Trong a s trng hp u thu nhit.

4

- Bc 2: Xy ra tng tc gia cc tiu phn ca cht tan v cc phn t dung mi to thnh dung dch, tc xy ra qu trnh solvat ha. Qu trnh ha tan l to nhit. Nhit ho tan ca dung dch, chnh l tng nhit ca cc gi tr trn.

Ty qu trnh no chim u th m gi tr H (hiu ng nhit ca phn ng) c th (+) hoc (-) hay ni cch khc qu trnh ha tan thu nhit hoc ta nhit. Thc t, nhit ha tan NaCl trong nc khong 3KJ/mol.

ha tan ca mt cht l nng bo ha ca dung dch cha cht tan mt nhit xc nh. ho tan thng c k hiu l S (S tnh theo mol/l hay g/l).

Dung dch bo ha l dung dch cha lng cht tan bng ha tan ca n ti mt nhit xc nh.

Dung dch qu bo ha l dung dch cha lng cht tan ln hn ha tan ca n ti mt nhit xc nh. b. ha tan cht kh trong nc

S c mt ca cht kh ho tan c vai tr rt ln i vi cc c tnh ho, sinh hc ca nc, c bit l oxy v kh cacbonic.

S tn ti ca cc cht kh trong nc lm bin i pH v tnh th oxy ho kh (Eh) ca nc, gy nh hng xu n cht lng ca nc.

S ha tan ph thuc vo bn cht ca cht kh, nhit ca nc, khong ha ca nc v p sut ca cht kh .

Xt s gia tng tan ca kh trong dung dch: gi s ang xy ra cn bng gia cht kh v cht lng, ngha l s phn t kh thot ra hoc tan vo dung dch vi tc bng nhau. Nu ta tng p sut, s phn t kh trong mt n v th tch s gia tng, kh s tan vo dung dch vi tc ln hn tc thot ra, lc ny nng kh tan trong dung dch s tng cho n khi mt cn bng mi c thit lp v d nhin lc ny dung dch s cha mt lng kh tan nhiu hn. S ph thuc ha tan ca cht kh v p sut ring phn ca kh trn mt nc tun theo nh lut Henry:

ho tan ca mt cht kh trong cht lng mt nhit khng i t l thun vi p sut ring phn ca kh tc dng ln cht lng.

Gi Si (mg/l) l ha tan ca kh i: Si = k.Pi (1-2)

k : hng s Henry, ph thuc vo bn cht ca cht kh v nhit . Bng 4. S ph thuc ca hng s Henry (k) vo nhit

k Nhit (oK)

O2 ( 10-5) CO2 ( 10-5) H2S ( 10-5)

273 67,45 3349,51 6825,24

275 63,48 3000,97 6397,08

277 60,52 2788,34 5998,05

279 57,47 2602,91 5626,21

291 43,83 1736,89 3966,99

298 38,76 1407,76 3332,03

5

nh lut Henry ch ng trong trng hp kh tan khng phn ng vi dung mi, v d: ng vi trng hp kh O2 tan trong nc, nhng khng ng vi trng hp HCl tan trong nc v y xy ra s phn ly ca HCl. V d: Xc nh ho tan (S) ca O2 279oK di p sut p = 0,2163.105 N/m2. Tra bng 4 ta c: k = 57,47.10-5

S = 57,47.10-5 . 0,2163.105 12,43 mg/l. ha tan cc cht kh trong nc tng khi nhit nc gim. Cht kh l hp cht c ha tan ln hn cht kh l n cht do cc phn t

hp cht thng l phn t phn cc. Thng thng khi khong ca nc tng ln th ha tan ca cc kh gim. ha tan ca O2 s gim 25% khi tng lng mui ha tan trong nc ti 40 gam.

c. ha tan cht lng trong nc Cht c cng bn cht phn cc ha tan tt vo nhau, m s phn cc ca mt

cht c xc nh bi cu trc ca n (dng lai ho, s tng tc gia cc electron lp v in t,...). Do , gia cu trc v tan c mt mi quan h xc nh.

Thng thng cc cht lng phn cc thng d ha tan trong nc; Cc cht hu c thng kh ha tan trong nc. 1.1.3. nh lut tc dng khi lng a. Tc phn ng

Trong qu trnh phn ng nng cc cht phn ng gim dn cn nng cc cht sn phm tng dn. Phn ng xy ra cng nhanh th trong mt n v thi gian th nng cc cht phn ng gim xung v nng cc cht sn phm tng ln cng nhiu. Nh vy, c th dng bin thin nng ca mt cht bt k trong phn ng theo thi gian lm thc o tc phn ng.

Tc phn ng l i lng c trng cho din bin nhanh hay chm ca mt phn ng. N c o bng bin thin nng ca cc cht phn ng hay sn phm phn ng trong mt n v thi gian. Xt phn ng: A = M + N

Tc trung bnh ca cht A: 1 2 1 22 1 1 2

C C C C Cvt t t t t

= = =

:v

vn tc trung bnh trong khong thi gian t t1 n t2;

C: s bin thin nng mol ca cht kho st t C1 n C2.

Khi t 0 th v tc thi l: dCvdt

=

Tng qut: aA + bB cC + dD th tc phn ng: dtdCBAKV ba == ][][

b. Hng s cn bng Cn bng ho hc l trng thi ca mt qu trnh thun nghch khi vn tc ca

phn ng thun bng vn tc ca phn ng nghch v do nng cc cht khng bin i na.

6

Xt phn ng cn bng:

A(K) + B(K) C(K) + D(K)k1

k2 Gi k1 v k2 ln lt l hng s vn tc ca phn ng thun v nghch. Gi s

phn ng thun cng nh phn ng nghch u thuc loi n gin (ngha l phn ng ch xy ra trong mt giai on, bc phn ng ring phn ca mi cht bng h s t lng nguyn ti gin ng trc mi cht trong phn ng).

Thc nghim cho thy, vi phn ng ng th th: vn tc ca phn ng t l thun vi tch nng cc cht tham gia. Vn tc phn ng thun l: v1 = k1[A][B] Vn tc phn ng nghch l: v2 = k2[C][D]

Gi s lc bt u phn ng, ch c A, B hin din. Vn tc phn ng thun v1 lc u rt ln, vn tc phn ng nghch v2 bng khng. Phn ng cng xy ra lu, nng cc cht A, B cng gim , nng cc sn phm C, D cng tng.

Nh vy, v1 gim dn theo thi gian, cn v2 tng dn theo thi gian. Sau mt thi gian vn tc phn ng thun v1 s bng vn tc phn ng nghch v2, lc ny phn ng t trng thi cn bng:

v1 = v2 k1[A][B] = k2[C][D]

k1k2

KC = =[C][D][A][B]

V k1 v k2 l cc hng s vn tc phn ng, ch ty thuc nhit (v ty thuc bn cht ca phn ng), cho nn ng vi mt nhit xc nh (v mt phn ng xc nh), ta c:

k1k2

KC = =[C][D][A][B] CB

CB: cn bng, ch nng cc cht C, D, A, B lc t trng thi cn bng. KC c gi l hng s cn bng ca phn ng lin h n nng (mol/l). Kc ch

ph thuc nhit v bn cht ca phn ng, m khng ph thuc vo nng cc cht trong phn ng.

Tng qut: mA + nB pC + qD

k1k2

KC = =[C]p[D]q

[A]m[B]n k1, k2 : c trng cho hng s tc ca phn ng thun nghch, ch ph thuc

vo bn cht phn ng v nhit . Vi [A], [B], [C], [D] l nng ca A, B, C, D lc cn bng. Kc: hng s cn bng ch ph thuc vo bn cht h phn ng v nhit .

c. Hng s in ly S in ly ca cht in ly yu l mt qu trnh thun nghch nn ta c th p

dng nh lut tc dng khi lng.

Gi s c cn bng: HA H+ + A-

(1-3)

7

Ta c: ][

][][HA

AH +=

[H+], [A-]: nng mol Hng s cn bng K gi l hng s in ly hoc hng s ion ha, c trng cho

kh nng in ly ca mt cht; K cng ln th s in ly cng mnh, cht in ly phn ly ra cc ion cng nhiu v ngc li. K ph thuc vo bn cht ca cht in ly, dung mi v nhit . d. Tch s ha tan

Gi s c cht in li t tan AnBm trong nc c cn bng:

AnBm Am+ + Bn- (cht rn) (trong dung dch)

Qu trnh thun nghch t trng thi cn bng, ta c dung dch bo ha cht in ly t tan. p dng nh lut tc dng khi lng vo cn bng d th ta c:

.m nn mn n

A B A BT a a+ =

Nh vy, trong dung dch bo ha cht in ly t tan tch s hot cc ion vi s m tng ng l mt hng s, hng s ny c tn l tch s tan.

V cht in ly kh tan c nng rt nh, nn a s trng hp c th coi hot bng nng , khi biu thc tch s tan c dng n gin:

[ ] [ ]n m

m n n mA BT A B

+ =

Gi tr T cng b th cht in li cng t tan v ngc li. Gi tr T l mt hng s xc nh mt nhit xc nh. Tch s tan l hng s cn bng, ln ca n ch ph thuc vo bn cht cht

tan, nhit . Tch s tan c lin h cht ch vi tan S (biu th bng nng mol) ca cht

in ly t tan.

. .n m

m n m nA BT m n S

+=

Bng 5. Tch s tan ca mt s hp cht in ly t tan (T) 20oC

Cht in ly T Cht in ly T

AgCl 2,8.10-10 BaSO4 1,5.10-9

AgBr 5.10-13 BaCO3 1,6.10-9

AgI 8,5.10-17 BaCrO4 1,2.10-10

Ag2S 1.10-50 Al(OH)3 5.10-33

Ag2CrO4 1,9.10-12 Cr(OH)3 7.10-31

AgCNS 1.10-12 CaSO4 2,4.10-5

ZnS 1.10-20 PbS 4.10-26

(1-4)

(1-5)

(1-7)

(1-6)

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1.1.4. Hot cc ion trong dung dch a. Khi nim v hot

Hot l nng hiu dng ca cc ion trong dung dch. Biu thc: a = f.C (1-8) Trong : C: l nng thc

a: l hot . f : l h s hot .

- Vi dung dch long cht in ly yu, tng tc cc ion khng ng k: f = 1, a = C.

- Vi dung dch cht in ly mnh hoc dung dch m c ca cht in ly yu: 0 < f < 1 v a < C.

Thc t cho thy nng cc ion trong dung dch cng ln th s tng tc tng h gia chng cng mnh, s khc nhau gia a v C cng ln, f cng nh v ngc li.

Trong nc t nhin, nng cc ion thng rt nh (< 200 mg/l) nn s khc nhau gia a v C thng khng ln. Nhng vi nc c khong ha rt cao (>500mg/l) th s khc nhau gia a v C li l ln. b. Cch tnh h s hot

* Tnh lc ion: Lc ion c trng cho cng trng lc chung ca cc ion trong dung dch.

Nhm nh gi tc ng tnh in ca cc ion trong dung dch.

K hiu:

Lc ion c tnh theo cng thc:

2 2 21 1 2 2

1

1 1( ...2 2

n

i ii

C Z C Z C Z=

= + + = (1-9)

Trong Ci: nng th i v Zi: in tch ca n. V in tch ca cc ion l khng i, hm lng cc ion trong nc thng c

tnh bng mg/l nn ngi ta c th chuyn i cng thc trn bng cng thc:

)...(21

2211 nn KCKCKC +++= (1-10)

C1, C2, Cn: nng mi loi ion (mg/l). K1, K2, Kn: h s t l chuyn i c tnh sn.

Bng 6. Gi tr h s t l chuyn i K

Ion K Ion K

K+ 0,026.10-3 NO3- 0,016.10-3

Na+ 0,043.10-3 HCO3- 0,016.10-3

Ca2+ 0,10.10-3 Cl- 0,028.10-3

9

* Tnh h s hot : Trong thc t ch c th xc nh h s hot trung bnh f . Chng hn vi

cht in ly AmBn:

)( .nm nm fff + + = (1-11)

Cng thc thc nghim tnh h s hot f ph thuc vo gi tr ca 298oK nh sau:

- Vi dung dch c lc ion 0,01, h s hot f tnh theo cng thc:

2.5,0lg =f (1-12)

- Vi dung dch c lc ion 0,01 < 0,5 th h s hot f c tnh theo cng thc:

+

=

1.5,0

lg2

f (1-13)

- Vi dung dch c lc ion > 0,5 th h s hot f c tnh theo cng thc:

Ar

f ++

=

.10.33,01.5,0

lg8

2

(1-14)

r : bn knh ion ngm nc tnh theo cm. Ch : Mt s ion c bn knh ngm nc vo khong 3.10-8 cm, th d OH-, F-, Cl-, Br-, I-, NH4+, Ag+,; r 4.10-8 cm cho: Na+, H2PO4-, Pb2+, SO42-, PO43-,...

Mt s ion ngm nc khc c bn knh to ln, c bit ion H+ ngm nc c bn knh rt ln: 9.10-8 cm.

A: h s thc nghim ph thuc vo bn cht ca cht in ly v dung mi.

Bng 7. Gi tr h s hot f theo v in tch ion Z

H s hot f

Z = 1 Z = 2 Z = 3 0,000 1,00 1,00 1,00

0,001 0,96 0,87 0,72

0,002 0,95 0,81 0,63

0,003 0,92 0,72 0,48

0,010 0,9 0,63 0,35

T bng gi tr trn cho thy: lc ion cng ln v ion c in tch cng ln, th h s hot f cng nh, tc l s khc gia nng v hot cng ln. Cn khi lc ion nh, vi cc ion c in tch thp, th f 1. Mt s v d

V d 1: Xc nh lc ion ca dung dch hn hp HCl 0,1M v CaCl2 0,2M. Gii:

HCl v CaCl2 trong dung dch coi = 1.

