BH

CbAPTElt - Ill

A new s;:ncLcophotome1:ric method is proposed .for

th( detPrt:".ination of r:1icro-s:Hounts of fluoride in water.

Mf'V:o•i is baseo on me,,surin1·, ttJe drcrernent in the colour

ol· th0riua-cnromotrope 2B complex on addition of fluoride.

r'luoride reacts ,.,itf, thoriun:-ct,rorr.otrope 2B complf'x and

LJrms stable thorium fluoride and simultaneously displaces

tLe red coloured ciJrorr.otro,:;e 2D dye. T11e Beer's l<.w is

o ne.Y Eo>d ir, ttl e r!.nt;e of. 0.? 5 to 2 • Q ;Jpn, o.f .fluoride.

l·.i'i'ec~ o=· ir.tcri'ercnce end other c,m,lyticol oar&rr.eters

!J<.<vc i.Jun stuc:ied. r~roposed metl.od is successl'LJlly

a;J;:>li£'d for th£ rieterminatlcn oc. fluoricle in polluted

river "''"tcr >one cfiluent o." a fertilizer factory.

u • ( 1 Lh) p. 228.

89

SPECTROPHOTC.JIV,ET!UC f)ETE!U'dNATlOi• O.r' i''LUO!UDE IN WATEH

Fluoride is one of the important air and water

pollutant. It is ubiqutous, detectable traces occur in

almost all substances. ~'luorine is potentially toxic

trace el~:·ment present in numan and animal nutrition(1,2).

Industrial plants processing fluoride containing minerals,

manufacturing phosphatic fertilizers and aluminium are

the most imoortant sources of fluoride. Fluorides are

used in steel making, opacifying of glass and enamels,

electroplating, insecticide, petroleum, plastic, dye and

pharmaceutical industries. BricK and ceran,ic works also

contribute to considerable pollution from fluoride (3-6).

lts wide distribution in nature and frequent exposure to

men, animals and vegitation has stimulated extensive

research on its toxicology and estimation in environment.

Several reports are available on the toxicology

of fluoride (7-11). The toxic effects of the organic

fluorine compounds are unlike those of inorganic fluorides.

J'he organic compounds like f l.Uorocarboxylates are the

most important and their m~tbbolic activity have been

studied extensively. An extraordinary feature of their

activity is that tt10se members of the series /(CH2 )nCOOH

for which n is odd are extremely toxic, wh£reas those

for which n is E>Ven h2ve 11 t tle or no toxicity at all.

The first rr.en.ber, monoflu .•rfocetic acid is widely used in

the ~·orm of its sodium salt to kill rodents and other

pests (12). Pesticidal activity of polyfluorinated

esters have also been reported (13). Fluoride compounds

in the atmosphere are generally considered to be toxic

to plants and animals (14,15). It is highly reactive

and complexes with many organic and inorganic biological

components and alters the metabolism of tissues (16).

The acute toxic effects of fluoride include vomiting,

abdominal pain, diarrhoea, convulsion, generalized and

rr.uccular weakness, collapse, dys,;nea, paresthesia,

difficulty in articulation, thirst, weakness of pulse,

disturbed colour vision, loss of consciousness and motar

unrest. Inhalation of fluoride dust and fumes causes

irritation of nasal passages and can cause pulmonary

90

~:·dema which is the cause of death (17, 18). Eyes and skin

burns are the possible hazards caused by fluoride in

contact with skin and eyes (11) • .A chronic skill disease

acne rosacea is also reported to be related to fluoride(19).

N e;-Jhri tis, hemorrhagic gastroenteritis and more or less

definite pathologic damage to other organs are found to

...;e caused by acute poisoning of fluoride. It reduces the

calcium content of blood and thus interferes with the

clotting of blood. It also acts as an inhibitor of certain

interacellular enzyo:es conceorned in anerobic glucolysis

of many type of cells, plant as well as mamalion (5,20) •

The chronic toxic pfiects are fluorosis, identi­

fied by skeletal ajnormalities, or damage ranging !rom

rr•wmatism to ;:>errr anent crippling, anaroxia, muscular

weakness and impairn.ent of dental development (21),

However, the symptoms of preskeletal fluorosis, which

91

are usually ignored and incorrectly attributed to other

causes are headache, polyuria, polydipsia, epigastric

distress, severe osteoasthritic bone pain, painful nodules

on the extren.i ties and disturbed kidney function (22). It

has been studied that the incidence of fluorosis is parti­

cularly high in parts of south India where calcium intake

is low (23,24).

