SHORT COMMUNICATIONS 333

T&W Vol. 23, pp. 333-335. Pergamon Press, 1976. Printed m Great Britam

ANALYTICAL CHEMISTRY OF UNUSUAL OXIDATION STATES : THE POLAROGRAPHY OF VANADIUM(IV) IN

CYANIDE MEDIUM

J. HERNANDEZ MENDEZ, A. SANCHEZ MISIEGO and F. LUCENA CONDE

Department of Analytical Chemistry, Faculty of Science, Analytical Chemistry Section of the Consejo Superior de Investigaciones Cientifrcas (C.S.I.C.) Salamanca, Spain

(Received 28 September 1973. Revised 28 June 1975, Accepted 3 September 1975)

This paper, which reports the results of a continued investi- gation of the analytical chemistry of some unusual oxi- dation states,” is concerned with the polarography of vana- dium(IV) in cyanide medium. Irreversible oxidative and reductive waves have been reported for v~adi~(IV) in strongly alkaline medium.*-4 Lingane and Meites5 studied the polarography of several oxidation states of vanadium in a number of supporting electrolytes, and at different pH’s. Vanadium(II1) was found to give two rather poorly shaped waves with E, at -1.17 and - 1.77 V vs. SCE, being reduced to an unknown species of vanadium(H). There are no previous reports of the polarography of vana- dium(IV) in cyanide medium.

Apparatus

EXPERIMENTAL

A Sargent Model XVI polarograph was used together with a dropping mercury electrode (DME) and a saturated calomel electrode (SCE) in the m~surement cell. The characteristics of the capillary were: m = 2.83 mg/sec, r = 3.2 set, at 25” in IM potassium cyanide with ionic strength adjusted to 3.0 by addition of sodium sulphate, and with height of column = 58 cm. Nitrogen was bubbled first through a solution of vanadium(H) to remove’oxygen, then through a phosphate-buffered cyanide solution to saturate it with hydrogen cyanide in equilibrium with that pH, then through the solution under investigation.

Procedure

A-stock solution of vanadium(W) was prepared by dis- solvinn VOSO,. 5H,O in 009N suluhuric acid. A suitable volume of this.wastaken, potassium cyanide was added, first to neutralize the acidity and then to give the desired cyanide con~ntration for the experiment. Sodium sulphate was added to adjust the ionic strength to 3.0 (or other value as required) and a drop of 1% gelatin solution to suppress a small maximum. Nitrogen was bubbled through the solution, and after 1520min the polarogram was recorded, with the polarograph and a potentiometer.

RESULTS

Effect qf the vanadium concentration

Over the range 8 x lo-‘-1.5 x 10-‘M for vanadium, the wave-height is strictly proportional to concentration, with a slope of 7.36pA.l.mmole-’ (standard deviation 4%). The temperature coefficient is about 1YJdeg. The pro- portionality to the square-root of the height of the mercury column confirmed that the electrode process was diffusion- controlled.

These results and their reproducibility, together with the good development of the waves and their broad diffusion

range, allow us to propose a new polarographic method for the determination of vanadium, the recommended pro- cedure being that used in this paper. The precision and accuracy are within the usual limits for classical polaro- graphy, and minimal for 10-5-10-ZM vigil in >0.5M cyanide medium at lO-SO”, the ~larography being performed within an hour of the addition of cyanide. It is necessary to work at constant ionic strength.

An experiment involving couiometric reduction of the vanadium with repeated scanning of the polarograms between electrolyses revealed that two electrons were in- volved in the reduction. From this figure, and the capillary parameters already mentioned, the diffusion constant D in the IlkoviS: equation was calculated to be 6-2 x 1O-6 cn?/sec.

EfSect of the cyanide concentration

A 2 x lo- ‘M vaMdium(IV) solution gave a wave-height increasing steadily from 14 nA in 051%f cyanide to 15 ,uA in -1.5M cyanide, thereafter remaining constant (Table 1). In 0.2M cyanide the polarogram was ill-defined, and in 0.4&f the wave-height was 11 PA. As the pH is quite high (l&11) it is likely that the vanadium(W) is partially oxi- dized before the nitrogen is passed, when there is not much cyanide present, and in some of these solutions a turbidity suggested that the vanadium was precipitating.

