C H A P T E R - III

Dfi:T'~RMUATIOli OF THIOCYANATE BY PRECIPITATION OF SILVER

THIOCYANATE FROM HOMOGENEOUS SOLUTION

DETERMINATION OF THIOCYANATE BY PRECIPITATION OF SILVER

THIOCYANATE JROM HOMOGENEOUS SOLUTION

SUMMARY

76

Silver thiocyanate has been precipitated 1n a

granular form from homogeneous solution by taking advantage

of the lon~ induction period for the formation of the

precipitate in strong ammonloal solution. The method is

based on the solubility of silver thiocyanate in ammonioal

solution due to the formation of diamino silver (I). On

heating the solution, ammonia escapes, and free silver ions

slowly enter the solution. The same can be achieved by

diffusing nitric acid, when ammonia is neutralised resulting

in precipitation of silver thiocyanate. The optimum pH

range for complete precipitation is found to be 7.0 - 9.0.

35 - 175 mg of thiocyanate has been conveniently determined

using this method. Anions like sulphate, acetate and nitrate

have no effect on the determination. Good reoov,Jries can be

made in presence of copper (II), cadmium and magnesium.

,_ '

DE'J!ERMIN:\TION OF THlUCYAUTE BY PltEOIPITA'riON 0!' SILVER

THIOCYANATE FROM HOMOGENEOUS SOLUTIO.III a-

A survey of literature reveals that very few

methods are available for the determination of anions by

precipitation from ho1110geneous solution. The determination

of thiocyanate by precipitation as silver thiocyanate from

homogeneous solution was therefore undertak~n.

77

For the determination of thiocyru1ate the available 1 gravimetric methods are by precipitation as silver thiocyanate ,

ouprous thiocyanate 2-4 and barium sulphate5. The precipitate

formed in the cuprous thiocyanate method is ourdy but the

solubility of the precipitate inoreases by the decrease of 6 pH • Nitrate, mercurous and. lead ions interfere. The

interference of bismuth, ru1timony and tin oan be prevented

by the addition of tartaric acid. Cobalt, nickel, manganese,

zinc, cadmium, ferrous, arsenic, alkali metals and alkaline

earth metals, in sulphate - free solutionsdo not interfere.

The determination,as barium sulphate involves the oxidation

of thiocyanate to sulphate and then precipitating as barium

sulphate. Evidently, the method is applioeble only when

sulphate and other sulphur compounds are not present.

Further, the presence of mineral acids gives erratic results

as the solubility of barium sulphate varies with the nature

of the mineral acid and its concentration. The precipitate

will have to be ignited.

The oonventional llilver thiocyanate method was

reported l

by van Name •

and is almost ideal for

The precipitate formed is

filtration and washiag7•

preoipi tation from homogeneous solution is expected to

produce a crystalline precipitate, the method can surely

be improved. With this object, the present investigation

has been taken up. The solubility of silver thiocyanate

in ammonia, resulting in the formation of a soluble silver

ammonia complex, and its repreczipitation by gradual

removal of ammonia, forms the basis of this method. This

78

oan be achieved by decreasing the pH either by volatalising

ammonia slowly or by gradually neutralising ammonia with

nitric acid by diffusion. In both these methods, the silver

ions are liberated insi tu. from the diamino silver (I) complex.

The use of the diamino silver (I) as a reagent was first

reported by Firsching for the separation and determination 8

of equimolar mixtures of chloride, bromide and iodide and

for the determination of phosphate9 • Varughese and Vaidya10

and Varughese and Somasekhara Rao11 also employed this reagent

for the determination of chromate and tungstate respectively.

•

EXPERIMENTAL

REAGENTS 1-

Stl!.!ldard Thiocyanate Solution 1-

About 4.6 gme of Ammonium thiocyanate (B.o.H. 1 AnalaR' grade) we:t·e acou.rately weighed and transfered

79

to one litre !lask. It was dissolved in water and diluted

upto m&rk. ~he thiocyanate content was checked by Volhard' a

method.

Ree.gent a-

Reagent solution was prepared by adding 20 ml of

concentrated ammonia (Sp.gr.0.91) to 40 ml of 1M silver

nitrate solution and diluting to 100 ml with distilled

water. 10 ml of the reagent was used for precipitation.

All o the.r chemicals used were of A. R. grade.

EQUIPMENT a-

Colorimetric meaaurementswere made on Bausch &

Lomb spectronic -20 colorimeter.

Thermogravimetric curve was recorded by Cahn RG

electrobalanoe.

pH measurements were made on Beckman pH - meter, H2•

'· '

Pho tomicrogre.pha were tekea on Oarl Zeiaa Zena

Amphive.l with mt. matic Automatic exposure oontrol.