V<'getation is also <ofi£cted by fluoride pollution,

It is amongst the rtJost phytotoxic of common air pollutants.

~'luoride enters the leCJVes throup-,b stomata in their surfaces

fron' the atmosphere. The symptoms of acute fluoride injury

to monocotyledon plants appear at ti;e leaf tips, which

become broc.n and necrotic and in dicotyledon plants the

necrosis is initially confined to the margins of the

leaf (25). Chronic injury of fluoride appears as an inter­

veinal chlorosis. Large seed fruits, grapes, pines, maize,

snd gladioli are most sensitive to fluoride, showing up as

over-ripening with separation of the fleshy parts of the

fruit. rluori:ie also causes wide spread damage to live­

stock. The e:f ~·ects from euting ccnt;;minated forage are

abnormal calcification of bone and to,th structures, milk

production is a.L>'O aecr·eased, animal lose weight, acquire

a stiff posture and hair coat becomes rough (9,26).

Fluoride also plays a r.J<>jor role in deteriorating the

acuatic ecosysterr (27).

In the absence of indus trial exposure the chief'

source of fluoride is drinking water. The deleterious

effects on bones and teeth are observed in community

consuming high concentration of fluoride but 0.8 to

1.~ ppm of fluoride is artificially added to potable

water sup_.;liea to reduce the incidence of dental caries

( 5, 28) without producing adv£rse effect on bones and

physical health {11,29). fluoride is mainly accumulated

in bones and teeth and is excreted by kidney. Thus

urinary fluoride levels are v.idely regarded as one of the

best indicators of fluoride intake (30). Fluoride

reduces the rate of dissolution of bone. Recently,

fluoride has been used in the treatment of osteoporosis

in an attempt to rebuild bone lost due to this disease

and has a profound effect on osteoblasts and bone forma-

tion (31).

Fluoride level in Drinking Water (32)

Parts per rtillion of fluoride

Effect

-------------------------------------------------------Less than 0.5

1

Greater than <! .5

10 to 20

High incidence of dental caries, no mottling

Low incidence of dental caries, little or no mottling.

Disfiguring mottling

Severe ~.ottling or fluorosis

-------------------------------------------------------

92

The threshold limit value as recommended by American

Conference of Governmental Industrial Hygienists (ACGIH)

is 3 ppm ( 7) • Due to its critical level and importance

in drir.king water the accurate determination of fluoride

has great importance with the growth of fluoridation of

public water supply.

Several methods for the determination of fluoride

have been reviewed in the literature (33-35). Fluoride

does not form coloured compounds thus only few direct

methods are re~orted which are based on the formation of

ternary complexes of fluoride with cerium, lanthanum or

zirconium and alizarin complexan (.36-39), eriochrome

cyanine R (40), quinalizarin (41), sulfonated alizarin

93

blue (4~,43). Ternary complexes with zirconium and xylenol

orange or methyl thymol blue (44), chromo tropic bisazo (45)

and thorium-chromeazurols-cetyltrimethyl ammonium (46),

etc. are also reported.

GEnerally all the other methods are indirect and are

based on the liberation of dye or chelating anions from

its metal complex with the formation of more stable metal

fluoride complex. The colour of the liberated anion

dHfers sufficiently with its metal chelate and permits

tHe determination of fluoride concentration. The reaction

can be generalized as : F- + MeR~ MeF + R- where

MeR is metal complex of chelating dye, MeF is a metal

fluoride complex and R- is a libera~ed dye anion. Various

chelating dye form coloured complex with metals such aa

\

94

zirconium, thorium, lanthanum, aluminium, etc. are used

for spectrophotometric determination of fluoride, the

widely used systems are alizarin with zirconium (47),

or thorium (48}, quinalizarin with zirconium (49),

eriochrome cyanine R with zirconium or aluminium (50,51},

SPADNS with zirconium or lanthanum (52,53), chloroanilic

acid with thorium (54}, hemotoxyline with aluminium (55).