10

HCl = H+ + Cl- CaCl2 = Ca2+ + 2Cl-

[H+] = 0,1M; [Ca2+] = 0,2M; [Cl-] = 0,1 + 0,4 = 0,5M.

7,0)2.2,01.5,01.1,0(21 222 =++=

V d 2: Tnh hot ca cc ion K+ v Cl- trong dung dch KCl 0,01M. Gii:

KCl = K+ + Cl-

Lc ion ca dung dch:

01,0)1.01,01.01,0(21 22 =+= 1,0=

H s hot ca cc ion:

Mff ClK 05,01,0.1.5,0lglg2 === +

Mff ClK 9,01,0.1.5,02 == +

Hot ca cc ion: Maa ClK310.901,0.9,0 === +

V d 3: Tnh hot ca nhng ion trong dung dch hn hp: KCl 10-3M v MgSO4 10-3M.

Gii: Lc ion ca dung dch:

33232322 10.5)10.210.210.1310.1(21 =+++= 210.7 =

H s hot ca cc ion:

Mff ClK 035,007,0.1.5,0lglg2 === + M920ff ClK ,== +

MffSOMg

14,007,0.2.5,0lglg 224

2 === + Mff SOMg 72,0242==

+

Maa ClK43 10.2,910.92,0 === + ;

MaaSOMg

410.2,724

2==

+

V d 4: Trong 1 lt dung dch CaCO3 bo ho c cha 0,292 gam Mg2+ v 0,851 gam Cl- th hm lng CO32- ( tnh theo hot ) l bao nhiu ? Bit .10.6,3 93

=CaCO

Gii: CaCO3 ho tan theo phng trnh:

CaCO3 Ca2+ + CO32-

].[]..[. 232

23

223

23

+++ == COfCafaa COCaCOCaCaCO

Trong dung dch: ][][ 232 + = COCa fff COCa == + 232

11

nn ta c: 2223 .][3 fCOCaCO=

fff

CO5

2

9

223

10.610.6,3][

==

=

V 3CaCO

rt nh nn nng cc ion Ca2+ v CO32- rt nh. Khi tnh trong trng hp ny c th b qua cc ion Ca2+ v CO32- m ch quan tm ti 2 ion Mg2+ v Cl-.

== + 012,024292,0292,0][ 2 gamg

== 024,05,35

851,0851,0][ gamCl

036,0)1.024,02.012,0(21 22 =+=

036,01036,02.5,0

lg2

+=f 48,0=f

lmgf

CO /5,710.5,1248,0

10.610.6][ 555

23 ====

1.2. Nc thin nhin 1.2.1. Gii thiu chung

Nc thin nhin chim 1% tng lng nc trn tri t, bao gm cc loi nc c ngun gc thin nhin nh nc sng, ao, h, sui, bin, i dng. Nc thin nhin l mt h d th nhiu thnh phn, v nc thin nhin lun cha nhng lng cc cht tan v khng tan c ngun gc t v c v hu c.

Tng tr lng nc t nhin trn Tri t khong t 1.385.985.000 km3 n

1.457.802.450 km3. Nc t nhin tp trung phn ln bin v i dng (trn 97,61%), sau l cc khi bng cc (1,83%), ri n nc ngm (0,54%). Nc

12

ngt tng mt chim mt t l khng ng k (0,02%),... Lng nc trong kh quyn chim khong 0,001%, trong sinh quyn 0,002%.

Nc cn cho nhu cu sng ca mi c th. Nc chim ti 80 - 90% trng lng sinh vt sng trong mi trng nc v 44% trng lng c th con ngi. Nc p ng cc yu cu a dng ca con ngi: ti tiu cho nng nghip, sn xut cng nghip, to ra in nng v t thm v p cho cnh quan.

Hng nm c khong 5 triu km3 nc bay hi t t v cc ngun nc mt (sng, h, i dng,...) sau ngng t v ma xung, lng nc do khi nc trn bay hi hp th xp x gn 3.1020kcal/nm.

Bng 8. Th tch cc ngun nc t nhin trn th gii

Ngun nc Th tch, 1000 km3 %

i dng 1.348.000 97,312 Bng 29.000 2,093

Nc ngm 8.000 0,577 H, sng sui 200 0,014

Nc chy trn mt t 40 0,003 Tng cng 1.385.240 100

Lng nc ngt c con ngi s dng c ngun gc ban u l nc ma c chng 105.000 km3, trong khong 1/3 chy ra sng, cn li 2/3 quay tr li kh quyn do bc hi b mt v thot hi nc thc vt. Nu xem 1/3 lng nc ma k trn (khong 40.000 km3) l ngun nc cung cp tim nng cho con ngi th vi s dn hin ti, mi ngi mi ngy nhn c trung bnh 16 lt nc. 1.2.2. Cc qu trnh hnh thnh thnh phn ha hc ca nc thin nhin a. Khi nim chung v s hnh thnh thnh phn ha hc ca nc thin nhin

S hnh thnh thnh phn ha hc ca nc thin nhin ph thuc vo iu kin hnh thnh, qu trnh di chuyn ca n, thnh phn v cc c tnh ha hc ca cc cht m n tip xc. Bao gm cc qu trnh ch yu sau:

- Qu trnh ho hc (ho tan, hp th, oxy ha kh,). - Qu trnh sinh ha (quang hp, phn gii,). - Qu trnh vt l (bc hi, ng bng, ma, tan bng,).

S tc dng tng h ca nc vi mi trng xung quanh c th lm tng hm lng cc cht trong nc, c th lm gim hm lng cc cht trong nc v cng c th thay i thnh phn ha hc ca nc thin nhin. b. Cc yu t nh hng n qu trnh hnh thnh thnh phn ha hc ca nc thin nhin

* nh hng do qu trnh phong ha ca : Cc tnh ton ca cc nh a cht cho thy: trng lng cc nhm trong v tri

t c t l phn b: macma 65%, bin cht (mentamorphic) 25% v trm tch 10%. macma gm hai loi: xm nhp v phn xut c xut x t qu trnh

hot ng ca ni la.

13

trm tch l kt qu tc ng ca nhng cu t (c thnh phn trong kh quyn hoc thy quyn) ln b mt v tri t v l kt qu ca qu trnh lng. Nhng trm tch quan trng l nham thch, ct v vi.

mentamorphic c to thnh t macma v trm tch do tc ng ca

p sut v nhit cao. Trong xy ra cc qu trnh ha hc v vt l l nhng qu trnh dn ti nhng khong bn nhit v c t trng c bit.

Qu trnh phong ho ca bao gm: - Phong ha vt l - Phong ha sinh hc - Phong ho ho hc

Nhng qu trnh phong ho ha hc tng i n gin xy ra nh: Qu trnh ho tan cht kt tinh, qu trnh phong ho do CO2, qu trnh thu phn, qu trnh phong ho oxy ho kh.

Kt qu ca cc qu trnh phong ha : sn phm c ho tan trong nc v b cun tri theo dng chy v lm cho thnh phn ha hc ca nc thay i. Tuy nhin, trong qu trnh di chuyn trong nc, tnh di ng ca cc nguyn t rt khc nhau:

- Cc cht di ng mnh: Cl, Br, I, S - Cc cht d di ng: Na, K, Mg, Ca - Cc cht t di ng: SiO2, P, Mn - Cc cht di ng rt yu: Fe, Al, Ti

Do tnh di ng ny m ta thy nc bin tp trung cc cht c tnh di ng mnh nht nh Na+, Cl- chim u th.

* nh hng ca nc khi tip xc vi t: Thnh phn ha hc ca t ph thuc vo tng loi t v v tr loi t tn

ti.

Nc trong t lu thng nh nhng rnh nh vi ng knh trn 10 m. Khi nc thm qua cc loi t khc nhau th s thay i thnh phn ha hc ca nc cng khc nhau.

14

Nc khi tip xc lu di vi t, ngoi kh nng ha tan cc cht trong t cn c kh nng trao i cht vi t v nh vy cng lm thay i thnh phn ho hc ca nc. C th:

- t hp ph cc cation ca nc: t l mt h keo, cc ht keo u mang in tch m. V vy keo t c kh nng hp ph cc cation ca nc rt mnh.

- t c kh nng trao i cc ion vi nc: Khi hm lng mt loi cation no ca nc kh cao th c th trao i vi cation khc trong t.

* nh hng ca cht hu c: Cht hu c trong nc c hiu l tn tch hu c ang phn hy, tn ti trong

nc c hai dng ha tan v l lng, khng bao gm sinh vt sng, tr sinh vt c kch thc rt nh nh vi sinh vt v to n bo, cng khng bao gm cc tn tch hu c th, c kch thc ln, c th ly ra khi nc.

Cc cht hu c sng v cc sn vt m n sinh ra trong qu trnh sng c nh hng rt ln n thnh phn ha hc ca nc t nhin.

* nh hng ca kh hu: Kh hu c th lm thay i cc qu trnh phong ha; thay i hot ng sng ca

cc vi sinh vt; thay i qu trnh oxy ha kh trong nc

* nh hng ca nc ngm: Nc ngm tip xc vi nham thch nhiu nht, nn thnh phn ha hc ca

nc ngm ch yu ph thuc vo tng nham thch v t cha n. Thnh phn ha hc v khong ha ca nc ngm tng su bin i theo

ma t hn nc mt. Nc ngm di su c th hu nh khng cha cht hu c v vi khun. Tuy nhin, kh nng nhim cc nguyn t vi lng trong nc ngm cao hn nc mt, in hnh l nhim Fe, Mn, As, F-, Br-, S2-, 1.2.3. Th kh ha tan v nng ion hyro (H+)

Ni chung cc cht kh trong kh quyn u c mt trong nc do kt qu ca hai qu trnh c bn l khuch tn v i lu.

C nhiu loi kh ha tan trong nc v c nh hng ti thnh phn ha hc ca nc. Chng ta nghin cu mt s kh ch yu ha tan trong nc. a. Kh oxy

Oxy l loi kh t ha tan trong nc v khng tc dng vi nc v mt ha hc. Oxy ha tan trong nc dng phn t t do gi l lng oxy ha tan, vit tt l

DO (Dissolved Oxygen). Oxy ha tan trong nc l iu kin ch yu cho s tn ti ca nhiu cht hu c sng trong nc v nhiu qu trnh ha hc khc nhau xy ra trong nc.

Mt s quy lut v s ha tan oxy t pha kh vo nc: - C hai nhm ch yu c tc dng lm giu oxy trong nc: oxy trong khng kh

ha tan vo nc v oxy c to ra do qu trnh quang hp. - Cc ngun ch yu lm gim oxy ha tan trong nc: oxy cn oxy ho cc cht v

c, hu c c trong nc; oxy cn cho sinh vt sng s dng h hp; oxy b gii hp khi nc.

15

- Trong nc, oxy tham gia ch yu vo cc qu trnh: Oxy ha cc cht hu c bng vi sinh vt, oxy ho cc cht nit bng vi sinh vt v oxy ho cc cht ha hc khc.

- nhit nht nh, nng bo ha ca oxy trong nc ch ph thuc vo p sut ring phn ca oxy trong pha kh (tun theo biu thc nh lut Henry).

- Khi nhit cng tng, nng mui tng, qu trnh h hp tng dn n ha tan ca oxy cng gim.

Bng 9. nh hng ca nhit n ha tan ca oxy trong nc

toC Nng bo ha (mgO2/l) toC Nng bo ha (mgO2/l)

0 14,65 20 8,84 10 11,00 30 7,29

- Hm lng oxy ha tan gim dn theo chiu su ca nc. - Khi mui tng, ha tan oxy gim.

Bng 10. nh hng ca nng mui n ha tan ca oxy trong nc

toC 0 10 20 30 S gim tan ca oxy (mg/l) ng

vi bin i mi mui (1g/l) 0,08405 0,06217 0,04777 0,04085

- Hm lng oxy trong nc t nhin thng dao ng t 0 n 14 mg/l. - Trong nc ngt lng oxy ha tan (DO) 0oC v 1 atm l 14,65 mg/l. Ngi ta

thng ly DO 250C, 1 atm bng 8 mg/l. - Mc tiu th oxy trong nc bi cc cht nhim c biu th qua ch s nhu

cu oxy sinh ha (Biochemical oxygen Demand - BOD) hay nhu cu oxy ha hc (Chemical Oxygen Demand - COD). b. Kh cacbonic

H CO32- - CO2 trong nc ng vai tr rt quan trng v n tham gia vo qu trnh trao i gia kh quyn v nc trn lp nc b mt.

Mt s quy lut v s ha tan cacbonic t pha kh vo nc: - Ngun cung cp CO2 ho tan trong nc: t khng kh, sn phm ca qu trnh

oxy ha cc hp cht hu c v s thi ra trong qu trnh h hp ca vi sinh vt. - Ngun lm gim CO2 trong nc: qu trnh gii hp; qu trnh xm thc ca mui

cacbonat v qu trnh quang hp sinh vt tiu th CO2. - CO2 l mt kh d ha tan trong nc v ha tan trong nc tng ln theo chiu

gim ca nhit . Nng CO2 lp b mt tun theo nh lut Henry. - Trong nc, kh CO2 c th tn ti dng t do hoc dng kt hp vi nc.

CO2 kt hp vi nc ph thuc vo gi tr pH ca nc.

Khi pH < 5: CO2 + H2O H2CO3

Khi pH 5: H2CO3 H+ + HCO3-

Khi pH 8: HCO3- H+ + CO32- - Trong thc t c th tng hoc lm gim hm lng ca kh CO2 trong nc b mt.

