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CHEMICAL PRESIPITATION
Presipitation could happen if multiplication of molarconcentration the ions more than solubility-product
constant (Ksp)
Presipitation stages :
- nucleation
-particle growth
ExampleCaCO3 Ca
2++ CO32-
If concentration of solid is considered as a constant Ks,
so ;
[Ca2+
][CO32-
] = K.Ks = Ksp = 5 x 10-9
(25 C)
Ksp : solubility-product constant- if multiplication of concentration molar ion Ksp
solution is super saturated, precipitate is formed
][
]][[
3
2
3
2
CaCO
COCaK
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Ksp is not enough, need to know factors that affect
chemical precipitation ;
1. Common ion effect;
repression of the solubility of a substance due to anexcess of one ionresult in a decrease in a solubility
Exp: solubility of CaCO3 in pure water is 13 mg/l; in
water containing 100 mg/l alkalinity of CO32-
the
solubility only 0,5 mg/l
2. Indifferent electrolyte effect ;Formation or dissolution of a precipitate always occur
in the presence of indifferent electrolyte. Although the
ions do not participate directly in the solubility
equilibrium reaction, they do affect the behaviour of
the precipitate.
concentration of indifferent electrolyte ionic strength
of solution value of activity coefficient value of
(Kc)eq solubility of the solution
2c
meqa
eqa
(aq)(aq)(s)
)(
)()(Kaswrittenbecan
[A][C])(Kor
(C)(A))(K
ACCA
m
eqaeq
m
K
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3. Other solutes effect;
a. Competing acid-base equilibria
If anion could form weak acid (CO3-, S2-, PO4
3-), exp ;
If acid added (H+)increase in a solubility
b. Effect of complex ion formationComplex formation increase in a solubility
c. Effect of hydrolysis;
- metal ion exist in aqueous solution as hydrated ions
(water-bound, exp Al(H2O)63+)
- in aqueous solution complex-forming reactions betweenhydrated metal ion with hydroxyl ion occurs lead to the
formation of hydroxocomplexes which contain one or more
metal ions (mono or polynuclear complexes)
CaCO3(s) Ca2+
+ CO3-
+
H+
HCO3-
+
H+
H2CO3
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EXAMPLE :
Complex formation :
ZnCO3 Zn2++ CO3
2+
+NH3
Zn(NH3)2+
+
NH3
Zn(NH3)22+
+
NH3
Zn(NH3)32+etc. untilZn(NH3)4
2+formed
Hydrolysis :
Following generalized equations is used to to describe hydroxocomplexes
formation for trivalent metals ion (when onlymononuclear complexes formed
1. Me3++ OH- MeOH2+
2. Me3++ 2OH- Me(OH)22+
3. Me3++ 3OH- Me(OH)32+
4. Me3++ 4OH- Me(OH)42+
Since hydrated metal cations are weak acid called cation acids (asam-2logam), the formation of metal hydroxocomplexes also considered as the
ionization of these cation acids.For trivalent metals ion that forms only
mononuclear complexes :
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1. Me(H2O)63+ Me(H2O)5(OH)
2+ + H+
or
Me3+ Me(OH)2+ + H+
2. Me3+ Me(OH)22+ + 2H+
3. Me3+ Me(OH)32+ + 3H+
4. Me3+ Me(OH)42+ + 4H+
These type of ionization occur whenever salts containing metal ions dissolved
in water, since H+
ions are produced resulting solution will be acidic.increase the solubility & should be considered in solubility calculations
Hydroxocomplexes formation constants for Al and Fe table
A log [spesies] vs pH diagram can be used to describe the effect of metal
hydroxocomplex formation on the solubility. Diagrams will be constructed by
using the plotting equations ;
1. Al(OH)3(s) Al3+
+ 3OH-
Ksp = [Al3+][OH-]3
Log Ksp = log [Al3+] + 3 log [OH-]
Log Ksp = -32,34
dari ionisasi air, diketahui log [OH-] =pH14
substitusi dua hal tsb di atas didapat :
-32,34 = log [Al3+] + 3pH42
atau
log [Al3+] = 9,66 3pH .1)
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2. Al3+ Al(OH)2+ + H+
-5,02 = log [Al(OH)2+] + log [H+] - log [Al3+]
log [Al(OH)2+] = 4,44 2pH .2)
Dengan cara yang sama didapatkan :
3. 2Al3+ Al2(OH)24+ + 2H+
log [Al2(OH)24+] = 13,05 4pH .3)
4. 6Al3+ Al6(OH)153+ + 15H+
log [Al6(OH)153+] = 10,96 3pH .4)
5. 8Al3+ Al8(OH)204+ + 20H+
log [Al8(OH)204+] = 8,58 4pH .5)
6. 13Al3+ Al13(OH)345+ + 34H+
log [Al13(OH)345+] = 28,19 5pH .6)
7. Al3+ Al(OH)4- + 4H+
log [Al(OH)4
-] = pH 13,91 .7)
3
2)(
Al
HOHAlK
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Log [spesies] vs pH diagram for the Al(OH)3(s)-H2O system
(equations 1 to 7) is presented bellow.
Fe(OH)3(s)
-H2
O system can be found in the same way
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APPLICATION OF CHEMICAL PRECIPITATION INENVIRONMENTAL ENGINEERING
A. Defluoridation
Defluoridation applicaions: - water treatment
- wastewater treatment
In wastewater treatment, fluoride is precipitate by addinglime
B. Metals removal
- Mostly metals can be precipitated as metal carbonate&
metal hydroxide, some in the form of metal sulfide
- Need to consider process condition, especially pH to
achieve minimum solubility
- Some metals is amphoter (amfoter), exp ; Cr & Zn
Ca(OH)2+ 2HF
CaF2(s) + 2H2O pH 11 - 12
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D. Water softening (removal of hardness)
- Precipitation reaction using lime soda ash
C. Penurunan kesadahan
Reaksi presipitasi dengan
kapursoda abu :
1. CO2 + Ca(OH)2 CaCO3(s)+ H2O
2. Ca(HCO3)2+ Ca(OH)2 2CaCO3(s)+ 2H2O Ca-C
3. Mg(HCO3)2 + Ca(OH)2 CaCO3(s)+ MgCO3+ H2O Mg-C
MgCO3 + Ca(OH)2 Mg(OH)2(s)+ CaCO3(s)
4. MgSO4 + Ca(OH)2 Mg(OH)2(s)+ CaSO4 Mg-NC
5. CaSO4 + Na2CO3 CaCO3(s)+ Na2SO4 Ca-NC
- Chemical needed:
(concentration of species in meq/l)
Discription Lime Soda Ash
CO2 1 x -
Cacarbonate 1 x -
Canon carbonate - 1 x
Mgcarbonate 2 x -
Mgnon carbonate 1 x 1 x
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Limesoda process ;
a. Excess lime treatment
To establish optimum precipitation, especially
Mg presipitation, excess lime is added in amount of
35 mg/l CaO (1,25 meq/l)
b. Split treatment
A fraction of raw water is treated in excess-lime
process, and neutralized the excess lime using (part of)
untreated water
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