The half-wave potential

Logarithmic plots of log i&-i) against potential gave straight lines with slopes of about 9OmV, and Et values varying from - 1.15 V in 05M cyanide to - l.iOV in 2.86&f cyanide. Experimental values are tabulated in Table 2. The slopes suggested that the reduction was irrc$crsible. Mats&a and Ayabe have deduced a general expression for the current-voltage curve for all reactions, reversible and irreversible,6 and have shown how it can be used for the analysis of polarograms.’ Over most of the wave, their expression simplifies to

E = (Et k,, - 2.3 :;$. log & (1) I

The factor G( is termed the transfer coefficient for an irreversible electrode reaction. The variation of (E& with ligand concentration is given by

‘(E+k,, = 2.3 ES (kY)f, log D + 3 log t - 0.053 N

- 10 PO ; - w - P)log~xCx) I Qf

where ($) is the rate constant for the slow reaction at the electrode, N is the number of ligands bound to the

334 SHORT COMMUNlCATlONS

1 Table 2. Kinetic characteristics of the polarographic reduction of V(W) in cyanide medium

cc;-1, -Wi,, 1% L mV a: W - PI*

9

;

-c

0.50 - 0.228 1149 0.31 0.75 - 0.228 1140 0.31 1.00 - 0.228 1135 0.30 1.25 - 0.228 1132 031 1.50 - 0.228 1130 0.29 1.75 - 0.228 1121 0.29 2.00 - 0.228 1115 0.31 2.50 - 0.228 1106 0.31 2.86 - 0.228 1100 0.31

* Arithmetical mean of three measurements

1

regarding the species involved in the reactions, though the figures do suggest that one cyanide ion is added to the complex species when reduction takes place in about 2M potassium cyanide solution.

1 , 1 I I

-1ao -1-20 -1.40 -160 -1.80

E, v

The rate constant @ for the reduction step can be found from equation (2) and takes the value lo- “‘2 cm/set for 2M cyanide solution.

Fig. 1. Polarograms of V(‘“) in cyanide l.OM temperature 25°C gelatine 0.05x, Vu”’ concentration: (5)4 x 10-3M; (6t2 x 10-3M; (7)-1.5 x 10-3M; (BtlO-‘M;

(9)--0.8 x 10-3M.

central ion in the species predominating in the solution and p the number in the species undergoing reduction after the dissociation step,f, and D, are the activity coefficient and diffusion coefficient respectively of the species MXN andfx and Cx are the activity coefficient and bulk concen- tration of the ligand X. The drop-time is denoted by t. As the limiting current is strictly diffusion-controlled in this case the term &/id becomes unity and its logarithm zero. From equation (I), assuming n = 2, we can solve to find a, which in this case is around 0.3 (see Table 2).

By combining equations (1) and (2) we get the simple relationship

Values for (N - p) obtained by using this equation are also presented in Table 2. The considerable variation in these values makes it hard to come to any conclusion

Table 1. Influence of CN- concentration on I,

CKCNI, 1, M PA

0.20 5.8’ 040 11.0 0.50 14.1 0.75 14.0 1.00 14.6 1.25 14.4 1.50 14.9 1.75 15.2 203 15.0 2.50 151 2.86 150

- 0.48 -0.28 - 0.23 -0.52 -0.85 - 1.33 -086 - I.07

Two simple experiments were carried out to determine the formal potential (E”)s for the V(IV)/v(II) system in cyanide solution. In the first, the potential was measured after mixing equal volumes of equimolar vanadium(H) and vanadium(IV) solutions in the presence of 2M cyanide, and in the second, vanadium(I1) in 2M cyanide solution was titrated potentiometrically with hexacyanoferrate(II1) solu- tion. In both experiments, the formal potential was found to be -0.70 V us. SCE. The difference between this and the (E&,, of - 1.1 V certainly indicates the highly irrevers- ible nature of this reduction.

The rate constant (kO)s of the overall process is given by

W& =&%ev[-$Wh] and has the value lo-“*’ cmjsec, positive proof that the reduction is irreversible.

The literature on cyanide complexes of vanadium is not very extensive, and what there is contains many contradic- tions. Virtually all the facts relate to solid species, usually produced by precipitation from non-aqueous media, and statements concerning ionic species are largely hypothesis. So much faith is placed in spectroscopic observation and prediction, in spite of lack of confirmation by analytical datas that it has taken X-ray structure analysis in one case’ to prove that the complex cyanide of vanadium(III) is in fact V(CN):-. Solid potassium hexacyanovanadate(II) K,V(CN)6 is well characterized” but reacts with water to give a brown precipitate. The heat of formation of a soluble vanadium(H) cyanide complex has been reported” but without any proof of the constitution of the species.

Rivenq found that a solution of vanadium(W) titrated with cyanide gave a precipitate, with a cyanide to vana- dium ratio of 2:1, which subsequently dissolved in an excess of cyanide. I2 He suggested the formula VO(CN)z- for this species, but this does not lie up with the formula Cs,VO(CN), reported for a blue crystalline salt described by Bennett and Nicholls.i3

If the N - p value of - 1 means anything, it would fit the requirements for the reaction

VO(CN):- + 2e + H+ + V(CN)z- + OH

between hypothetical ions predicted on the basis of two known solid compounds. A reduction of an oxo-species * Ill defined polarogram.