16

- trm tch, CO2 tham gia phn ng:

CaCO3 + CO2 + H2O Ca(HCO3)2 Ca2+ + 2HCO3-

Qu trnh ny dn n s thay i pH ca mi trng. c. Cc kh khc

i vi cc loi kh khc ha tan trong nc th H2S l loi kh ng ch . Trong lp nc mt c oxy ha tan th hm lng H2S ch tn ti trong mt

khong thi gian ngn v n b oxy ha rt mnh. Trong nc ngm, hm lng H2S c khi t mc cao, lp nc ngm cng

su hm lng H2S cng cao. d. Nng ion H+

Trong nc nguyn cht tn ti cn bng:

H2O + H2O H3O+ + OH-

n gin ta vit: H2O H+ + OH-

2980K: [H+] = [OH-] = 10-7M. Tch s ion ca nc: KW = [H+][OH-] = 10-14. Ta c: pH = - lg[H+] (1-15)

Mi trng axt: pH < 7 Mi trng baz: pH > 7 Mi trng trung tnh: pH = pOH = 7 Trong nc thin nhin, nng ion H+ khng phi ch ph thuc vo s in ly

ca nc m ch yu ph thuc vo t l hm lng H2CO3 ; HCO3- v CO32- v mt s ion khc c trong nc. Cn bng cacbonat ha tan (h ng)

i vi dung dch h cacbonat n gin, c 6 cu t ha tan: CO2, H2CO3, HCO3-, CO32-, H+, OH-. H gm 4 phng trnh quan h:

H2CO3 CO2(aq) + H2O ][)]([

32

2

=C

aqC (1-16)

H2CO3 H+ + HCO3- ][]][[

32

332

=

+

CC

COH (1-17)

Tng CO2 ha tan ([H2CO3*]): [H2CO3*] [CO2 (aq)] + [H2CO3] (1-18)

(t: 1-18) *][

]][[

32

31

=

+

CC

HCO3- H+ + CO32- ][

]][[

3

23

2

+

=CC (1-19)

H2O H+ + OH- ]][[2+ = HOH (1-20)

Lc ny trong h gm c cc cu t: CO2 (aq), H2CO3, HCO3-, CO32-, H+, OH-. Nng tng hp cc hp phn ca h (CT) c biu din di dng:

17

CT = [H2CO3*] + [HCO3-] + [CO32-]

M ][

*][][ 3213 +

=

CHKHC ;

232213212322

3 ][*][

][*][

.][][

][][

++++

=

=

=

CHKKCHKHCC

CT = [H2CO3*] + [HCO3-] + [CO32-] = [H2CO3*] )][][1( 2

211++

+

+KK

TCCH = *][ 32 [ ]2211 ][][1 ++ +

+KK -1

(1-21)

Cc cng thc trn cho php nh lng phn trm tng dng cacbonat c trong nc. Chng hn: Trong ngun nc bin, gi tr K1 o c ti 220C v p sut 1atm l 4.10-7, ln hn bn bc so vi gi tr K2 (4,2.10-11). Bi vy, s phn ly ca axt cacbonic ch yu l phn ly bc mt. Khi :

][*][

][3

321

+

=HC

CHK

iu ny cho thy nng ion [H+] ph thuc ch yu vo nng bicacbonat [HCO3-], theo quan h t l nghch.

Nu ngun chnh to ra ion HCO3- l do phn ly ca cc mui Ca(HCO3)2, Mg(HCO3)2,khi lm tng nng HCO3-, nh th lm gim nng H+, tc l tng pH. Gi s trong dung dch cha mt lng ng k ion Ca2+:

HCO3- H+ + CO32-

+

+

==23

3

3

23

22C

C

C

C

a

aa

a

aa

CaCO3 Ca2+ + CO32- +

+ == 2

23

23

2 2.Ca

CCCa aTaaaT

++

+

==

322

3 ... 22 CCaCaC aa

Ta

T

aa

m a =f.C +++

= 2233

...2CaCaHCOHCO

CfCfT

a (1-22)

Nhit v p sut thy tnh c nh hng ng k n pH ca nc. Nu nhit (hoc c p sut) tng th hng s phn ly ca H2CO3 v H2O tng ln, dn n pH tng. Nu nhit tng th ha tan ca kh CO2 trong nc li gim, khi pH tng.

250C, hng s phn ly axt H2CO3, HCO3-:

H2CO3 H+ + HCO3- 65,6

32

31 10][

]][[ +

=

=C

C

HCO3- H+ + CO32- 33,103

23

2 10][]][[

+

=

=CC

][][][

3

32

1

+

=

CC

][][][

23

3

2

+

=

CC (1-23)

18

Bng 11. Gi tr hng s nng K1*, K2* ca axit cacbonic trong nc bin

Nhit %oCl

10 14 20 24 28 30 K1(10-8)

15 0,74 0,8 0,89 0,95 0,99 1,01 20 0,8 0,87 0,97 1,04 1,07 1,10

K2(10-9) 15 0,6 0,66 0,76 0,83 0,9 0,93

20 0,8 0,89 1,02 1,12 1,21 1,26

(K1*, K2*: hng s nng bin i theo mui) T biu thc trn cho thy: K1, K2 l hng s, t l vi nng h cacbonat ph

thuc vo nng ion H+, tc l ph thuc vo pH nc. - Biu thc K1:

pH = 6,35, 165,610][ K== + [HCO3-] = [H2CO3 ]

pH < 6,35, 65,610][ + > [HCO3-] < [H2CO3 ]

pH > 6,35, 65,610][ + [H2CO3 ]

- Biu thc K2:

pH = 10,33, 233,1010][ K== + [HCO3-] = [CO32- ]

pH < 10,33, 33,1010][ + > [HCO3-] > [CO32- ]

pH > 10,33, 33,1010][ +

19

- Trong khong pH thng gp trong thm lc a (4,35

20

c. Trong cc h nc ngt hm lng SO42- cao hn trong nc sng. Trong nc bin, hm lng SO42- gim dn theo su. c. Ion kim loi kim (ch yu l K+, Na+)

Ngun gc nguyn thy ca cc ion kim loi kim K+, Na+ ha tan trong nc l t s phong ha, ha tan cc macma trn b mt Tri t.

Trong nc t nhin, hm lng ion Na+ thng ln hn ion K+ v ion K+ tham gia vo chu trnh thc vt mnh hn, mt khc, trong mi trng nc, ion K+ cn phn ng vi axt silixic ha tan v cc thnh phn khc hnh thnh cc khong th sinh nh t st,

Trong nc i dng, ion Na+ c hm lng ln nht trong cc cation, cn ion K+ c hm lng nh nht trong cc cation. Cc ion cn li c hm lng rt t. Chng hn, trong nc bin hm lng Rb khong 0,2 mg/l; Li ch khong 0,1 mg/l.

Hm lng trung bnh trong nc i dng: Na+: chim 77,34% tng cc cation (tnh theo ml/kg), ion K+ ch chim 1,64%.

Ion Na+ c trng cho nc c khong ha cao. Khi hm lng Na+ t vi g/l th n l ion ch yu c trong nc. d. Cc ion Ca2+, Mg2+

Ngun gc nguyn thy ca cc ion Ca2+, Mg2+ trong nc thin nhin l t s phong ha ha hc cc loi v khong vt trn b mt Tri t bi nc ma, nc ngm, (cha axt cacbonic v cc axt khc):

CaCO3 + CO2 + H2O Ca2+ + 2HCO3- Nhng khi trong nc c hm lng HCO3- cao s din ra qu trnh to kt ta.

y l nguyn nhn chnh gy ra cc nham trm tch vi. Cc ion Ca2+, Mg2+ c ha tan vi hm lng ln, chng l cc ion chnh ca

nc thin nhin. Trong nc c khong ha cao th hm lng Ca2+ nh hn hm lng Mg2+.

Trong nc i dng, hm lng trung bnh ca Ca2+ chim 3,37% trong tng cation; Mg2+ chim 17,62% (tnh theo ml/kg). e. Cc ion HCO3-, CO32-

HCO3-, CO32- trong nc thin nhin c ngun gc t kh CO2 trong kh quyn, trong khng kh t v khong cacbonat trn b mt tri t, c bit l trong nc c khong ha thp.

Trong nc lc a, c bit l nc c ngun gc kh quyn, nc ngm trong cc thnh phn to trm tch cacbonat v nc c ngun gc macma, ion HCO3- c hm lng ln nht trong cc ion chnh (v d: nc khong Vnh Ho, c hm lng HCO3- ti 2445mg/l). Ion HCO3- cng l dng tn ti thng chim phn ln trong h cacbonat ca nc t nhin. Ion CO32- trong nc(ch tn ti pH 8,3) t gp hn v nu c, cng thng c hm lng nh hn so vi ion HCO3-, Cl-, SO42-. Cc ion ny xut hin trong nc do qu trnh ha tan cc mui cacbonat c tc dng ca CO2:

MCO3 + CO2 + H2O M2+ + 2HCO3- f. Hp cht sunfua

Trong nc thin nhin, cc hp cht sunfua c th tn ti th kh H2S ha tan, dng HS-, S2- ca axt H2S v ca cc mui ca hai dng . S in ly ca H2S:

21

H2S H+ + HS- 72

1 10.91,0][]][[ + =

=SS

HS- H+ + S2- 152

2 10.2][

]][[

+

=

=

S

S

1.2.5. Cc nguyn t vi lng v cht hu c trong nc thin nhin a. Cc nguyn t vi lng trong nc thin nhin

Cc thnh phn vi lng trong nc l nhng thnh phn tn ti vi hm lng < 1mg/L, tc l thuc c hm lng phn triu, phn t (ppm mg/L, ppb g/L) hoc nh hn na. Cc thnh phn vi lng c tnh hm lng c hai dng: dng ha tan (bao gm c kh ha tan) v dng l lng.

C th chia thnh phn vi lng trong nc thnh hai nhm sau: - Cc thnh phn v c vi lng: Hg, As, Se, Cd, Pb, Cu, Zn,,, Br, I, F, xianua,

sunfua, tn ti trong nc nhiu dng nh cation, anion, phc v c, phc hu c, kh (dng ha tan); ion hp ph vo cc cht l lng v cht keo trong nc, cc vi kt ta ca cc khong th sinh c hnh thnh trong nc (dng l lng v dng keo).

- Cc thnh phn hu c vi lng: phenol v dn xut, thuc bo v thc vt, cht ty ra, du, m,

Cc nguyn t thuc nhm halogen (tr clo), l nguyn t vi lng c trong nc thin nhin. Chng c tc dng rt ln trong qu trnh sng ca sinh vt. V d nh flo, nu khng c flo hoc c qu nhiu flo (> 1,5 mg/l) u gy cc bnh v rng. Nu thiu it gy cc bnh ni tit

Cc nguyn t thuc nhm kim loi nng (Cu, Ni, Co, Pb, Hg, Cd ) c rt t trong nc thin nhin.

Nguyn t phng x c tnh cao nht trong nc thin nhin. Hm lng ca chng rt nh nhng li rt d nhn thy. Cc nguyn t phng x thng c trong nc thin nhin l Radon v Radi.

Ty theo hm lng v dng tn ti ca cc thnh phn v c, hu c vi lng trong nc c th l yu t c li hoc gy nguy him cho s sng ca sinh vt v con ngi. b. Cht hu c trong nc thin nhin

Trong nc thin nhin tn ti cc cht hu c c th chia lm hai loi. - Loi 1: Bao gm cc cht hu c cu to nn c th cc loi sinh vt sng trong

nc nh cc loi ng vt, thc vt sng trong nc. - Loi 2: Bao gm cc cht hu c do sinh vt thi ra trong qu trnh sng ca chng

hoc trong qu trnh phn gii cc cht hu c t c th cc sinh vt sau khi cht. Cc cht hu c thng c thnh phn rt phc tp. Ngoi cc nguyn t chnh

nh cacbon, hyro, oxy cn c nhiu loi nguyn t khc nh N, P, K, Ca, Fe, Na, Tt c nhng cht ny u c th l ngun cung cp cc cht cho nc thin nhin.

Hm lng v mc khong ha cht hu c trong nc c xc nh v nh gi bi hng lot cc thng s sau:

- Cc thng s biu th gin tip: oxy ho tan (DO) v bo ha oxy; nhu cu oxy ha hc (COD); nhu cu oxy sinh ha (BOD); t s COD/BOD; s tiu th thuc tm ( oxy ha); hm lng cc ion nit v c; hm lng phophat ho tan,

22

- Cc thng s biu th trc tip: tng cacbon hu c (TOC - total organic carbon), cacbon hu c ha tan (DOC - dissolved organic carbon); nit hu c; photpho hu c, 1.3. Ha hc nc sng 1.3.1. c im chung

Thnh phn ha hc ca nc sng c quyt nh bi c tnh vn c ca con sng y. c tnh y bao gm cc yu t sau: Ngun nc cung cp, cu to a cht, kh tng, thu vn, kh hu, tc dng chy,

Bng 13. Thnh phn ho hc trung bnh ca nc sng h

Thnh phn % trng lng Thnh phn % trng lng

CO32- 35,2 Ca2+ 20,4 SO42- 12,4 Mg2+ 3,4

Cl- 5,7 Na+ 5,8 SiO2 11,7 K+ 2,1

NO3- 0,9 (FeAl2)O3 2,7

khong ha ca nc sng thng nh hn cc loi ngun nc khc v nc sng tip xc vi cc tng nham thch trong thi gian rt ngn do nc sng lun chuyn ng, kh nng ha tan cc cht rt thp. 1.3.2. ng thi ca cc ion ch yu nht

Da vo khong ha ca sng, ngi ta chia sng lm 4 cp: - Sng cp I c khong ha rt thp: < 200 mg/l.

- Sng cp II c khong ha trung bnh: 200 500 mg/l.

- Sng cp III c khong ha cao: 200 500 mg/l. - Sng cp IV c khong ha rt cao: > 1000 mg/l.