MIN 8 microaoope 'With lo'\ol pow3r objective 9x

and micrometrio eye piece 5x were used for measuring the

orystE&l. sizes.

PROCEDURi~ I -

1) By Volatalisation of Ammonia •-

80

25 ml of ammonium thiocyanate solution, containing

oa. 1.45 mmolea, was pipetted out into a 500 ml pyrex beaker

and 30 ml of concentrated ammonia and 25 ml of lM ammonium

nitrate solution were added. The volume of the resulting

solution was adjusted to about 200 ml by adding distilled

water. 10 ml of the reagent solution were now added. l ml

of O.l\( thymol blue indicator waa also added. ThP. beaker

was heated on a steam bath. The evaporation losses of the

solvent duriag heating were made up by adding necessary

quantities of distilled water from time to time. The

completeness of precipitation was i~dicated by the change

of colour of the supernate from blue to yellow. The time

taken for precipitation was found to be 3 - 4 hours. It

was observed that the precipitation commences at pH 10.6.

The pH of the supernate after complete precipitation was

found to re.nge between 7. 0 and 9. 0 at room temperature.

• 81

~he precipitate was filtered throu~h a G4 sintered

~lass oruoible and washed twice or thrioe with small amuunta

of distilled water. The pnoipi tate was l'L•ally driad at

120°0 in an oven to a constant wei~ht and wei~hed as Ag SCN.

ii) By Diffusion Method ,_

25 ml of the thiocyanate solution were pipetted

out into a 250 ml beaker. 30 ml of oonoentre.ted ammonia

and 25 ml of lM amrronium nitrate solution were added and

diluted to about lOOml. 10 ml of the reagent solution

were now added and the beaker was kep• in a deaiooator.

About 200 ml of oonoentrated nitrio acid were take.:1 in

another beaker and placed in the same desiccator. The

desiccator was set on a shaker for eentle shakinB• The

precipitation was ge11era1ly complete in 6 - 7 hours.

The precipitate was filtered, dried at 120°0 and weighed

as .A.g SON.

( Conversio£1 factor, SCN/Ag SCN .. o. 34998)

Determination of the amount of thiocyanate present in the filtrate

Colorimetry a-

As a cheok on the filtrate, the excess of silver (I)

was precipitated as silver chloride and filtered off. Then the

amount of thiocyatlate present in the filtrate was detel'lllined by

copper pyridine oomplex method12• The absorbance being measured

at 410 nm.

RESULTS AND DlSCUS'HON

Table 1? and Table 13 summarise the results

obtained for the determination of thiooyanate as silver

thiocyanate by pL·ecipi tation from homogeneous solution

employing the two developed procedures.

TABLE 12

Determination of thiocyanate as silver thiocyanate by

volatalisation of ammonia method ,_

Thiocyanate Thiocyanate Difference Error taken found (mg) (mg) (mg) ( ~)

34.590 34.608 ~0.018 0.0521

34.590 34.579 -0.011 o. 0318

34.590 34.602 +0.012 o. 0347

51.890 51.924 +0.034 0.0655

51.890 51.885 -0.005 o. 0094

51.890 51.858 -0.032 0.0617

86.480 86.612 +0.132 0.1526

86.480 86.561 +0.081 o. 0937

86.480 86.425 -0.055 0.0636

86.480 86.367 -0.113 0.1307

86.480 86.446 -0.034 o. 0393

86.480 86.512 +0.032 0.0370

82

,

Table 12 Oontd.

172.960

172.960

172.960

172.900

173.080

172.780

-0.060

+0.120

-0.160

0.0347

0.0694

0.0925

•

It is clear from Table 12 that the thiocyanate can be

determined with fair e.oouraoy in the concentration range

studied using vo1ataliaation of ammonia method.

TABLE 1:5

Determination of thiocyanate as silver thiocyanate by

diffusion method ,_

Thiocyanate taken

(mg)

86.480

86.480

86.480

86.480

86.480

86.480

Thiocyanate found

(mg)

86.492

86.4~6

86.501

86.427

86.451

86.524

Difference Error

(mg) ( ~)

+0.012 0.0139

-0.046 0.0532

+0.021 0.0243

-0.053 0.0613

-0.029 0.0335

+O. 044 0.0509

8 .~

Table 13 illdioetes that the ditfusion methoJ. is equ1<ll7

aoourate for the determination of thiocyanate.