SHORT COMMUNICATIONS 335

to a simple metal complex would be expected to be irre- versible. But there is nothing to suggest why reduction should be to vanadium(H) instead of to vanadium(II1).

6.

7. 8.

REFERENCES 9. 1. F. Lucena, J. H. Mendez, A. S. Misiego and J. A.

Marcos, InJ Quim. Anal., 1973, 27, 164. 10. 2. W. R. Crowell and H. L. Baumbach, J. Am. Chem. 11.

Sot., 1935, 57, 2607. 3. W. R. Crowell and D. M. Yost, ibid., 1928, 50, 374. 12. 4. J. J. Lingane, ibid., 1945, 67, 182. 13. 5. J. J. Lingane and L. Meites, ibid., 1951, 73, 2165.

H. Matsuda and Y. Ayabe, Bull. Chem. Sot. Japan, 1956, 29, 134. ldem, Z. Elektrochem., 1959, 63, 1164. J. J. Alexander and H. B. Gray, J. Am. Chem. Sot., 1968, 90, 4260. R. L. R. Towns and R. A. Leveson, ibid., 1972, 94, 4345. R. Nast and D. Rehder, Chem. Ber., 1971, 104, 1709. F. H. Guzzetta and W. B. Hadley, Inorg. Chem., 1964, 3, 259. F. Rivenq, Bull. Chem. Sot. France, 1947, 14, 971. B. G. Bennett and D. Nicholls, J. Chem. Sot. (A), 1971, 1204.

Summary-The polarographic reduction wave for vanadium(IV) in cyanide medium has been studied and the effect of cyanide concentration on the diffusion current and half-wave potential has been interpreted, using the theory of irreversible but diffusioncontrolled reduction. Coulometric experiments suggest a two-electron transfer, and the shift of half-wave potential corresponds to the addition of one cyanide ion during the reduction step. The proportionality of wave-height to vanadium concen- tration in the used alkaline cyanide medium indicates a possible analytical unity. The half-wave poten- tial is about - 1.1 V.

Talonta. Vol 23, pp. 335-336. Pergamon Press, 1976. Prmted m Great Braam

A REFINED CHEMICAL ANALYSIS OF SnF, . AsF,

B. SEDEJ

J. Stefan Institute. University of Ljubljana, Ljubljana, Yugoslavia

(Received 20 August 1975. Accepted 12 September 1975)

Studies of the systems MF,-AsF,-HF. where M is any bivalent metal, carried out in this laboratory, have resulted in the isolation of a number of compounds of the type MF, AsF,. Compounds of this particular composition might be discussed either in terms of ionic formulations such as M2+F-AsF;, MF+AsF; or in terms of covalent, fluorine-bridged structures FMF. AsF,. The true nature of the compounds can only ultimately be determined by a cl ! \tal structure investigation but so far, it has not been possible to isolate crystals suitable for crystallographic work.

However, by chemical analysis simple direct evidence for the presence of free fluoride on the one hand and hexa- fluoroarsenate on the other can be obtained, since both can be determined separately. Known methods of analysis are not directly applicable to this problem and we describe here the modifications made to the methods for chemical analysis of SnF, AsF,, which was taken as the first test compound since on the basis of its vibrational spectra it was formulated as a salt of the fluorine-bridged polyca- tion (Sn-F):+ with the AsF; anion,’ and more recently as [Sn-F]+[AsF,]-.’

EXPERIMENTAL

Reagents and apparatus

Tetraphenylarsonium chloride solution, 6.7% w/v. Cupferron solution, 5%. Sodium hydroxide solution. Ammonia solution.

Sulphuric acid, 75% v/v. Thorium nitrate standard solution, 0.0125M. Methylthymol Blue, 0.2% solution in 75% methanol. TISAB buffer. CBS buffer. All reagents and standard solutions were of analytical-

reagent grade. The fluoride content was also determined with a fluoride ion-selective electrode (Orion No. 94-09) in conjunction with a reference electrode (Orion No. 9@01) and digital pH-meter (Orion Model 801). The ana- lytical methods employed were all checked against official standards.

Procedures

SnF, AsF, is a very sensitive material and rapidly hyd- rolyses in moist air. Samples were therefore weighed into air-tight Teflon containers in a dry-box. Before the hy- drolysis in alkaline solution these were cooled to liquid- nitrogen temperature in order to moderate the violent reaction with water.

Total fluorine in the solution of the hydrolysed sample was determined by a modified distillation method.3-5 The sample was decomposed by 75% sulphuric acid in the pres- ence of silica sand within 6 hr in a distillation apparatus which permitted recycling of water vapour. The distillate containing sodium hexafluorosilicate was titrated with 0.0125M thorium nitrate in the presence of Methylthymol Blue indicator. The results obtained in several parallel determinations were within the permitted experimental error (see Table 1).