Hu ht nc cc con sng trn tri t u l sng cp I hoc cp II. C hai ngun cung cp nc cho sng l ngun nc trn mt t (nc ma, nc

tuyt v bng) v ngun nc ngm (nc trong tng t bi, nc trong tng ngm). Kh hu c nh hng rt ln ti ngun cung cp nc cho sng, lm cho lng

nc cung cp cho sng thay i, thnh phn ha hc ca cc ngun nc cung cp cho sng thay i v do lm cho thnh phn ha hc ca nc sng cng thay i. Nh vy, mi con sng u c ng thi thy ha c th.

Nhn chung i vi cc con sng c khong ha trung bnh v thp, khi khong ha tng ln th hm lng cc ion cng tng ln tng ng, c bit l hm lng Cl- v HCO3- tng rt nhanh. i vi cc con sng c khong ho cao, khi khong ha tng ln th hm lng cc ion cng tng theo, nhng c bit hm lng HCO3- gim. 1.3.3. ng thi ca cc hp cht hu c

Cc cht hu c trong nc sng tn ti di dng trng thi cc cht mc nt b xi t t v m ly chy vo sng v cc sn vt c phn gii t cc cht hu c to ra. a. oxy ha ca nc (COD - Chemical Oxygen Demand)

23

xc nh hm lng ca cc cht hu c tn ti trong nc ngi ta dng i lng ch oxy ha ca nc, vit tt l COD.

oxy ha ca nc l s miligam oxy cn thit oxy ho ht cc cht hu c c trong 1 lt nc. Nh vy qua lng oxy cn dng c th tnh ra hm lng cc hp cht hu c c trong nc. b. Phn loi nc sng

Da vo oxy ho ngi ta chia nc sng ra cc loi:

- Nc sng c COD rt thp: COD 2 mg/l.

- Nc sng c COD thp: COD = 2 5 mg/l.

- Nc sng c COD trung bnh: COD = 5 10 mg/l.

- Nc sng c COD hi cao: COD = 10 20 mg/l.

- Nc sng c COD cao: COD = 20 30 mg/l. - Nc sng c COD rt cao: COD > 30 mg/l.

1.3.4. ng thi ca cc cht kh ha tan v ca ion H+ a. ng thi ca cc cht kh ha tan

Hm lng ca cc cht kh ha tan vo nc ph thuc vo cc yu t: - Bn cht ca cht kh; - Nhit ca nc; - Cng ca cc qu trnh quang hp trong nc; - Thnh phn ca ngun nc b sung cho sng; - p sut ring phn ca cc kh trn b mt nc.

Thng thng hm lng oxy trong nc sng khng vt qu 15 mg/l v hm lng CO2 khng qu 30 mg/l.

* Kh oxy: Hm lng oxy trong nc sng ph thuc vo nhiu yu t.

Nhn chung v ma h, hm lng oxy trong nc sng vo khong 6 8 mg/l v nhit cao; v ma ng, th khong 8 12 mg/l v nhit ca nc thp. Hm lng ny cn ph thuc vo ngun sinh vt sng trong nc v kh hu ca khu vc sng chy qua.

* Kh CO2: ng thi ca kh CO2 trong nc sng tng t ng thi ca oxy. V ma ng hm lng CO2 cao v sinh vt trong nc t hot ng nn lng

CO2 tiu th t, ng thi ngun nc chnh cung cp cho sng v ma ng l nc ngm thng c nhiu CO2.

V ma h, nhit cao, hot ng ca cc vi sinh vt nhiu nn hm lng ca CO2 gim nhiu, ch vo khong 1 - 3 mg/l. Thm ch c lc khng cn CO2 trong nc, nn vi sinh vt phi ly CO2 trong khng kh. b. ng thi ca ion H+

24

Hm lng ion H+ trong nc sng ph thuc nhiu vo ma v. - Gi tr pH ca hu ht cc con sng trong ma ng t 6,8 - 7,8 cn trong ma h

cao hn, t 7,8 - 8,8. - Cc con sng c b sung bng ngun nc ca cc m ly th thng c pH thp

(pH < 6). Cc con sng khu vc nhit i v ma h pH c th tng ln ti 9. 1.4. Ha hc nc ngm 1.4.1. c im chung

Nc ngm tip xc trc tip v hon ton vi t v nham thch: nc ngm c th l cc mng mng bao ph cc phn t nh b ca t, nham thch; l cht lng c cha y trong cc ng mao dn nh b gia cc ht t, ; nc ngm c th to ra cc tia nc nh trong cc tng ngm nc.

Cht lng nc ngm ph thuc vo cu trc a tng m nc thm qua. Do vy nc chy qua cc tng a tng cha ct hoc granit thng c tnh axit v cha t cht khong. Khi chy qua a tng cha vi th nc thng c cng v kim hydrocacbonat kh cao. Ngoi ra, cc c trng chung ca nc ngm l:

- c thp. - Nhit v thnh phn ho hc tng i n nh. - Khng c oxy, nhng c th cha nhiu kh H2S, CO2, - Cha nhiu cht khong ho tan, ch yu l st, mangan, canxi, magie, flo. - Khng c s hin din ca vi sinh vt.

Nc ngm cng c chia thnh cc tng, lp khc nhau v thnh phn ha hc ca cc tng lp cng khc nhau.

nh hng ca kh hu i vi nc ngm khng ng u. Thnh phn ca nc ngm khng nhng chu nh hng v thnh phn ha hc

ca tng nham thch cha n m cn ph thuc vo tnh cht vt l ca cc tng nham thch .

Nc ngm t chu nh hng ca sinh vt nhng chu nh hng nhiu ca vi sinh vt. tng su do khng c oxy v nh sng nn vi sinh vt ym kh hot ng mnh, chi phi nhiu n thnh phn ha hc ca nc ngm. V vy thnh phn ha hc ca nc ngm cha nhiu cht c ngun gc vi sinh vt.

Nc ngm khng c oxy. Khi pH < 5,5, trong nc ngm thng cha nhiu kh CO2. y l kh c tnh n mn kim loi v ngn cn vic tng pH ca nc. Thng nc ngm c nng kh CO2 khong 30 - 50 mg/l, nhiu ni nc ngm mch su (200 300 m) nng CO2 c th ln n 320 mg/l. Cc bin php lm thong c th ui kh CO2, ng thi thu nhn oxy h tr cho cc qu trnh kh st v mangan. Ngoi ra, trong nc ngm c th cha kh H2S c hm lng n vi chc mg/l. y l sn phm ca qu trnh phn hy ym kh cc cht hu c c trong nc. 1.4.2. S phn tng ca nc ngm a. Nc tng trn

T mt t thm xung to ra tng nc ngm ny, sau li c tho tiu ra sng, h. V vy nc tng ny giao lu mnh.

25

Thnh phn ha hc ca nc tng trn ph thuc vo: thnh phn ha hc ca ngun nc mt, thnh phn ha hc ca lp tng t cha n, kh hu, thi tit

Do s bay hi ca nc mt t, nc ngm tng trn c th theo cc mao dn ln gn hoc thm ch ln ti mt t mang theo nhiu mui khong, khi nc bay hi li phn mui khong ny cho t. y l mt trong nhng yu t quan trng lm cho t kh hn bin thnh t chua mn. b. Nc tng gia

Nc tng ny nm bn di mt gc xm thc, ph thuc cht ch vo mc bc l v a cht thu vn.

Nc t trn t thm xung ti tng nc ngm ny thng rt chm, c bit l vi tng nham thch t thm nc. V vy, nc tng ny l nc chm giao lu. Trong tng ny, thnh phn nguyn thu ca nc ban u b thay i nhiu do nc v nham thch tc dng mt thit vi nhau hoc do s trn ln cc ngun nc khc. c. Nc tng di

Nc tng ny cch bit hon ton vi mt t v hu nh khng giao lu. C nhiu gi thit khc nhau v ngun gc ca tng nc ny:

- Do nc tng trn thm xung. - Do nc bin thm vo m c. - Do nc t rt lu b chn vi bi cc tng nham thch hnh thnh m c. - Do di p lc rt ln m nc trong cc nham thch hay bn tch ra m c

1.4.3. Nc khong v nc c thnh phn c bit a. Nc khong

Nc khong l loi nc ngm c cha nhiu cht c tnh cht kch thch v phng din sinh hc. Cc loi nc ny c dng trong vic iu dng sc kho v cha bnh.

Nc ngm c khong ha ln hn 1,0 g/l c th gi l nc khong. Khi khong ho ln hn 50 mg/l th gi l nc mn.

Ngi ta thng cn c vo hm lng mt s cht c tc dng kch thch sinh hc trong nc khong phn loi nc khong.

Hm lng CO2 t do ti thiu c gi l nc khong l 0,250 g/l. Hm lng CO2 t do xp x bng 0,750 g/l mi c gi l nc khong cacbonic.

Thc ra gii hn gia nc nht v nc khong khng r rt. C nhng loi nc c khong ho rt thp, nhng c cha mt s cht c tc dng cha bnh nn vn c gi l nc khong. Ngi ta cng c th phn loi nc khong theo nhit : nc lnh (nhit ca nc < 253oK), nc m (nhit ca nc t 253 - 310oK), nc nng (nhit ca nc > 315oK) b. Nc c thnh phn c bit

Trong nc ngm c loi nc cha rt t cc ion v khng n nh. c bit trong loi nc ny c cha hm lng ion H+ cao nn c gi l nc axt v gi tr pH thng nh hn 4.

Ngun gc ca cc loi nc ny l do cc hp cht ca S trong nham thch b oxy ho dn dn to thnh H2SO4 tan trong nc. Qu trnh ny thng xy ra tng

26

nc ngm su di t, cng c th xy ra trong nham thch trm tch lp trn cng do hot ng ca ni la cung cp. 1.5. Ha hc nc bin 1.5.1. c im chung

Nc bin chim 99% ton b lng nc trn tri t, n bao ph 71% b mt tri t. Th tch nc bin khong 1304.106 km3.

V bin v cc i dng thng nhau nn thnh phn ha hc ca nc bin tng i ng nht theo thi gian v khng gian.

Nc bin c khong ho rt cao, thng vo khong 35 g/l. Nng mui trong nc bin ln hn trong nc ngt 2000 ln.

V bin v cc i dng thng nhau nn thnh phn cc cht trong chng tng i ng nht. Hm lng mui c th khc bit nhiu trong cc i dng nhng t l v nhng thnh phn chnh th hu nh khng i.

Din t theo ngn ng ha hc th nc bin l dung dch ca 0,5 mol NaCl, 0,05 mol MgSO4 v vi lng ca tt c cc nguyn t c mt trong ton cu. 1.5.2. Cc ion ch yu c trong nc bin

khong ho ca nc bin hon ton ph thuc vo lng cc ion ch yu c trong nc bin.

Trong nc bin Na+, Cl-, Mg2+ chim 90%; K+, Ca2+, SO42- chim 3%; cc cht cn li chim 7% tng lng cc cht.

Bng 14. Cc thnh phn ch yu ca nc bin

STT Cht Hm lng trung bnh (mg/l) STT Cht Hm lng trung

bnh (mg/l) 1 Na 10500 16 S 2460 2 K 380 17 Cl 18980

3 Be 0,00005 18 Br 65 4 Mg 1270 19 I 0,06

5 Ca 400 20 Fe 0,01 6 Ba 0,03 21 Zn 0,01

7 B 20 22 C 28 8 Al 0,01 23 Cu 0,003

9 Si 3 24 Mn 0,002 10 Sn 0,003 25 Co 0,005

11 Pb 0,003 26 Ni 0,005 12 N 0,6 27 Ag 0,0003

13 P 0,07 28 Cd 0,0001 14 As 0,003 29 Cr 0,00005

15 Bi 0,0002 30 Hg 0,00003

27

Hm lng cc ion ch yu c trong nc bin c mt th t xc nh. C khong 11 thnh phn ion chnh - chim khong 99,99% tng hm lng cc cht khong ha tan.

- V anion: Cl- (55,04%) > SO42-(7,68%) > HCO3- + CO32-. - V cation: Na+ (30,61%) > K+ > Mg2+ (3,69%) > Ca2+.

Bng 15. Hm lng cc nguyn t vi lng ha tan trong nc bin tn ti di dng cc hp cht khc nhau:

Nguyn t Dng tn ti chnh Nguyn t Dng tn ti chnh

Fe Fe2+, FeCl+, Fe(OH)3 N NO3- Cu CuCO3, Cu(CO3)22-, Cu(OH)+ F F-

Zn Zn2+, ZnCl+, ZnSO4 P HPO42-, NaHPO4, MgHPO4, PO43-

As As(OH)3, As(OH)4- Br Br-

Pd PdCl42- I IO3- Cd CdCl+, CdCl2 Si H4SiO4, H3SiO4-

Pb PbCl-, PbCl2, PbCO3 Te TeO32-, HTeO3- Hg HgCl42-, HgCl3-, HgCl2

Nhn chung hm lng ion trong nc bin rt n nh v mt t l gia chng. Nguyn nhn ca tnh n nh l do nc bin c tnh giao lu rt mnh nn kh nng trn ln l rt cao. V li, th tch nc bin rt ln, mt s thay i nh, cc b s khng nh hng n thnh phn ha hc ca nc bin.

kim: L tng nng anion ca cc axt yu c mt trong nc bin. Trong cc loi ion ch yu ca nc bin th HCO3- v CO32- ca axt cacbonic (H2CO3) v H2BO3- ca axt boric (H3BO3) bi chng c hm lng ln nht. Cc anion khc c hm lng khng ng k nn thng b b qua, ch c tnh n trong mt s trng hp cn thit.

nhng vng ca sng h s kim (= kim/ mui) thng rt cao (khong 760), i dng (khong 660 - 680), khu vc cc ngun nc lc a ra bin cng nhiu th h s kim cng tng (c th ln n 1000). 1.5.3. mn ( mui) ca nc bin

mn ca nc bin l i lng c trng nh lng cho lng cc cht khong rn ha tan (cc mui) trong nc bin.

mui ca nc bin c th bin i trong nhng gii hn kh rng, nhng t l khi lng gia cc thnh phn chnh ca n hu nh khng i mi vng bin trn th gii, tr cc vng ca sng, m ph, vng vnh kn v cc bin km trao i vi i dng. iu ny c Marxet (pht hin nm 1819) v Ditmar (khng nh nm 1876) trn c s nghin cu thnh phn mui nc bin nhiu vng trn th gii v tng kt thnh quy lut c bn:

Trong nc i dng xa b, t s gia nng ca cc ion chnh lun khng i, khng ph thuc vo tr s tuyt i ca mui.