some other reagents are chlorosulphophenol S (56),

salicylflavone (57), 4-(2 pyridyl azo) resorcinol (58),

Arsenazo III (59,60) and chrome azurol S (61) with zirconium,

salicylamide-iron (62), racetophenone oxime-uranium (63)

and ti tanyl oxalate (64), pyrocatechol violet-alumin£um

(65), carboxyarsenazo-thorium (66), rufigallic acid­

zirconium (67), sulphoalizarin-scandium (68), 2-hydroxy-

1-(2-hydroxy-4 sulfo-1-naphthylazo) 3 naphtholic acid­

aluminium (69), chromotrope-2R-thorium (70), rutin­

zirconium (71) and metal oximates (72). Solid reagents

have also been proposed for the determination of fluoride

(73-76). Other available methods for determination of

fluoride are titrimetric (77), fluorimetric (?6,79),

potentiometric (80,81), ion selective electrode (82,83},

ion chromatography (84,85), activation analysis {86),

molecular absorption spectrometry (87,88) and flow

injection analysis (89,90). The colorimetric determina­

tion of fluorides has been incorporated in automated

procedure for the analysis of fluorides.

In the present investigation a new rapid and sensi­

tive method for the determination of fluoride in water is

developed. The method is based on the bleaching action of

fluoride on the purple coloured thorium-chromotrope 2B dye

complex ( ~ max 550 nm) and measurement of the decrease in

colour intensity at 570 nm i.e., the wavelength of maximum

difference. The pure dye chromotrope 2B has maximum absor­

ption at 510 nm and it forms a purple coloured complex with

thorium. On addition of fluoride to tile dye-metal complex,

fluoride forms more stable compound with thorium and

displaces the red coloured dye which is measured spectro­

photometrically. The method is .successfully applied for

the determination of fluoride in polluted river water and

effluent water. Reaction can be given as under:

Chromotrope 28.

Thorium- chrotnotrope 28

complex

OH

0 N-«5' N ---fh- OH 2 '\::::!/" II 6 OH

NJOO HO S ~SOH

3 3

Thorium - chrornot rope 28 complex

Chromot rope 2 8

2-+ThFs + nH20

95

EXPERIMENTAL

Apparatus

All spectral studies were made with ECIL spectro­

photometer and a Carl :Leiss spekol using 1 em matched

glass cells.

Reagents:

Standard solution of fluoride - 0.211 g of A,R. grade

sodium fluoride was dissolved in 100 ml of glass distilled

water, Working standards were prepared by appropriate

dilution of the stock containing 1 mg of fluoride per ml.

Chromotrope 2B (p-ni trobenzene azo chromotropic acid)

solution A 0,002 M solution of chromotrope 2B dye

was prepared in glass distilled water.

Thorium nitrate solution - 0.001 M solution of thorium

nitrate was prepared in glass distilled water,

Mixed reagent - Equal volume of 0.002 M chromotrope 2B

solution and 0,001 M thorium nitrate were combined to

give red-violet complex. Mixed reagent was prepared

daily,

Reference solution - 1 ml of mixed reagent and 0.2 ml

9G

of 0,02 M hydrochloric acid were taken in 10 ml volumetric

flask and volume was made upto the mark with glass distilled

water.

Diverse ions solution - 'Solutions of diverse ions were

prepared by the reported method (91),

All reagents used were or analytical reagent

grade.

Procedure

An aliquot of water sample containing 2 .:> to 20 ;Ug

of fluoride was taken in 10 ml volumetric flask. To this

1 ml of mixed r•-agent was added and the pH maintained at

2.5 to 3.0 by adding 0.02 M hydrochloric acid. Then its

volume was made upto the mark with glass distilled water.

;,fter 1 min absorbance was measured at 570 nm against the

distilled water as reference. Absorbance of blank or

reference solution was also measured at 570 nm against

the distilled water.