8t

Pho tomiorographs of the preoipi tatea obtained b7

the conventional wethod13 and the two developed procedures,

are presented in Figure 6. Magnification is x96o It is

observed that the particle size of the preoipi tates obtaiHed

b7 two developed prooedure8 was considerably improved in 13

comparison to that o btait!ed by ooavetltional '1'1e thod • It

is also no ted that the crystallin1 ty of the particles is

more pronounced 1n the ease of precipitate formed by diffusion.

It can be .seen from the figure 611 that in the preoipitate

formed by volatalisation of ammonia method, the crystals

are rod like and they are priSlllatic whereas in that of

diffusion method (Fi~·e 6C), they are well developed,

Rhomb shaped and also having triangular out lines and no

orystallini ty ir1 the oase of oonventional method (Figure 6A).

Figure 6D is that of the precipitate obtaitled by volatalisation

of ammonia method when ammonium nitrate is not used in which

the crystals are less developed.

The sizes of the crystals formed by two procedures

were measured micrometrical17• The length of the bigger

crystal formed by volatalisation is found to be o. 688 Dill

and its breadth is 0. 416 mm. The lell8th of the smaller

orystal formed in this fashion is 0.112 mm and breadth is

0.08 mm. But the visual percent of smaller orystale is 60.

/

. . -:J 'I ·-~ ' . '

l• --'1: -~. . ~'.J:Ji£

~ .. ,__~ ... -.. ,.~~\,· .. ' . ~ ~ ,, . . ..... ~. ' . . \,. .. , '

~ ~ ,.. ...,.a~- ., ~ ·~" ~ . .. . .. . ~- .. ' .... · ... ~ ~ <:1, • 7~ . . . ' . .

·---~- *'- '-\ ~ . . . .,. .. -~ , " ' '\ # ~:>- , • .,. ~· '- ' • '., •• '.I ,_ . ......... • ~ \.

I • . " . 'a_ -~~ • - - . ... • -~ \ -41.. .

1. ·~ . . ·o ,,.. ) )..._,.., .... - ' ' ~ ' . . . ~ I - ' . I "

·'~ ... •, ~\t .. ~_t· ' . 1 ·, ! ' ~' . .. " '- ." .. . • .. ' l. < .:;;,. A..

•

The bigoer crystal formed by diftusioa method is having

length of 0.416 mm an~ breadth is 0.320 mm whereas smaller

crystal. is having 1e.1gth of o. 096 mm and o. 08 mm breadth.

The visual percent of smaller orystala is 20. Smaller

crystals are very less in the preoipitate formed by the

diftusion method whe11 compared to volatalisation method.

85

Some experiments we1·e carried out to etu.dy the

effect of pH on preoipi tation and to determined the pH at

which quantitative precipHation is affected Using

volatalisation of ammonia method. The extent of preoipi tation

in each case was determined by finding out the amount of

thiocyanate in the filtrate oolorimetrically. The results

are recorded in Table 14.

TABLE 14

Determination of the amount of thiocyo.nate in the filtrate

oolorimetrically ,_

pH

10.0

9.7

9.4

g. 2

9.0

Thio oyo.na te in filtrate (mg)

18.22

12.~6

7.71

1.05

0.15

Table 14 Contd.

8.6

8.2

7.8

7.4

7.0

0.04

0.03

0.02

0.03

0.03

•

The precipitate commences to form at pH 10.6. The results

of Table 14 shows that the preoipi tation is quantitative

between the pH range of 7.0 and 9.0. A clear idea of the

effeot of pH on preoipi tation oan be had from Figure 7.

The calibration ourve is given in Figure 8 and the data

is given below.

TABLE 15

Calibration curve data for determining thiocyanate a-

pg of thiooyenate

100

250

500

750

1000

Optical density

0.04

0.11

0.22

0.33

0.44

R6

• • < 0 4

.. r- T-

1-I-· -,

I '

. I I ' i

('I

H I ' -• I

"LJ I

I I

-~-~-

7.0 76 B ' B S "" 1(1 0 10 ]

Fjg.7. l?tf ec t of pH on p " precipitation of s1 t ve r thiocyanate.

o.&o

04 0 - - / /

/

~ y

' I

0 - --

/ !

i ----· v ·--r-

_l~ I i

/ I 0

200 400 GOO soc 1000 concentration ol th1 oc yr. at~ tugl

Fig.&. calibration curve tor determining thiocyanate /\410mm

Studies weL·e made to fiadi!l6 out the amount of

silver aitrate required !or the oomplete pL·eoipi tation of

86.4 mg of thiooya.aate. The resu.lta are presented in the

Table 16.