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T quy lut ny c th xc nh chnh xc hm lng ca mt loi ion no , ri bng tnh ton n gin mi lin h bit th d dng suy ra hm lng ca cc ion khc v do vy c th xc nh c mn ca nc bin. Ion Cl- c chn cho mc ch ny v s c mt ca n trong nc bin vi nng ln nht.

Nng trung bnh ca Cl- trong nc b mt i dng l 19,3534 g/kg. Trn c s cc ngh quyt ca hi ngh quc t v hi dng hc nm 1889 v

1901 ti Stckhm (Thu in), M. Knudsen v cng s khng nh: mui ca nc bin l trng lng cn kh tnh bng gam (cn trong chn

khng) ca mt kilogam nc bin, vi iu kin tt c cc halogen trong c thay th bng clo tng ng, nhng mui cacbonat c thay th bng oxt v cc cht hu c b phn hy ht 4800C.

clo nc bin l tng lng (tnh bng gam sau khi quy i tng ng sang clo) ca cc halogen c trong 1 kg nc bin.

i vi nc i dng v cc bin trao i tt vi i dng, mi quan h gia mui (tnh bng g/kg, k hiu S) v clo (tnh bng g/kg, k hiu Cl) nh sau:

S = 0,034 + 1,8050.Cl (1-24) Nhng nm sau ny, mt s tc gi cn xy dng nhng cng thc mi v mi

lin h gia tng nng cc ion (tnh bng g/kg, k hiu I) chng hn:

Lymen v Fleming (1940): I = 0,069 + 1,8112.Cl Kocx (1963): S = 1,80655.Cl

Kocx (1966): I = 1,81578.Cl ;...

Thc nghim chng t I%o gn vi mui thc ca nc bin hn gi tr S; song s sai khc ca chng khng ng k, ch vo khong 0,004 khi mui nc bin nm trong khong 30 - 40.

Tuy nhin mn ca cc i dng cng c dao ng. S dao ng ny ph thuc vo qu trnh cn bng ca qu trnh bc hi v ngng t ca nc bin. Qu trnh ny ph thuc vo nhiu yu t nh v tr a l, kh hu, ma, Chng hn vng nhit i c mn S = 35,7 ; cc bc v cc nam S = 34,5.

mn ca nc bin cn thay i theo su. Ngi ta nhn thy rng t mt ca i dng ti su khong 5000m mn dao ng trong khong t 34,9 - 36,5. 1.6. Phng php nghin cu ho nc v yu cu s dng nc 1.6.1. B tr h thng trm o c cht lng nc

Ty theo nhu cu s dng nc m ngi ta b tr cc trm o c cht lng nc. Vic b tr v tr v s lng cc trm o phi lm sao o c c nhng c trng nht ca cht lng ngun nc phc v cho cc nhu cu trn v phi theo di s bin i cht lng cc ngun nc c bin php x l cn thit, kp thi. 1.6.2. Phng php ly mu nc a. Cc loi mu nc

Ty vo mc ch phn tch khc nhau (ho hc, sinh vt, vi sinh vt,) m cc phng php, thit b ly mu v cch x l mu khc nhau.

29

- Mu n: l loi mu gin on, thng c ly th cng hay t ng t nc trn b mt, hoc mt su nht nh hoc di y. Mi mu thng ch i din cho cht lng nc thi im v a im ly mu.

- Mu gin on (theo thi gian v theo v tr). - Mu lin tc (ly lu lng nh trc v lu lng thay i): Cha mi thnh

phn ca nc trong sut giai on ly mu, loi mu ny c th pht hin c s thay i cht lng nc.

- Mu lot (theo chiu su hoc theo din tch): Tc l ly theo su khc nhau cng mt v tr hoc mt v tr nht nh ca mt vng nc nhiu v tr khc nhau.

- Mu t hp: Tun th vi mt s mc gii hn c da trn gi tr trung bnh ca cht lng nc.

Khng nn dng mu trn xc nh hm lng ca nhng ch tiu ca nc d b thay i nh pH, cc kh ha tan. b. Mt s ch dn khi ly mu nc

V tr ly mu ph thuc vo mc ch nghin cu. Thi gian v tn sut ly mu ph thuc vo mc bin ng ca cc cht, c th theo cc thi gian trong ngy. Trong ngy nng, pH ca nc c th tng, c bit l nc c kh nng m thp do kh nng trao i CO2 ca thc vt thy sinh v nng photpho c th s b gim i.

Lng nc cn ly ti thiu m bo cho cc php phn tch, thng 1 - 2 lt. Dng c ng mu phi c lm sch bng cc bin php cn thit nh ra bng cc cht ty ra hoc dung dch axt. i vi phn tch vi sinh vt th dng c ng mu cn phi c v trng. Ngoi ra vt liu lm bnh cha mu phi tr v mt ha hc v khng c kh nng hp thu cc cht c trong mu nc.

Khi ly mu i vi ngun nc chy nh sng, sui,...th nn ly mu ch nc chy mnh nht.

Mu ly cc h thin nhin, h cha, cc ao, m,... th nn ly mu cc v tr v su khc nhau. Khng nn ly mu trung bnh i vi loi mu ny.

Mu ly t cc mch, cc loi ging, cc b h cha nc nhn to th nn ly nhng su cn thit.

Mu nc ma phi ly vo lc tri ma v ghi r thi im ly mu. Mu t cc trm v cc vi nc sinh hot thng thng c ly trc tip ti

cc ng dn nc ra. Mu nc thi nn ly trung bnh, trc khi ly mu cn nghin cu k yu cu,

mc ch s dng nc v cc iu kin sinh hot khc. c. Bo qun v x l mu nc trc khi phn tch

Trong khong thi gian t lc ly mu nc n khi phn tch, hm lng ca cc hp phn c th b thay i vi mc khc nhau.

Nhit v pH ca nc l hai i lng bin i nhanh nht, v vy hai ch tiu ny cn c xc nh ngay ti ni ly mu. Hm lng mt s kh cn phi c xc nh ngay nh: O2, CO2, H2S, Cl2.

i vi vic xc nh kim loi nng trong nc th cn tun th theo nhng quy trnh nghim ngt bo qun mu.

30

Vic bo qun v x l mu nc trc khi phn tch thnh phn ca n phi tun theo tiu chun vit nam TCVN 5993 - 1995 v ISO 5667 - 3 - 1985.

Bo qun mu nc l nhm gi gn cc yu t, ng thi duy tr tnh cht v tnh trng mu nc trong mt khong thi gian ngn trc khi em phn tch.

* Ly mu phn tch ch tiu ha l: Trng hp ly mu phn tch ch tiu ha l th cn phi np mu y bnh sao cho khng c khng kh trn mu. iu hn ch s tng tc gia pha lng vi pha kh v s lc khi vn chuyn. Cc loi bnh polyetylen (PE) thch hp cho vic ly mu thng thng xc nh cc thng s vt l, ha hc ca nc t nhin. Bnh PE ly mu c ra sch v trng qua ngun nc c ly mu 3 ln.

* Ly mu phn tch ch tiu vi sinh: Trc khi ly mu nc, cc dng c, chai l cn thit phi c tit trng. Dng chai thy tinh c nt mi vn cht. Th tch chai l 250ml. Ra sch chai sau sy kh kh trng 160oC trong thi gian 2 gi.

- Ly nc ti vi: t ming vi nc cn thn bng bng tm cn vi ngn la to. M nc, cho chy tht mnh trong vng 1 pht, sau vn nh tia nc chy thm vi pht, ri hng ly 2/3 chai (trong chai cn c khong trng lc mu trc khi lm xt nghim).

- Ly nc ti ging, sng, h: Buc chai ly mu vo dy qung ri qung xung ging, sng, h su cch mt nc 0,5m. Sau khi nc vo y chai (khng cn bt kh ni ln) th ko ln, kh trng ming chai, bt nc ri y np.

Bng 16. Dng c cha mu, v iu kin bo qun mu nc ca mt s thng s (trong 1 lt mu nc)

TT Phn tch Chai ng iu kin bo qun Thi gian

bo qun ti a 1 TSS PE Lnh 4oC 4 gi 2 pH PE Khng 6 gi 3 kim PE Lnh 4oC 24 gi

4 Oxy ha tan (DO)

TT C nh ti ch (Winkler) 6 gi

5 BOD PE Lnh 4oC 4 gi 6 COD PE Lnh 4oC 24 gi

7 NH 3 PE Lnh 4oC, H2SO4 c v 23 ml CHCl3

24 gi

8 NO3- PE Lnh 4oC 24 gi 9 PO43- TT Lnh 4oC; 1-2 ml CHCl3 24 gi 10 Cd PE 5 ml HNO3 c - 11 As PE 5 ml HCl - 12 Hg PE 1 ml HNO3 c -

13 Pb PE 3 ml HNO3 c v 2 ml CH3COOH c

-

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Ghi ch: PE: Chai polyethylen; TT: Chai thu tinh

1.6.3. Yu cu s dng mt s ngun nc a. Nc dng cho cng nghip

* Cng nghip lm giy: - c v mu ca nc phi thp giy lm ra c trng cao. Trong nc

khng c c cht l lng mt giy nhn, khng sn. - Hm lng st trong nc phi thp giy khng b vng theo thi gian. - cng trong nc phi thp giy d dn v khng dn in. - pH ca nc phi trung bnh, nu qu cao th kh lm, cn nu thp qu giy

d mn, mc.

* Cng nghip dt: - pH = 6,8 7,5; - Hm lng st < 0,1 mg/l.

b. Nc dng cho nng nghip

Thng thng nc c khong ho trong khong 0,8 1,0 g/l l nc thch hp cho nng nghip. Nu khong ha ln ti 4 g/l l nc chua, mn khng dng cho nng nghip c.

* H s ti Ka: theo thc nghim ph thuc vo hm lng cc cht trong nc:

- Nu [Na+] < [Cl-]: ][5

288

=Cla

- Nu [Na+] > [Cl-] v [Na+] < [SO42-] + [Cl-]:][][

288+ +

=ClNaa

- Nu [Na+] > [Cl-] + [SO42-]: ][9][5][10288

24+

=SOClNaa

Nng cc ion tnh theo mlg/l.

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Da vo Ka ngi ta chia nc s dng cho nng nghip thnh 4 loi: - Ka > 18: nc rt tt.

- Ka = 18 6: nc dng ti c. - Ka = 5,9 1,2 : nc dng c th ti c nhng khng tt. - Ka < 1,2: khng th ti c.

* Cc cht l lng trong nc: thng l cc ht bn, ct, ph sa, cc cht ny c th lm cho t ph nhiu hoc c th lm cho t chua mn dn. Kch thc cc ht cht l lng trong nc hp l l 0,2 0,005 mm.

* Nhit : c nh hng ln n cht lng ca nc ti. Tt nht l nc ti c nhit thch hp cho tng loi cy trng.

* Cc yu t khc: - Dng nc ti khng c ln cc axt t do nh HCl, H2SO4. - Nng cc loi mui: ZnSO4, Na2SO4, Na2CO3, NaCl...phi thp v nu hm lng

ca chng cao s gy keo t t, lm cho t cng li khng thch hp cho cy trng. - Nc thi u khng th dng lm nc ti.

c. Nc dng cho nui c - Nhu cu v oxy: Hm lng oxy ha tan trong nc cng cao cng tt. Gii hn

cho php hm lng oxy ho tan trong nc (DO) l 3 mg/l. Nu nh hn th c bt u kh th v nu DO < 0,5 mg/l th c s cht.

- Cc cht khc: Cc thnh phn ho hc khc phi c to cho cc sinh vt trong nc pht trin tt, lm ngun thc n di do cho c.

- pH: thch hp cho vic nui c t 7 8. Nu pH < 6 lm gim tc dng h hp, gim kh nng hp th thc n, c s chm ln. d. Nc dng cho ni hi

- Tc dng lm han g ni hi: Ni hi b g ch yu l do trong nc c axt v oxy ha tan gy ra. Nc dng cho ni hi yu cu hm lng cc loi mui cacbonat khng qu 15 mg/l v hm lng oxy khng c qu 0,1 mg/l. c bit vi ni hi cao p phi loi tr ht kh oxy ha tan.

- Tc dng lng ng cn: nh hng ca lp cn dn n nhit ca ni hi km, gy lng ph nhin liu ng thi lm thnh ni hi c nhit khng ng nht.