RESULTS AND DISCUSSION

Spectral characteristics

The absorption spectra of chromotrope 2B show that

the dye has a maximum absorption at 510 nm. The dye metal

complex has maximum absorption at 550 nm. The differen­

tial absorbance of the chromo trope 2B-thorium complex with

various concentrations of fluoride against a reference

solution shows maximum difference at 570 nm. which was

used for spectrophotometric determinations (Fig. 1).

hffect of varying reaction conditions

The optimwL pH for maximum colour development -a

found to be in the range 2.5 to 3.0. At lower pH the dye

97

0·7

0·6

0·5

0-4

C1> <..)

c 0-3 tU .0 .... ~ .0 0·2 <

0·1

98

""" ~-6 8

20 Wavelength , nm

FIG.l. ABSORPTION SPECTRA

A. CHROMOTROPE 2 B DYE AGAINST WATER

B. CHROMOTROPE 2 B DYE + THORIUM

AGAINST WATER

was liberated even without the addition of fluoride and

at higher pH the dye was not liberated at all.

The reaction was rapid at room temperature. No

change was observed upto 15°- 40°C. The liberated dye

was found to be stable for rJ 12 hours. The effect of

tiffie on reaction of fluoride with thorium-dye complex was

evaluated and it was observed that only one min. was

required to complete the reaction and later constant

results were obtained.

Effect of molar ratios of chromotrope 2B and

thorium was studied. Absorbance values were maximum for

molar ratio of 1 : 1 of 0.002 M dye and 0.001 M thorium.

At lower ar;d higher molar ratios decrease in absorbance

values were observed.

Beer's law, molar absorptivity, Sandell's sensitivity

and Reproducibility

The Beer's law is obeyed in the range of 2.5 to

20 fUg of fluoride per 10 ml {0 .25 to 2 ppm) giving a

99

standard curve which is straight line of negative slope.(Fig.2).

~olar absorptivity and Sandell's sensitivity were

5. 7x103 lit mol-1cm-1 (.! 100) and 0.0033 )Ug cm-2 respect­

ively.

Reproducibility was checked by determining 10 ,ug

of nuoride per 10 ml, over a period of seven days. The

results show that the method is reproducible. The stand­

c.rd deviation and relative standard deviation were found

•

o.a

Q.7

0-6

E c 0-5 0 ...... In

• O.lj. Cll u c , .Q

... 0-3 0

"' .Q

<1: 0-2

0·1

.0

100

2.5 5 7·5 10 12-5 15 17-5 20 Cocent ration of tluori de = ,ug 110m l

FIG.2. CALl ERATION CURVE FOR THE DETERMINATION

OF FLUORIDE WITH CHROMOTROPE 29 AND

THORIUM

to be .!: 0 .010!:1 and 1.492 % respectively for a solution

containing 10 fllg of fluoride (Table !) •

Effect of diverse ions

101

Effect oJ various diverse ions normally found in

water along with fluoride was studied. Most of the common

ions such as sodium, potassium, nitrate, nitrite, chloride,

bromide, acetate, carbonate, sulphite, etc. do not inter­

fere with this method. Cations such as calcium, magnesium

and aluminium which also form complexes with fluoride

interfere with this method. These were removed by cation-

exchange resins. Sulphates if present can be removed after

precipitation as benzidine sulphate (92). However, most

of the interferences are reu.oved by distillation of

fluoride prior to its determination as recommended(93)

('.l.'abl e II) •

Application of the method

The present method was applied for the determina­

tion of fluoride in drinking water and waste water. As

the drinking water ,eamples were found to be free of

fluoride, known amounts of fluoride were added to the

samples and analysed by proposed method. as well as SPAJli1S method.

To check the applicability of the method polluted

water samples of Kharoon river which receives effluents

from a superphosphate fertilizer plant were analysed.

r-:ethod was also applied to determine fluoride in effluent

water from fertilizer plant. Samples contained a number

of anions and cations interferents along with large

TABLE - I

HEPROOOCIBILITY OF 'IHE ME'IHOD

Concentration of fluoride • 10 ;ug/10 ml

102

--------------------------------------------------------No, of Days * Absorbance

&570 nm

--------------------------------------------------1 2 3 4 5 6 7

Mean = Standard deviation =

Relative standard deviation =

0.570 0.565 0,580 0.575 0.570 0.565 0.570 0.570

+ 0.0105 - 1.492~

--------------------------------------------------~ Mean of three repititive analysis.