TABLE 16

Percent of thiocya.aate precipitated with varying emounte ot

ailver nitrate a-

Silver .ui trate taken

(mmoles)

o. 50

0.75

1.00

l. 25

1.50

1.75

2.00

Thio oyana te taken

(mg)

86.46

86.48

86.48

86.48

86.48

86.48

86.48

Thio oyana te found

(mg)

29.06

43.41

58.12

73.52

86.48

86.48

86.48

% of Thiooyane~e preoipi tated

33.6

50.2

67.2

85.0

100.0

1oo.o 100.0

The resu.lts show that 1.5 mmo1es of silver nitrate is

required for complete precipitation. Fi~~· 9 is the

graph of the oonoentration of silver nitrate verses the

percent of thiocyanate precipi tawd. The etfeot of exoess

87

of the reagent was also studied. No interfere:1oe was observed

upto 50 mmoles of silver nitraw.

... 1 "

" '

I I

j ' _j

25 0 04 •• ' ' SIL~er n.trote (mmole~l

' 6 '0

Fig 9 Effect of reagent concentratiOn on precipitatiOn of

Silver thtocvanate.

' I I

A·AtnmonJJfi'O~etot•

B- Only cll'lmonlo C- Ammonium nitroh

~ ~ 0- Ammonium sui Ph ate

~ ~ ~ E- Ammon_!•~!_! __

~ !

• ~ ~ ~ :---------

f\. '

' ~

6

' I ' I I

'" eo 240 Time I rni n.)

f--

-- -

A

• ' c D

Fig.10. Efhct of t1me on pH using different ammonium salts during prec1prtation of silver thiocyanate

Ammonium nitrate is pnerall:y ueed 1n the

methods 1nvolvini the precipitation of silver salta

like phosphate, chromate, tungstate, vanadate end

arsenate by the vo~alisatioa of ammonia method. It

was thcught worth while to study the effect of other

ammonium salts also. j number of experiments conducted

to determine the pH decrease with time 1n presence of

ammonium acetate, ammonium sulphate, ammonium nitrate

and ammonium oxalate. The results are given in Table 17.

A plot of pH decrease with time is shown in Figure 10.

It is observed that the required pH range of 7-9 is

obtained 1n a lesser time when ammonium sulphate,

ammonium nitrate or ammonium oxalate is used. As

ammonium nitrate has the same anions as silver nitrate,

it is pre fared to ammonium sulphate. As ammonium oxala-te

interfere the dete.rm1nat1on, it is not sui table.

88

The effect of the oonoentration of ammonium

nitrate is not critical. In the absence of ammonium

nitrate also the precipitation was found to be quantitative

but the crystallinity was not so improved, which oan be

seen from Figure 6D, end the pH decrease takes more time.

In order to improve the orysta.llini ty and to reduce the

time, 25 mmoles of ammonium nitrate was used in routine

procedure. Excess amounts upto 50 mmoles have no effect

on the precipitate.

The influence of various forei&n anions were

investigated.

TAB

I.E l.7

Ette

ct o

r d

lr!ere

nt am

monium

sa

l.ts on the

pH d

ecrease with

tlllle s-

s. N

o. su

bstan

ce ()

30

6

0

90

1

20

1.6 0

l80

2

10

2

40

2

70

3

00

3

30

3

60

~.

No

liiiUIIO

nlum

sa.Lt

~1..6

~.L.O ~0.8 1

0.6

10

.4 1

0.1

9

.3

8.6

8

.1

8.1. 8

.1

7.9

7

.9

2.

Am

mon illlll n itl' a

te

10

.4

10

.2

9.6

9

.3

9.0

8

.6

8.2

7

.0

6.6

6

.6

6.6

6

.6

6.6

3.

Am

monium

su

lph

ate

10

.2

10

.0

9.3

9

.1

8.8

8

.4

8.0

7

.2

6.4

6

.4

6.4

6

A

6A

4.

Am

monium

aceta

te

u.s

ll..O 1

0.9

J.0.7

~0.6 lO

A

9.8

9

.1

8.7

8

.3

8.3

8

.1

8.1.

5.

Am

mon illlll

ox

ala

te

10

.4

9.8

9

.5

8.9

8

.8

8.1

7

.7

7.2

7

.2

7.2

?.2

7

.2

?.2

T Jae

1s

.fD a.to

ute

s.