V bn cht ca cn lng ng l do nhit cao nn cc mui canxi, magi v mt s kim loi khc bin thnh kt ta. V d:

Ca(HCO3)2 CaCO3 + CO2 + H2Ot0

Mg(HCO3)2 MgCO3 + CO2 + H2Ot0

CaCl2 + MgSO4 = CaSO4 + MgCl2CaCl2 + Na2CO3 = CaCO3 + 2NaClCaSO4 (tan) + Na2SiO3 = CaSiO3 + Na2SO4MgSO4 + Na2SiO3 = MgSiO3 + Na2SO4 FeCl3 + 3H2O Fe(OH)3 + 3HCl2Fe(OH)2 Fe2O3 + 3H2Ot

0

33

- Tc dng si bt: khi trong nc c cc mui kali, mui natri, x phng, du m , chng thng lm gim sc cng b mt ca nc, d to ra cc bt kh tan c tc dng gy ra p sut khng u trn b mt cc tuabin hi, lm gim tui th tuabin. e. Nc dng cho b tng

Tc dng ca nc i vi b tng ch yu l qu trnh cc cht trong nc tc dng vi cc cht c trong b tng lm cho b tng b ph hu dn. Tnh xm thc ca nc c th chia lm 5 loi:

- Xm thc ca CO2 ho tan: Lng CO2 d s ho tan cc mui cacbonat trong b tng lm cho b mt b tng b ph hy dn.

CaCO3 + CO2 + H2O = Ca(HCO3)2MgCO3 + CO2 + H2O = Mg(HCO3)2

- Xm thc do kh kim: Khi hm lng [HCO3-] < 131 mg/l th s xy ra hin tng cc mui cacbonat v Ca(OH)2 trong b tng tan ra cng lm cho cng trnh b ph hy.

- Xm thc ca cc loi axt: khi nc c pH thp. S xm thc ca axt ch yu l qu trnh ho tan cc mui cacbonat trong b tng lm cho b tng b ph hy dn.

- Xm thc ca cc loi mui sunfat: Khi hm lng SO42- trong nc cao th SO42- cng vi nc thm vo b tng to ra CaSO4.2H2O. Mui ny c h s gin n cao lm cho b tng b rn nt.

- Xm thc ca mui magi: tng t SO42-. f. Nc dng cho sinh hot

Nc dng cho sinh hot ca con ngi l nc p ng y cc yu cu sau: - Nc khng c thnh phn c hi: l cc ion kim loi nng, cc cht khc nh

H2S, NH4+, NO2-, v cc cht hu c. - khong ho ca nc phi thp: cng ca nc thch hp vi i sng l

20H0 (0H), trng hp c bit khng c vt qu 40H0. V rng nc c cng cao hay khong ho cao s c nhiu tc hi cho i sng.

n v o: - c (oH): 1oH = 10mg CaO/l nc. cng (mg CaCO3/L) = cng tng cng - cng canci. cng < 50mg CaCO3/l : nc mm ; 50 - 150mg CaCO3/l : nc trung bnh cng 150 - 300mg CaCO3/l : nc cng; cng > 300mg CaCO3/l : nc rt cng. Hm lng Fe trong nc mt d c t cng lm cho cht lng nc gim i

nhiu. Khi hm lng Fe > 1mg/l th nc bt u vn c, c v v mi kh chu. Khi hm lng Fe qu ln lm nc c mu vng v c vng ni ln rt kh s dng.

- Tnh cht vt l ca nc phi tt: khng c mi, v kh chu, phi khng c mu sc v trong sut.

34

Chng 2: HA HC PHN TCH NH LNG 2.1. Phng php phn tch th tch 2.1.1. Khi nim chung a. Nguyn tc

Da vo th tch v nng cc cht tc dng va vi nhau (tng ng nhau) xc nh hm lng cc cht cn tm. V d: Mun xc nh nng dung dch NaOH ta ly V th tch dung dch NaOH cho tc dng vi dung dch HCl bit nng . Gi V l th tch dung dch HCl va dng ht, khi : V . NNaOH = V. NHCl (2-1)

- Dung dch tiu chun: l dung dch bit chnh xc nng ca n. - Dung dch chun : l dung dch cha bit nng . Ta dng dung dch tiu

chun xc nh n. - im tng ng: l im khi dung dch tiu chun v dung dch chun tc

dng va vi nhau. Ngi ta xc nh im tng ng bng cht ch th mu. Trong thc t im dng chun c th khng trng vi im tng ng v iu gy ra yu t sai s chun . b. Cc phng php phn tch th tch

iu kin phn tch: - Cc phn ng phi xy ra hon ton theo mt phng trnh ha hc xc nh. - Tc phn ng phi nhanh. - Khng c phn ng ph xy ra. - C th xc nh c im tng ng.

Trong thc t ngi ta chia ra lm 4 phng php chnh: - Phng php trung ha. - Phng php kt ta. - Phng php phc cht. - Phng php oxy ho kh.

c. Nng dung dch trong phng php phn tch th tch biu th thnh phn ca mt dung dch, ngi ta dng nng dung dch.

Vy nng dung dch l lng cht tan c trong mt lng xc nh dung dch hoc dung mi, lng cht tan ln to dung dch m c, ngc li l dung dch long.

* Nng phn trm theo khi lng (%): s gam cht tan trong 100 gam dung dch: 100% =

dd

Ct

mm

C (2-2)

V d 1: Dung dch NaOH 20% ngha l c 100g dung dch th c 20g NaOH tan trong .

* Nng % theo th tch: Biu th s ml cht tan c trong 100ml dung dch.

35

V d 2: Ancol etylic 700 ngha l trong 100ml dung dch ru ny cn c 70ml C2H5OH nguyn cht v 30ml nc.

* Nng mol: S mol cht tan c trong 1 lt dung dch:

)(lVn

Cdd

ctM = (2-3)

* Nng ng lng: Mt loi nng khc thng c s dng tnh ton trong cc phng php phn tch th tch l nng ng lng c nh ngha l s ng lng gam ca cht tan trong mt lt dung dch.

VnCN

'= (2-4)

V: th tch (lt) n: s ng lng gam cht tan c trong dung dch.

V d 3: Dung dch HCl 2N: l dung dch c cha 2 ng lng gam hoc 236,5g HCl nguyn cht.

p dng nh lut ng lng cho cc phn ng trong dung dch: Gi s phn ng : A + B C

Gi: NA, NB : nng ng lng gam ca 2 dung dch A v B VA VB: th tch ca 2 dung dch A v B phn ng va vi nhau Theo nh lut ng lng ta c:

NA .VA = NB .VB (2-5) y l biu thc ton hc p dng nh lut ng lng cho dung dch.

* S lin quan gia cc loi nng . - S lin quan gia cc nng phn trm (C%): Trn m1 gam dung dch c C1%

vi m2 gam dung dch c C2%, c dung dch c C%, ta c: m1.C1 + m2.C2 = (m1 + m2)C (2-6)

Nu pha long bng nc th coi nng C% ca nc bng 0. - S lin quan gia cc nng mol (CM):

V1.C 1M + V2.C 2M = (V1 + V2)CM (2-7)

Nu pha long bng nc th coi nng CM ca nc bng 0. - Gia nng mol v nng phn trm:

MdCCM

%10= (2-8)

M: khi lng phn t cht tan CM: nng mol ca dung dch d: khi lng ring ca dung dch C%: nng % ca dung dch

36

- Gia nng ng lng v nng phn trm ca dung dch:

DdCCN

%10= (2-9)

D: ng lng gam - Gia CM v CN:

CN = n.CM (2-10) n: s e m 1 ptg cht trao i

d. Cc cch chun trong phn tch th tch

* Cch chun trc tip: Thm t t dung dch chun vo dung dch nh phn, thuc th R tc dng trc

tip vi cht nh phn X (dung dch chun ) theo phn ng: R + X Q + Y

Da vo th tch v nng ca dung dch chun. Tnh lng cht X. V d 1. Xc nh NNaOH bng dung dch tiu chun HCl.

Ti im tng ng: VNaOH . NNaOH = VHCl . NHCl V d 2. chun ha dung dch NaOH, ngi ta ho tan 1,2600 gam axt oxalic H2C2O4.2H2O v nc v thm nc n 500,00 ml. Chun 25 ml dung dch thu c ht 12,58 ml NaOH. Tnh [NaOH]. Gii:

- Tnh theo nh lut hp thc (lht): aA + bB cC + dD

n0 (tng mol ban u) CA.VA CB.VB

To phn ng cc i: bVC

aVC BBAA ..

max ==

H2C2O4 + 2NaOH Na2C2O4 + 2H2O

n0 50025260,1

422

OCH

1000

58,12.C

max 1000.258,12.

50025.

00,126260,1 C

=

Suy ra: )/(07949,058,12.00,500.00,126

1000.00,25.260,1.2][ lmolNaOH ==

- Tnh theo ng lng (lg):

OHOC 2422 2. = )(00,63200,126

2.

2422 2 gamM OHOCH ==

Nng ng lng ca:

[H2C2O4] = N04000,000,5001000.

00,632600,1

=

37

Nng ng lng ca NaOH = nng mol

Suy ra: 422422 OCHOCHNaOHNaOH

NVNV =

12,58. NNaOH = 25,00. 0,04000

NNaOH = 0,07949N = 0,07949M

* Cch chun ngc: Gi X: l cht cn xc nh nng , M l cht bit nng , c th tc dng

vi X to cht bn. Cho M d tc dng vi X: M + X MX Dng dung dch tiu chun Y xc nh lng M d:

Md + Y MY cng thc tnh nng cht X nh sau:

VM . NM = VX . NX + VY . NY

X

YYMMX V

NVNV =

V d 3. xc nh SO42- trong nc ta dng dung dch Ba2+ d bit nng cho tc dng vi SO42-.

Gii: Ba2+ + SO42- BaSO4

Sau chng ta dng dung dch tiu chun EDTA chun Ba2+ d: Ba2+ + EDTA = Ba. ADTA

EDTAEDTASOSOBaBa NVNVNV ... 242422 += ++

++

=

24

22

24

..

SO

EDTAEDTABaBaSO V

NVNVN

* Cch chun gin tip: Cht X l cht cn xc nh. Cho M tc dng vi cht X:

M + X MX Xc nh M trong MX bng dung dch tiu chun Y:

MX + Y Z t ta c: VX.NX = VY.NY = VZ.NZ

V d 4. xc nh Ca2+ trong mu nc ta cho ion C2O42- tc dng vi Ca2+ to kt ta bn: Gii:

Ca2+ + C2O42- CaC2O4 Sau chun bng dung dch KMnO4 trong mi trng axt mnh:

5CaC2O4 + 2KMnO4 + 8H2SO4 = 2MnSO4 + K2SO4 + 5CaSO4 + 10CO2 + 8H2O

38

V d 5. nh lng Cu ngi ta ho tan 1,080 gam qung ng. Thm KI d. Sau chun it gii phng ra ht 15,65 ml Na2S2O3 0,0950M. Tnh % Cu trong qung. Gii:

Phng trnh phn ng:

2Cu2+ + 5I- (d) 2CuI + I3- Cu2+ b th bng I3-.

Chun I3- bng Na2S2O3: 2S2O32- + I3- S4O62- + 3I- - Tnh theo lht:

2Cu2+ + 5I- 2CuI + I3-

no Cu

gm )(

max 2.546,63m

S mol sau phn ng, n

I3- + 2S2O32- S4O62- + 3I-

no 2.546,63

m 1000

65,15.9050,0

max 2.546,63m

2.100065,15.9050,0

lht: 2.1000

65,15.0950,02.546,63=

m m = 0,09448g;

%Cu = %75,8100080,1

09448,0=

- Tnh theo quy tc lg:

2Cu2+ + e + I- 2CuI

3I- - 3e I3-

2Cu + = M 2Cu + s lg 2Cu + = Cum = s lgI3-;

3I = 2

3I

M

I3- + 2e 3I-

2S2O32- - 2e S4O62-

3I = 2

3IM : khng thay i.

22 3OS = M 22 3OS [

22 3OS ] = 0,0950M = 0,0950N

S lg 3I = s 22 3OS s lg

+2Cu = s lg[ 22 3OS ] = 100065,15.0950,0

39

)(09448,01000

546,63.65,15.0950,01000

65,15.0950,0 gmCum

===

%Cu = %75,8100080,1

09448,0=

2.1.2. Phng php trung ha a. C s ca phng php

Phng php trung ha da trn c s ca phn ng trung ha. H+ + OH- = H2O

2980K, chng ta c tch s ion ca nc: KW = [H+][OH-] = 10-14 v pH + pOH = 14

Trong qu trnh chun th nng H+ v OH- thay i tc l pH thay i. Trong phng php ny, xc nh nng ca baz, ngi ta dng cht chun

l axt mnh; xc nh nng ca axt, ngi ta dng cht chun l baz mnh. b. Cht ch th mu dng trong phng php trung ha

Dng cht ch th mu nhn bit im tng ng. Cht ch th mu dng trong phng php trung ha l cht ch th axt - baz.

l nhng hp cht hu c biu l tnh axt yu hoc baz yu c th thay i mu theo pH ca dung dch.

Bn thn cc cht ch th ny l cc axt yu hoc cc baz hu c yu m dng phn t hoc ion chng c mu khc nhau. iu ny do s phn ly hoc lin hp ca cht ch th km theo s chuyn v cu trc bn trong dn n s i mu.

Phn ng in hnh ca ch th axt - baz di dng sau:

HInd + H2O H3O+ + Ind- hoc:

Ind- + H2O HInd + OH- Trong cc dung dch axt mnh, ch th HInd l dng chim u th, tng ng vi

mu axt v Ind- s l mu kim ca n. Trong cc dung dch kim cc ion Ind- s chim u th tng ng vi mu kim ca cht ch th . V d: Phenolphtalein l mt axt hu c yu, khng mu trong dung dch axt v c mu trong mi trng kim. Trong mi trng kim mnh mu ca chng b mt chm.