TABLE- II

EFFECT OF DIVERSE IONS

Concentration of fluoride = 10 jUg/100 ml

---------------------------------------Diverse ions * Tolerance limit

ppm ------ -------------------------------------------------

No; Phenol, formaldehyde, aniline, Fe3+

Ca2+, Mi+ rot Al~\

500

400

200

100

50

20

1

------------------------------·---* Amount o! diverse ions that causes an error of + ~. -

103

amounts of phosphates and sulphates. Therefore the

samples were distilled (9~) prior to the analysis.

Distillation of fluoride samples was carried out in

concentrated sulphuric acid at 180°c. The traces of

sulphate present in distilled sample was removed by its

precipitation as benzidine sulphate (Table III). The method has been compared with some commonly used spectrophotometric methods reported for the analysis of fluoride (Table IV). CONCLUSION

The proposed method is sensitive, rapid and easy

to carry out. It can be successfully applied for the

determination of fluoride in drinking water, polluted

water and effluent water.

TJ1BU. - III

APPLICATION OF THE Ml:.THOD

(a) Analysis o£ fluoride in drinking water

s:N~-.--;1~~;-i_d_e _______________ Fl~;!ct;-!~~ct*-----------

---------------------------------------added Present % SPAOOS % method t'l.ecovery method Recovery

pg pg ~g

--------------------------------------------------------1. 2, 3. 4.

3.0 8.5

12 .o 18.5

3.0 8.5

11.9 18.3

100.0 100.0 99.1 98.9

2. 95 8.4

12.0 18.5

98.3 98.8

100,0 100,0

--------------------------------------------------------*!<Jean of three repetitive analysis,

(b) Analysis of fluoride in polluted water

--------------------------------------------------------S.No.

* i''luoride found W,g) Present method SPAOOS method

--------------------------------------------------------1. 2, 3. 4. 5.

5.30 9.21

13.88 18.01 22.29

5.29 9.20

14.01 18.03 22.50

--------------------------------------------------------* Mean of three repetitive analyses.

(c) Analysis o£ fluoride in ef1·1uent water

--------------------------------------------S.No.

* Fluoride found Wg) Present method SPAOOS aethod

----------------------------------------1. 2. 3. 4.

3.40 6.85 8.21

10.40

3.46 6.85 8.19

10.50

-----------------------------* Mean of three repetitive analyses.

104

TA

BL

E

-IV

CO

MPA

IUSO

N

WIT

H

OT

HI!.R

S

PE

CT

H.O

PrlO

ID}',E

TR

IC

ME

TH

OD

S

--------------------------------------------------------------------------------------------s.N

o,

Reag

ents

II max,

nm

Range

of

determ

ine ti on

in m

g lit-

1

Tim

e for

co

lou

r d

evelo

p­

men

t

Rem

arks R

eference

--------------------------------------------------------------------------------------------1

.

2,

3.

4.

5.

b.

7.

L.irconiun.-s<

'AD

NS

570

Th

oriu

m-A

lizarine

520

Alu

min

ium

-Erich

rom

e -

cyan

ine

R

Zirc

on

ium

-4-(2

-py

ridy

l 540

azo) -re

sorc

ino

l

Ti ta

ny

lox

ala

te-

40

0

resacetop

hen

on

e o

xim

e

Ztrco

niu

m-A

rsenazo

III

665

Thorium

-chromo tro

pe

2B

57

0

o.o

-1

.4

0.0

5 -

10

1 -

6

0.2

-

1

2 -

24

0.1

-

1.4

0.2

5 -

2

5 m

in

-10

-15

m

in

5 min

-30 m

in

1 min

Method is

se

nsitiv

e,

rap

id an

d sta

ble

.

Inte

nsity

o

f co

lou

r ch

ang

es with

tim

e.

Method is

tem

peratu

re d

epen

den

t.

Heavy

meta

l, ta

rtara

te

and asc

orb

ate

inte

rfere

.

Less

sen

sitiv

e.

Sele

ctiv

e,

inte

rfere

nce

of eu

2+

and

Fe 3+.

Stm

ple

and ra

pid

.

52

48

. 51

58

64

59

Pro

po

sed

method

--------------------------------------------------------------------------------------------

""" 0 CJl

RE.l''EI{EN CES

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106

4. L. Hodges, "Environmental Pollution", Holt Rinehert and Winston, Inc., (1973), p. 47.

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41.

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