Polarographic behaviour of tungstate in ascorbic acid solution

1NORG.

NUCL.

CHEM.

POLAEOGRAPHIC

LETTERS

Vol. 7, pp. 1141-1145,

1971.

B E H A V I O U R OF TUNGSTATE

Pergamon Press.

Printed

in

Britain.

Great

IN ASCORBIC ACID SOLUTION

A . N a r a y a n a n + and F.Umland Anorganisch-Chemisches

Institut der Universit~t }£Unster,

44 ?£Unster,Westfalen(Federal

Republic

of Germany)

( R e c e i v e d 17 July 1971)

Polarographic

behaviour

entrated h y d r o c h l o r i c hydrated cult

oxide

of tungstate has been studied

acid(1-3).The

in dilute mineral

precipitation

in phosphoric

acid solution at the d r o p p i n g m e r c u r y electrode (4).Tungstate

(WO3A2)2- with ascorbic let±er the results behaviour

forms complexes

acid(5).We in ascorbic

has been rece-

of the type

are reporting

of our investigation

of tungstate

of the

acids makes the study diffi-

in these media. The b e h a v i o u r of tungstate

ntly reoorted

in conc-

in this

on the p o l a r o g r a p h i c

acid solution.

i~{ATERIALS AND METHODS Tun~state

stock solution was prepared by dissolving

acid E2WO4(Merck,A.R) oxide and diluted

in just sufficient

amount of sodium hydr-

to volume with water. The tungsten content

the solution was determined by the hydroxyquinoline ~scorbic

acid

(T~{erck,A.R) was dissolved

w a t e r and this solution was prepared The experimental

techniques

method

of (6).

in air-free distilled

freshly before use.

and the electrolysis

same as p r e v i o u s l y d e s c r i b e d ( 7 ) . R a d i o m e t e r used

tungstic

cell are the

type p o l a r o g r a p h was

and the capillary characteristics were m=2.8mg./sec.

t=3 sec.per drop and h=50 cm. RESULTS On p o l a r o < r a p h i n @ ascorbic

acid

AND D I S C U S S I O N

e solution

in the absence

of sodium tun{state

of supporting e l e c t r o l y t e s , a wave

nt~rts at -C.3 volt with a half-wave potential

+ A]exanc'er Von Humboldt

in excess of

Fellow

1141

around -0.5 volt

1t42

POLAROGRAPHIC BEHAVIOUR

Vol. 7, No. 11

&9 4,2 3,5

~-c

z8

~ 2,1 I'4t a7 -o,'3

-Q3

-q3

Q3

-Off-Of,

-0.7

-I.4

-1.5

Ede vs SCE in rolls

FIG.I Effect of ascorbic acid concentration on the reduction of tun~state O.01M tungstate solution;molar ratio of tungstate to ascorbic acid,a)1:1 b)1:2 c)1:3 d)1:6 e)1:10 and f)1:20 vs.saturated

calomel

rode. At constant concentration,the the wave

tion until tration

height

increases

sing ascorbic

elect-

tungstate with

of increa-

a limiting

of ascorbic

attained,beyond ther increase

e

concen-

4, 2 •

acid is 3,5

which no furthat

the molar ratio of tungstate

to ascorbic

<

2,8.

-~

2,1.

z3

wave

appears

to aseorbic

u

is formed.

acid ratio

complete

(Fig. ld).When

the ascorbic

concentration

is increased

still further no increase

-q3

acid in

is observed.

~ig.2 tungstate

shows the effect

concentration

(3

0,?

of 1:2

the molar ratio is 1:6

waveheight

b

1,4-

at a tungsten

(Fig. lb) and is almost when

C

o

acid is 1:1,no

characteristic The wave

d

is observed(Fig, l).

From fig. la it is evident when

/.,9 -

acid concentra-

on the

of

-o,7 Ed.e. vs. SC.E

-1,1 in v o l t s

FIG. 2 Effect of tungstste concentration upon the waveheight Nolar concentration of tungstste,a)O.O02 b)O.O04 c)0.006 d)O. OO$ and e)O.01 0.2N ascorbic acid.

Vol. 7, No. 11

POLAROGRAPHIC BEHAVIOUR

1143

waveheight. It is observed that at a concentration 0 to 0.01 molar tungstate ved between waveheight

a linear relationship

and concentration

at higher tungstate concentrations

3, 5

a

b

Q4

-Q3

is obser-

of tungstate,but

deviation

c

range of

occurs.

d

2,8 ~. 2,1 E

L.

1,4. 0,7i

-0,3 -Q5 -Q3

-QZ.