Trong dung dch n tn ti cn bng:

HO HO CCOOH

CCOO-

O O

+ H+

Cau tao phan t cua phenolphtalein

Khong mau o

40

Bng 17. Mt s ch th axt - baz quan trng

S bin i mu STT Cht ch th

Axt Baz Khong bin i

pH Loi ch th

1 Methyldacam: MO Vng 2,5 4,5 (2) 2 Methyl vng Vng 2,9 4,0 (2) 3 Phenolphtalein: PP Khng tm 8 10 (1) 4 Qu tm Xanh 6 8 (1) 5 Methyl : MR Vng 4,2 6,2 (2) 6 Phenol Vng 6,4 8,0 (1)

(1) - Ch th axt; (2) - Ch th kim c. Cc yu t nh hng n s i mu ca cht ch th

* nh hng ca cc ion trong dung dch: Cc cht in ly trong dung dch c tc dng n mu sc ca cht ch th trn hai kha cnh: Thay i cng mu ca mt trong hai dng mu ca cht ch th; nh hng n cn bng ca cht ch th. Do trong nhiu trng hp phi hiu chnh li pH ca khong i mu ca cht ch th.

* nh hng ca nhit : Khi thay i nhit ca dung dch s lm thay i s i mu ca cht ch th, v lm thay i kh nng phn ly ca dung mi, ca axt, baz, mui v c ca cht ch th.

* nh hng ca dung mi: Khi thay i dung mi th s phn ly ca axt - baz thay i. Ch th axt - baz do cng thay i theo dung mi. l nguyn nhn gy nn s nh hng ca dung mi n s i mu ca ch th. d. ng chun axt mnh bng baz mnh hoc ngc li

Nu cht chun v cht b chun l axt mnh v baz mnh th c th hnh dung phn ng axt - baz tng th bng phng trnh: H3O+ + OH- = 2H2O V d: Tin hnh kho st qu trnh chun 100 ml dung dch HCl 0,1N bng dung dch NaOH 0,1N.

Trong qu trnh ny, t buret chng ta nh tng git dung dch NaOH 0,1N xung bnh nn ng 100 ml dung dch HCl 0,1N. NaOH + HCl = NaCl + H2O OH- + H+ = H2O

* S bin i pH ca dung dch HCl trong qu trnh chun : n gin chng ta coi th tch chung ca dung dch trong qu trnh chun

l khng i. iu ny cng khng nh hng nhiu n kt qu phn tch. - Trc khi chun : trong dung dch ch c HCl l axt mnh in ly hon ton.

HCl = H+ + Cl- HCl 0,1N tng ng vi 0,1M.

[H+] = [HCl] = 0,1M pH = 1.

41

0

5

10

15

0 50 100 150 200 250

%

pH

pp

MO

- Trc im tng ng: Khi 90% dung dch HCl c chun , HCl cn li 10 ml.

.201,0100

1,0.10][][ ====+ pHMHClH

Khi 99% dung dch HCl c chun .

.3001,0100

1,0.1][][ ====+ pHMHClH

Khi 99,9% dung dch HCl c chun .

.40001,0100

1,0.1,0][][ ====+ pHMHClH

- Ti im tng ng: Khi 100% dung dch HCl c chun bi NaOH, trong dung dch ch c NaCl, mui ny c pH = 7.

- Sau im tng ng: Dung dch bt u d NaOH l baz mnh, in ly hon ton.

NaOH = Na+ + OH- [OH-] = [NaOH]

[NaOH] = 0,1N tng ng 0,1M. Khi 100,1% dung dch HCl c chun :

410.1100

1,0.1,0][][ === NaOHOH ; pOH = 4 pH = 10.

Khi 101% dung dch HCl c chun :

310.1100

1,0.1][][ === NaOHOH ; pOH = 3 pH = 11.

Khi 110% dung dch HCl c chun :

210.1100

1,0.10][][ === NaOHOH ; pOH = 2 pH = 12.

Khi 200% dung dch HCl c chun :

1,0100

1,0.100][][ === NaOHOH ; pOH = 1 pH = 13.

* Lp bng bin thin v v ng cong chun .

STT % chun pH

1 0 1 2 90 2 3 99 3 4 99,9 4 5 100 7 6 100,1 10 7 101 11 8 110 12 9 200 13

42

* Nhn xt: - im tng ng ca php chun c pH = 7 trng vi im trung tnh. - ng cong chun c mt on dc ng vi khong t 99,9% ti 100,1% dung

dch HCl c chun . on ny ngi ta gi l bc nhy ca php chun .

Trong phm vi bc nhy, ch cn thay i mt t dung dch tiu chun ( 0,1%) th gi tr pH thay i 6 n v t 4 n 10.

* Chn cht ch th: Nguyn tc chung l phi chn cht ch th c khong i mu trong phm vi

bc nhy ca php chun . Tt nhin nu trng im tng ng hoc gn im tng ng cng tt.

Vic chn cht ch th nh vy gip ta trnh c sai s ln trong php chun . Trong phm vi bc nhy ch cn thm mt dung dch tiu chun lm cho dung

dch chun c pH thay i t ngt v lp tc lm cht ch th i mu, bo hiu im tng ng v ngng chun li. Sai s ca php chun khng ng k.

Trong php chun trn chng ta c th dng cht ch th: - Chn MO cho php chun axt mnh bng baz mnh. MO c khong i mu

pH = 2,5 4,5, khi mu sc ca dung dch chuyn t mu hng sang mu vng th ngng chun . Ta thng hay chun qu 1, 2 git dung dch tiu chun nn cng gn im tng hn.

- Chn PP cho php chun baz mnh bng axt mnh, PP c khong i mu pH = 10 8. Ngng chun khi mu dung dch chuyn t tm sang khng mu. 2.1.3. Phng php kt ta a. C s ca phng php

Phng php chun kt ta da trn phn ng to thnh cht kt ta trong qu trnh chun .

Yu cu ca cc phn ng: - Cc cht kt ta to ra phi c tch s ha tan cng nh cng tt, tc l ha tan

ca cht rt nh. - Tc phn ng to kt ta phi nhanh. - Cc kt ta khng c kh nng hp ph cc cht khc c trong dung dch. - Khng xy ra hin tng cng kt. - Phi c kh nng xc nh im tng ng.

Vic ng dng nhng phng php chun kt ta trong cht chun khng phi l mui bc m l nhng hp cht khc cn tng i hn ch. b. ng chun kt ta

Trong qu trnh chun theo phng php kt ta th nng cc ion to ra hp cht kt ta s thay i lin tc. V vy trong phng php ny v ng cong chun , chng ta phi kho st qu trnh bin i cc ion to kt ta trong qu trnh chun .

43

Trong s cc phng php kt ta, ngi ta dng nhiu nht l phng php bc xc nh ion halogenua v SCN- (hoc ngc li).

Ag+ + X- = AgX X-: Ion halogenua v SCN-

V d: Kho st qu trnh chun 100 ml dung dch NaCl 0,1N bng dung dch AgNO3 0,1N. Bit TAgCl = 10-10.

Phn ng xy ra trong qu trnh chun : NaCl + AgNO3 = AgCl + NaNO3

y: [NaCl] = 0,1N tng ng 0,1M. [AgNO3] = 0,1N tng ng 0,1M.

Tch s ha tan ca AgCl: TAgCl = [Ag+][Cl-] = 1.10-10 thun li cho qu trnh tnh ton ta gi:

pAg = -lg[Ag+] ; pCl = -lg[Cl-] V vy: pAg + pCl = 10

* Kho st hoc pAg hoc pCl trong qu trnh chun : - Trc khi chun :

Trong dung dch ch c NaCl 0,1M: NaCl = Na+ + Cl-

[Cl-] = 1.10-1M pCl = 1. - Trc im tng ng:

Khi 90% dung dch NaCl c chun , NaCl cn li 10 ml.

.201,0100

1,0.10][][ ==== pClMNaClCl

Khi 99% dung dch NaCl c chun .

.3001,0100

1,0.1][][ ==== pClMNaClCl

Khi 99,9% dung dch NaCl c chun .

.40001,0100

1,0.1,0][][ ==== pClMNaClCl

- Ti im tng ng: Khi 100% dung dch NaCl c chun bi AgNO3, trong dung dch ch c

mui AgCl, nn:

;1010][][ 510 MAgCl AgCl + ====

pCl = pAg = 5 - Sau im tng ng:

Dung dch bt u d AgNO3 Khi 100,1% dung dch NaCl c chun :

44

AgNO3 = Ag+ + NO3-

Nn [Ag+] = [AgNO3] = 410.11001,0.1,0 = M

pAg = 4 pCl- = 6 Khi 101% dung dch NaCl c chun :

[Ag+] = [AgNO3] = 310.11001,0.1 = M

pAg = 3 pCl- = 7 Khi 110% dung dch NaCl c chun :

[Ag+] = [AgNO3] = 210.11001,0.10 = M

pAg = 2 pCl- = 8 Khi 200% dung dch NaCl c chun :

[Ag+] = [AgNO3] = 110.11001,0.100 = M

pAg = 1 pCl- = 9

* Lp bng bin thin v v ng cong chun .

* Nhn xt: - Bc nhy ca php chun c pCl = 4 6. - im tng ng ca php chun c pCl = 5.

* Cht ch th: - Phng php Mohr: Trong php chun ny ngi ta dng cht ch th l CrO42-

. CrO42- tc dng vi Ag+ cho kt ta Ag2CrO4 mu nu c ha tan 8,4.10-5 mol/L ln hn ha tan ca AgCl (1.10-5 mol/L): 2Ag+ + CrO42- = Ag2CrO4

Khi cho t t ion Ag+ vo dung dch chun cc ion Cl-, CrO42- th AgCl s kt ta trc, Ag2CrO4 s kt ta sau.

STT % chun pH 1 0 1

2 90 2 3 99 3

4 99,9 4 5 100 5

6 100,1 6 7 101 7 8 110 8

9 200 9

02468

10

0 50 100 150 200 250

%

pCl

45

nhn bit im tng ng ca php chun , chng ta phi tnh ton hm lng K2CrO4 c trong dung dch sao cho khi t ti im tng ng, tc l khi NaCl c chun hon ton mi xut hin kt ta Ag2CrO4 mu .

Chng ta bit: TAgCl = [Ag+][Cl-] = 10-10

12242 10][][42+ == CrOAgCrOAg

Ti im tng ng: [Ag+] = [Cl-] = 10-5 M khi [Ag+] = 10-5M mi c kt ta Ag2CrO4 th hm lng CrO42- c trong

dung dch c tnh ton theo tch s tan ca Ag2CrO4.

12242 10][][42+ == CrOAgCrOAg

2 424 2[ ] [ ]Ag CrOCrOAg

+

= ; MAg 510][ + =

nn: MCrO 22512

24 10)10(

10][

==

K2CrO4 = 2K+ + CrO42- [CrO42-] = [K2CrO4] = 10-2M

Nh vy chn nng K2CrO4 trong dung dch chun l 10-2M th ti im tng ng mi bt u xut hin Ag2CrO4 kt ta mu . S chun kt thc khi trong dung dch chun t kt ta trng bt u chuyn sang xut hin kt ta .

Cn ch n nng axt ca mi trng. Theo phng php ny cn phi c tin hnh trong mi trng trung tnh hoc gn trung tnh.

- Phng php Volhard: Da trn phn ng gia Ag+ v SCN-, dng ion Ion Fe3+ lm cht ch th.

Ag+ + SCN- AgSCN(rn) Sau khi t im tng ng, lng CNS- d s to vi Fe3+ phc sunfuaxianua

st c mu mu.

Fe3+ + 3SCN- Fe(SCN)3 Nh vy im tng ng ca php chun c xc nh cn c vo s to

thnh phc Fe(SCN)3 c mu . Php chun cn c thc hin trong mi trng axt ngn nga s kt ta Fe(III) di dng hiroxit.

Bng 18. Mt s phng php kt ta o bc

Cu t cn xc nh

Phng php xc nh im cui

Ghi ch

Cl-, Br- Phng php Mohr SCN- Phng php Volhard Khng cn tch mui bc

BH4 Phng php Volhard

bin dng

Chun trc tip theo lng Ag+ d trong phn ng: BH4 + 8Ag+ + 8OH- 8Ag(rn) + H2BO3 + 5H2O

Br-, Cl-, I-, SeO32- Ch th hp ph

46

2.1.4. Phng php phc cht a. Nguyn tc

Cng nh cc phn ng khc dng nh phn, cc phn ng to phc cn phi xy ra nhanh, hp thc v nh lng.

Cc phng php to phc da trn phn ng to phc ca cc cht. Trong phn tch th tch ngi ta thng dng cc phng php sau y: Phng php thu ngn (II), phng php bc, phng php complexon.

Phng php complexon: Da trn phn ng to phc ca cc ion kim loi vi nhm thuc th hu c c tn chung l complexon.

Phng php complexon l phng php phn tch chun da trn vic dng phn ng ca ion kim loi to phc vi complexon, to thnh cc mui ni phc, bn, t phn ly, tan trong nc. b. Chn cht chun

Complexon l cc dn xut ca axt aminopoli cacboxilic. Th d axt etylendiamin tetraaxetic hay complexon II, vit tt l EDTA:

HOOC - CH2 CH2 - COOH

HOOC CH2 CH2 - COOH

N - CH2 - CH2 - N

Tuy nhin EDTA t tan trong nc nn kh s dng. Ngi ta thng dng mui

dinatri ca EDTA gi l complexon III hay trilon B, vit tt l Na2H2Y. HOOC - CH2 CH2 - COOH

NaOOC CH2 CH2 - COONa

N - CH2 - CH2 - N

lm cht chun, v n c tinh khit cao, c cng thc ha hc bn iu kin thng thng.

Trong phn ng vi kim loi, mui ny phn ly hon ton to ra ion phc: Na2H2Y = 2Na+ + H2Y2-

Tnh cht qu gi nht ca cht chun EDTA l kh nng phn ng ca n vi cc ion kim loi theo t l 1:1 khng ph thuc vo in tch cation.