00

:.1:> •

90 •

TABLE l8

Preotpitation of silver thiocyanate in preaenoe of foreign

anions ,_

Foreign anion Concentration SCB-(mmolee) taken

(mg)

-SON Difference found

(mg) (mg)

Error

( IC)

NH4

( cH3ooo) 100.0 86. 480 86. 602 +0.122 0.1411

" 200.0 86. 480 86. 638 +0.138 0.1596

(NH4 ) 2 so4

100.0 86.480 86.424 -o. 056 0.0648

• 200.0 86.480 86.621 +0.14l 0.1630

Na( cH3coo) 100.0 86.480 86. 386 -0.094 0.1086

" 200.0 86.480 86. 390 -0.090 0.1041

K2so4 100.0 86.480 86. 523 +0.043 0.0497

" 200.0 86.480 86.564 +0.084 0.0971

Na No3 50.0 86.480 86. 3:1S -0.152 0.1758

K N03 50.0 86.480 86. 346 -0.134 0.1550

Na2 Sio3 0.5 86.480 86.542 +0.062 0.0717

The results, as given in Table 18, show that acetate, nitrate

and sulphate do not interfere 1n the determination even at

high concentration. Silicate ions interfere if present more

than 0.5 mmoles. In presence of halides, higher values were

obtained.

•

The effect of some cations were also studied and

the results are presented in !able 19.

TABLE 19

Preoipi tation of silve:L' thiocyanate in presence of diverse

cations a- (50 mg of eaoh oation was used)

Foreign cation

Cu(II)

Cd( II)

Mg(II)

Oa( II)

Sr( II)

Thiocyanate taken

(mg)

86.480

86.480

86.480

86.480

86.480

Thiocyanate foWl.d

(mg)

86.462

86.581

86.542

87.900

109.60

Difference Error

(mg) ( "'

- 0.018 0.0208

+ 0.101 0.1168

+ 0.062 0.0717

+ 1.420 1. 6430

+23.120 26.7300

In presence of coppe1"( II), cadmium( II) and magnesium( II)

good recoveries of thiocyanate would be made. It appears

that calcium interferes to a small extent, while strontium

interferes seriously.

Thermogravimetric curve of dried precipitate

is shown in Figw:·e 11. The curve is horizontal extending

up to 300°0. This horizontal portion tends 1 tself to the

automatic det·Jrmination of silver thiocyanate. At 300°0,

91

T • m p • ro I u r o, •c

'" "' "' ,., 540 600

i,,r

...

Figl1. Thermogravrmetne curve for srlver throeyqnate

"' I

,.,

i ! _ _l__ _ _J

two molecules share the sudden loss of one atom of sulphur

and the residue consists of the double salt Ag 3CN - Ag CN,

92

0 it yields equally a horizontal line extending from 30Q-450 a.

0 0 Beyond 450 c, cyanogen gas is evolved and Ag2s left at 660 c. After 660°0 the curve raises due to the formation of silver

sulphate accompanied by an inorease in weight.

CONCLUSIONS

The methods are simple and give accurate

results. The advantage of the method is that the

crystelline precipitates are obtained in place of curdy,

amorpJ:ihs precipitates formed by the conventional J!lethod. " . Heavy metals, if present, will get precipitated at the

higher pH and oan be separated by filtration while silver

thiocyanate remains in solution. Gravimetric factor

SCN/Ag SON • o. 34998 is favourable when compared to oopper .

thiocyanate method SON/Cu SOLi = o. 4 7757. The precipitate 0 can be dried upto 300 C without any weight loss. The

determination can be carried out in presence of oopper (II),

cadmium(II) and magnesium(II). However, thismethod oan not

escape the interference by halide and oyanide ions.

9.1

R E P E R E H C E S

1, G. van Name, Am.J,So1., !Q, 451 (1900),

2. M.L.Rivot, Com,pt.rend., ~' 868 (1864).

3, J,llodnar and V.To1nay, Z.anal,Chem., 120, :5:56 (1940),

4, R,lleloher and T. s.weet, .A.nal.Chim.Aota., _2, 337 (1952),

5. L.W.Winkler, Z,angrew.Chem., ]2, 160 (1920).

6. A.I.Vogel, "A Teat Book of Quantitative Inorganic

Analysis", Longmana Green & Oo,Ltd. :5rd,(l968) P,497.

1. R.ll.Piaoher and D.G.Petera, "Baaio Theory and Praotioe

of Quantitative Chemioal Analysis", W.B.Saunders

Company, 3rd., (1968) P.l84.

8, F,H,Pirsohing, Anal.Ohem., ig, 1876 (1963),

9. ------------, Ibid • , ~' 87:5 (1961),

10. K.Varughese and V.G.Vaidya, Ana1.Chim.Aota.,2Q, 176 (1970

11. -----------and K,Somasekhara Rao,, Ibid .,jl, 219 (1971

12. G,Char1ot,"Colorimetrio determination of Elements",

Elsevier, Amsterdam, (1964) P.2l6.

13. Ref,No. 6, P,569.