-Q8

I

Ede. vs. S.C.E. in volts

FIG.3 Effect of pH on the waveheight O. O07N tungstate solution,O.1M ascorbic acid. a)pH 3.1 b)pH 4.05 c)pH 4.4 d)O.O5M acetic acid-O. O5M ammonium acetate buffer and e)solution a)made alkaline to pH 11 and then reacidified to pH 3.1 by adding ascorbic acid. Fig.3 shows the effect of increasing of the solution by adding sodium hydroxide crease

the pH

or buffer. An in-

in pH of the solution shifts the wave to more nega-

tive potential

and the wave becomes

ill-defined,but

the mini-

mum upon the limiting current disappears.Fig. 3d shows the polarogram

in presence

of acetic acid-ammonium

acetate buffer

of pH 4.1,which is also ill-defined. On making the solution of the tungstate-ascorbic

acid complex alkaline(pH

11) an intense

blue solution is formed which shows an absorption

spectrum

with a maximum at 770 m~.This blue colour is not formed on adding tungstate

to an ascorbic

acid solution adjusted to

pH 11 with sodium hydroxide. On reducing the pH of the blue solution to 3.5 by adding ascorbic acid the colour slowly

1144

POLAROGRAPHIC BEHAVIOUR

Vo]. 7, No. l l

disappears. The p o l a r o g r a m of this solution is shown in fig. 3e. The cathodic wave of this wave

starts around -0.4V,but

the height

is much smaller than in fig.3a.

z.,2 (1

d

3,5

<

2,8

._c 2,1 u~ 1,4

O,7 J

-Q3 -Q5

-q2

-Q2 -Q3

-Q6

-0,9

-1,2

Erie vs, SCE, in volts

FIG.4 Effect of indifferent electrolytes and camphor a)O. OO7M tungstate,O. IM sscorbic acid;b)same as in a)+O. IM caesium chloride;c)same as in a)+O.2M sodium chlorided ) s o l u t i o n in a) saturated with camphor. Fig. 4 shows the effect of the addition of indifferent electrolytes

as well as that of the surface active

camphor. Both caeasium chloride distort

and sodium chloride

agent

seriously

the wave.lt was found that the addition of p o t a s s i u m

sulphate

or dilute mineral

is the same as observed state-ascorbic of the wave

acids had the similar effect which

in figs.3b-3c. On saturating

the tung-

acid solution with camphor, only a small portion

is o b s e r v e d , w h i c h shows that the reduction

is

associated with adsorption. The height

of the tungstate wave

is low compared

to the reduction

the pentavalent

state in hydrochloric

coefficient

of 1.75% per degree

acid(1,2).A

diffusion

column

(i~ h O'5)

acid

tungsten to temperature

as well as constancy of

the ratio of current to the square root of the m e r c u r y

in ascorbic

of the hexavalent

of the height

show that the current

controlled. The low diffusion

current

is

Vol. 7, No. 11

P OL AR OGR AP H I C BEHAVIOUR

1145

observed can be attributed to the large size of the tungstate-ascorbic acid complex molecule,resulting in smaller diffusion. Eventhough the current increases with tungstate concentration this method cannot be recommended in tungsten analysis as the wave is distorted in presence of indifferent electrolytes. The blue solution formed on making the tungstate solution containing ascorbic acid alkaline,can be attributed to the "tungsten blue" which is supposed to contain hexa- and pentavalent tungsten species(8).On making this solution acidic with ascorbic acid,on polarographing the resulting solution the wavehelght is lower than the normal tungstate-ascorbic acid complex(compare figs.3a and 3e).This is expected as in the formation of the blue,part of the tungstate is reduced to the pentavalent state and the concentration of the hexavalent tungsten which undergoes reduction is smaller in this solution.

One of the authors(A.N)gratefully acknowledges the Alexander Von Humboldt Stiftung for the award of a fellowship. REFERENCES 1.J.J.Lingane and L.A.Small,J.Am. Chem. Soc.71,973(1949) 2.M.v. Stackelberg,P.Klinger,W.Koch and E.Karth, Techn. Mitt. Krupp,A.Forschungsberichte,2,59~1939) 3.H.A.Laitinen,E.B.Cldham and W.A. Ziegler, J.Am. Chem. Soc.75,3048(1953) 4.E.M. Issa,B.A.Abd-el-Nabey and A.M.Hindawey, Z.Anal. Chem. 224,2638(1967) 5.V.S.Milienko,S.V.Dragan and Z.S.Desanka,Glasnic Hem. Drustva, Beograd 29,121(1964) 6.A.I.Vogel,Quantitative Inorganic Analysis, p.567,Longmans,London,1962. 7.A.Narayanan and F.Umland,Anal. Chim. Acta,54,368(1971). 8.N.V.Sidgwick,The chemical elements and their compounds, p.1046,Clarendon Press,Oxford,1952.