Ion H2Y2- rt d to phc rt bn v tan trong nc vi nhiu ion kim loi. Tt c cc cation phn ng vi EDTA, tr cc kim loi kim u to thnh cc phc bn, l c s cho phng php chun . V d: ng glicocoliat:

O = C - O O - C = O

Cu

H2N - H2N NH2 - NH2 H2Y2- ha tan vi cc ion kim loi nh Ca2+, Mg2+, Fe2+, Mn2+, Cu2+,...cc cation

kim loi ny k hiu l Me2+.

Me2+ + H2Y2- MeY2- + 2H+

47

Theo phng trnh trn th vic to phc MeY2- c quan h cht ch vi [H+], tc l pH ca dung dch. cn bng to phc trn chuyn dch theo chiu thun, ngha l to phc hon ton th pH ca dung dch phi cao (pH > 7). iu ny cho thy trong phng php phc cht phi s dng s to phc trong mi trng baz. c. Cht ch th dng trong phng php complexon

Cht ch th mu dng trong phng php complexon l cht ch th c kh nng to vi ion kim loi phc c mu khc vi mu ring ca cht ch th. Cht c gi l cht ch th kim loi. Nh vy php chun ion kim loi bng EDTA gm 2 giai on:

- Phn ng gia ion kim loi t do v complexon. - Phn ng gia complexon v ion kim loi trong phc.

Cht ch th ny c mt trong dung dch chun mt gi tr pH no c kh nng to phc tan, c mu vi cation kim loi cn xc nh. Nhng phc ny km bn hn phc to bi vi EDTA v cation kim loi .

Cht ch th thng c s dng rng ri trong php chun complexon l Eriocrm T en ( k hiu H2Ind- ): l mt axt a bc yu. Trong dung dch c pH < 7 tn ti di dng:

-O3S N = N

OH OH

NO2 Cht ch th ny c phm vi i mu khc nhau.

H2Ind- H2Ind2- Ind3-

o Xanh Vang da cam

pH = 6,3 pH = 11,6

Khi s dng cht ch th ny trong chun xc nh hm lng cc cation kim

loi thng dng pH = 6,3 11,6 nn tn ti dng H2Ind2- c mu xanh. H2Ind2- to phc tan, km bn vi cation kim loi c mu . Nh vy nu chnh dung dch chun c pH = 6,3 11,6 khi cho cht ch th Eriocrm T en vo dung dch s c mu nu trong dung dch c cc cation kim loi. Dng EDTA chun dung dch ny th vic kt thc chun khi dung dch chun c mu chuyn sang mu xanh.

Eriocrm T en to phc hoc hng vi ion kim loi Mg2+, Zn2+, Cd2+ thng c dng chun trc tip cc ion trong mi trng pH = 10 dng hn hp m NH3 - NH4+. V d 1: Chun cation Ca2+ v Mg2+ trong nc t nhin bng EDTA, cht ch th l Ericrm T en.

Trc tin ta dng dung dch m c pH = 10 iu chnh pH ca dung dch chun c pH l 10. Cho vi git cht ch th Ericrm T en vo:

HInd2- + Me2+ MeInd- + H+ Me2+ : Ca2+, Mg2+; MeInd- c mu .

48

Dng dung dch tiu chun H2Y2- chun dung dch ny. Ti im tng ng H2Y2- chim ht Me2+ y HInd2- ra c mu xanh:

H2Y2- + MeInd- MeY2- + HInd2- + H+ Vic kt thc chun khi dung dch chun i mu t tm sang xanh lam. C th dng cht ch th Murext (k hiu H4Ind-) trong php chun complexon.

Murext l mui amoni ca axt pupuric C8H5O2N5, l mt loi bt mu thm. Dung dch nc murext c mu tm thay i theo mi trng: pH 9 mu tm , pH = 9 - 10: mu tm, pH > 11 - tm xanh.

Trong mi trng axt mnh H4Ind- c cng thc cu to nh sau:

O = C C - N = C C = O

NH - C C = NH

NH - C C = NH

O- O

OO H4Ind- tc dng vi cation kim loi thnh phc mu hng. Dng EDTA chun

dung dch th ti thi im tng ng EDTA dnh ht cation kim loi y H4R- ra nn dung dch chun c mu tm hu.

Murext l cht ch th tt cho vic chun trc tip cc ion Ca2+, Cu2+, Ni2+, Ag+. V d 2: Chun dung dch Ca2+ trong nc t nhin bng dung dch tiu chun EDTA vi cht ch th murext.

u tin dng dung dch NaOH iu chnh pH ca dung dch chun c pH = 12. Cho tip mt t murext vo:

H4R- + Ca2+ CaH2R- + 2H+ Dung dch chun lc ny c mu hng. Dng dung dch tiu chun H2Y2- chun dung dch ny. Ti im tng

ng:

CaH2R- + H2Y2- CaY2- + H4R-

Dung dch lc ny c mu tm hu. Vic chun kt thc khi dung dch chun i t mu hng sang mu tm hu.

b. Qu trnh chun Trong phng php phc cht, kho st qu trnh chun ngi ta da vo

hng s khng bn Kkb ca phc MeY2-.

MeY2- Me2+ + Y4- ; ][

]][[2

42

+

=MeY

YMekb

Cng ging nh phng php kt ta, da vo Kkb c th tnh c [Me2+] hay [Y4-] thay i trong qu trnh chun . T v c ng cong chun .

49

2.1.5. Phng php oxy ho kh a. Nguyn tc

L phng php phn tch nh lng s dng cc phn ng oxy ha kh, tc l s dng cc phn ng trong c s trao i electron.

Ox1 + Kh2 Kh1 + Ox2 Phn ng ny bao gm 2 bn phn ng:

Ox1 + ne Kh1

Kh2 - ne Ox2 Yu cu ca phn ng:

- Cc phn ng xy ra theo mt chiu xc nh. - Phn ng phi xy ra hon ton. - Tc phn ng phi nhanh. - Phi xc nh c im tng ng.

b. Mt s khi nim c bn

* Th in cc ca kim loi: Khi nhng mt thanh kim loi Me vo nc hay dung dch mui ca n s xut

hin mt tng in kp trn b mt kim loi v lp dung dch st b mt kim loi. S chnh lch th trong tng in kp c gi l th in cc ca kim loi, k hiu l .

iu kin tiu chun v nng ca cation kim loi trong dung dch l 1 mol/l, th in cc c so snh vi in cc hiro tiu chun v gi l th in cc tiu chun ca kim loi, k hiu l 0.

iu kin khng phi l iu kin chun, nng cation kim loi thay i, th in cc ca kim loi c tnh theo biu thc ca Nerst:

]ln[/0/++= ++ nMeMeMeMe MenF

RTn

n (2-11)

* Th oxy ho kh: Khi nhng mt thanh kim loi tr v mt ha hc vo dung dch cha cp oxy ho v kh th trn b mt thanh kim loi v dung dch cng xut hin mt tng in kp v th chnh lch trong tng in kp c gi l th oxy ho - kh.

Th oxy ho - kh c k hiu l EOx/Kh. V d: nhng mt thanh Pt vo dung dch cha cp Fe2+ v Fe3+. in cc ny c k hiu: ++ 23 / FeFePt v th ca cp oxy ho - kh ny c k hiu: ++ 23 / FeFe .

Cng ging nh th in cc ca kim loi, xc nh th oxy ho - kh ngi ta cng phi to lp mt pin in gm mt in cc l in cc cn o th oxy ho - kh v in cc cn li l in cc hiro tiu chun.

Trong iu kin khng phi l tiu chun v nng cc dng oxy ho, kh u l 1 mol/l th E o c gi l E tiu chun, k hiu l Eo.

Trong iu kin khng phi tiu chun v nng cc dng oxy ho - kh khc 1 mol/l th E o c l th oxy ho - kh ca cp oxy ho - kh trong iu kin cho.

Chng ta c th tnh th oxy ha - kh qua biu thc Nerst:

50

][][ln0

KhOx

nFRT

+= (2-12)

vi [Ox] v [Kh] tnh theo mol/l. 2980K, F = 96500, chuyn ln sang lg, ta c cng thc:

][][lg0

KhOx

nFRT

+= (2-13)

Bng 19. Th oxy ho - kh tiu chun ca mt s cp oxy ha - kh:

Dng oxy ho Dng kh K hiu E0(vn) Fe2+

S I2

Cl2 MnO4-

F2 SO42-

Cu2+

Fe3+

Sn4+

Fe S2- I-

Cl-

Mn2+

F- SO32-

Cu Fe2+ Sn2+

Fe2+/Fe S/S2- I2/2I-

Cl2/2Cl-

MnO4-/Mn2+

F2/2I- SO42-/SO32-

Cu2+/Cu Fe3+/Fe2+ Sn4+/Sn2+

- 0,44 - 0,51 + 0,54 + 1,36 + 1, 52 + 3,03 - 0,58 + 0,17 + 0,77 + 0,15

V d 1: Th oxy ho - kh ca cp MnO4-/Mn2+ ph thuc vo [H+] theo phng trnh:

MnO4- + 8H+ + 5e Mn2+ + 4H2O

][]][[

lg5059,0

2

840

+

+

+=Mn

HMnO

* Hng s cn bng ca phn ng oxy - ha kh. Trong phn ng oxy ho - kh:

- Cp no c E ln l cp ng vai tr cht oxy ha. - Cp no c E nh hn l cp ng vai tr cht kh.

Trong qu trnh phn ng xy ra, E ca cp oxy ha gim dn, ng thi E ca ca cp kh tng dn, ti thi im EOx = EKh th phn ng t ti trng thi cn bng (c hng s cn bng Kcb).

V d 2: Xt phn ng: 2Fe3+ + Sn2+ 2Fe2+ + Sn4+ (*) Cp oxy ha: Fe3+/Fe2+; Cp kh: Sn4+/Sn2+

Ti thi im cn bng: ++++ = 2423 // SnSnFeFe

]][[]][[

23

42

++

++

=SnFeSnFe

C

51

TTCB: KhOx = o oOx Kh = n059,0

]][[]][[lg 23

42

++

++

SnFeSnFe

00KhOx = n

059,0 lgKc

hay )(.9,16lg 00 KhOxC nK =

Trong n l s electron trao i trong phn ng oxy ha - kh. Nh vy trong phn ng trn (*):

lgKC = 16,9.2(0,77 - 0,15) = 20,956 ; KC = 1020,956.

* Chiu ca phn ng oxy ha - kh: Chng ta da vo th oxy ha kh xc nh c chiu ca phn ng oxy ha kh. c. Chun bng phng php oxy ha kh

Trong qu trnh chun bng phng php oxy ho kh, nng cc cht oxy ha v cht kh lun thay i, nn th oxy ha kh ca cc cp oxy ho kh lun thay i. V vy trong phng php ny chng ta kho st s thay i E theo qu trnh chun . th biu din E trong qu trnh chun l ng cong chun . Da vo ng cong chun ta cng xc nh c bc nhy th, im tng ng, chn cht ch th thch hp cho phng php chun . V d 1: Chun 100 ml dung dch FeSO4 0,1N bng dung dch tiu chun KMnO4 0,1N trong mi trng [H+] = 1M.

Phn ng xy ra:

10FeSO4 + 2KMnO4 + 8H2SO4 5Fe2(SO4)3 + 2MnSO4 + K2SO4 + 8H2O Dng ion thu gn:

5Fe2+ + MnO4- + 8H+ 5Fe3+ + Mn2+ + 4H2O

* Kho st th oxy ha kh (E) trong qu trnh chun : V phn ng thun nghch, nn thi im no ca qu trnh chun , trong dung

dch chun lun tn ti cp oxy ha MnO4-/Mn2+ v cp kh Fe3+/Fe2+.

][

]][[lg

5059,052,1 2

84

+

+

+=Mn

HMnOOx

][][lg

1059,077,0 2

3

+

+

+=FeFe

Kh

Khi tnh ton E trong qu trnh chun , c th tnh theo 2 cng thc trn, tt nhin kt qu nh nhau. Thng thng trc im tng ng tnh theo EKh; cn sau im tng ng tnh theo EOx.

- Trc im tng ng.

Khi 50% Fe2+ c chun : V77,05050lg

1059,077,0 =+=

Khi 90% Fe2+ c chun : V826,01090lg

1059,077,0 =+=

52

Khi 99,9 % Fe2+ c chun : V947,010,0

9,99lg1059,077,0 =+=

- Ti im tng ng.

i vi cp kh v cp oxy ha: ][][lg

1059,077,0 2

3

+

+

+=FeFe (a)

][]][[

lg5059,052,1 2

84

+

+

+=Mn

HMnO

][][

lg1059,052,155 2

4+

+=Mn

MnO (v [H+] = 1M). (b)

(a) + (b) : ]][[]][[

lg059,052,1577,06 224

3

++

+

++=MnFe

MnOFe

Theo phng trnh ion thu gn trn, ta c: [Fe3+] = 5[Mn2+]; [Fe2+] = 5[MnO4-]

nn 1]][[]][[

224

3

=++

+

MnFeMnOFe

v th 6E = 0,77 + 5 1,52

V395,16

52,1577,0=

+=

T ta c cng thc tng qut tnh E im tng ng:

Vbaba OxKh 395,1.. 00

=+

=

a: s e trao i ca cp kh; b: s e trao i ca cp oxy ha. - Sau im tng ng.

E tnh theo cng thc: ][

]][[lg

5059,052,1 2

84

+

+

+=Mn

HMnO

Khi 100,1 % Fe2+ c chun : V485,1100

1,0lg5059,052,1 =+=

Khi 101 % Fe2+ c chun : V496,1100

1lg5059,052,1 =+=

Khi 110 % Fe2+ c chun : V508,110010lg

5059,052,1 =+=

Khi 200 % Fe2+ c chun : V52,110010