Potentiometric and Spectrophotometric Methods for Determination of Tungsten and Vanadium in Heteropoly Compounds Dorothy P. Smith and Michael T. Pope Department of Chemistry, Georgetown University, Washington, D. C. 20007
ALTHOUGH THE FORMATION of heteropoly compounds containing both vanadium and tungsten in the same anion appears to be well documented (I+, and although such species are probably involved in various analytical procedures (5,6), very little unequivocal evidence has been forthcoming to show that specific compounds can be isolated with integral numbers of vanadium and tungsten atoms. Reported analyses of crystalline tungstovanadosilicates and -phosphates often show nonintegral atomic ratios, e.g. Si: W: V = 1 :9.9:2.6,P: W: V = 1:9.5:2, instead of 1: 10: 2 (2, 7). Such figures have been ascribed to deficiencies in analytical procedures ( 2 , 7 ) rather than to impure compounds. In the course of an investigation of such “mixed” heteropoly tungstates, we have indeed found that the methods generally employed for analysis of tungsten, in the presence of both phosphorus and vanadium, were unsatisfactory. These methods were too insensitive (8), required unusual apparatus (9, IO), or were inconvenient amalgam reductions (2, 7, 11), some of which consistently gave low results. Potentiometric titration of tungsten(V1) with chromium (11) in 1 2 M hydrochloric acid at 90 “C has been reported (12). We find that, by using a wax-impregnated carbon indicator electrode (13), such a titration can be carried out at room temperature and vanadium can be used to determine tungsten, or tungsten in a heteropoly compound, without prior removal of phosphorus. In the case of a mixed heteropoly compound, vanadium can then be determined directly by the redox method of Rao and Rao (14) and tungsten found by difference. Alternatively, vanadium and tungsten may be determined spectrophotometrically as citrate complexes. Vanadium is determined directly, following reduction to the tetravalent state by excess citrate. Tungsten is determined on a separate sample, after removal of vanadium by a modification of the ion-exchange procedure described by Price and Maurer (15).
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(1) “Gmelins Handbuch der Anorganischen Chemie,” 8th ed., Verlag Chemie, Berlin, 1933, System Nr. 54 “Wolfram,” pp 390-2. (2) A. I. Kokorin and N. A. Polotebnova, J . Gen. Chem. USSR, 24, 1133, 1691 (1954). (3) A. V. Ablov and Ts.-B. Konumova-Frid,Rum. J. Inorg. Chem., 4, 263 (1959). (4) N. A. Po!otebnova, Russ. J. Inorg. Chem., 10, 815 (1965). (5) E. R. Wright and M. G. Mellon, IND. ENG.CHEM.,ANAL. ED., 9, 251 (1937). (6) D. K. Gullstrom and M. G. Mellon, ANAL.CHEM., 25, 1809 (1953). (7) A. I. Kokorin, J . Gen. Chem. USSR,24, 967 (1954). (8) A. S. Witwit, R. I. Magee, and C. L. Wilson, Talanta, 9, 495 (1962). (9) J. B. Headridge and E. J. Dixon, Analyst (London),87,32 (1962). (10) J. B. Headridge and M. S. Taylor, ibid., 88,590 (1963). (11) A. S. Witwit and R. I. Magee, Anal. Chim. Acta, 27, 366 (1962). (12) R. Flatt and F. Sommer, Helu. Chim. Acta, 27, 1518, 1522 (1944). (13) P. J. Elving and D. L. Smith, ANAL.CHEM., 32, 1849 (1960). (14) G. G. Rao and P. K. Rao, Talanfa,11, 603 (1964). CHEM., 35,595 (1963). (15) W. H. Price and R. H. Maurer, ANAL.
1906
ANALYTICAL CHEMISTRY
EXPERIMENTAL
Apparatus and Reagents. The potentiometric titrations were carried out in an atmosphere of nitrogen using 0.1-0.2M chromium(I1) sulfate, stored over amalgamated zinc in an automatic buret, The tip of the buret was inserted through the stopper of a four-necked flask, the other necks of which held the nitrogen inlet and outlet tubes, the salt-bridge, and the indicator electrode. The wax-impregnated carbon electrode was prepared as described by Elving and Smith (13) and mounted in a rubber stopper. It required merely rinsing with distilled water after each use. The spectrophotometric methods required the use of an ion-exchange column (22 X 1.5 cm) packed with Dowex 50W-X2in the hydrogen form. Absorbances of the citrate complexes were measured in 1-cm quartz cells on a Cary 14 spectrophotometer. Stock solutions of sodium metavanadate (Alfa Inorganics, Inc.), standardized by the method of Rao and Rao (14, and sodium tungstate, standardized by the cinchonine method (16), were used to test the procedures. A stock solution of A.R. citric acid, containing 0.50 g C s H ~ O 7 ~ 2 H ~ 0 per ml, needed for the spectrophotometric method, was freshly prepared each month. Procedures. (a) POTENTIOMETRIC TITRATION.The sample, which should contain 1-2 mg-atoms of tungsten(V1) vanadium(V), is dissolved in the minimum (10 ml) of water or dilute sodium hydroxide. If ammonium ions are present, they are removed at this stage by addition of 2 ml of 6M NaOH followed by digestion on a steam bath. To the resulting colorless solution is added 150 mi of 1 2 M HCl, which reduces vanadium(V) to a brown vanadium(1V) species. (Some sodium chloride may precipitate at this stage, but does not interfere.) The brown solution cannot be titrated reproducibly, even after thorough degassing, and must be digested on a steam bath to a clear light blue color (30-60 min). A further 150 ml of 1 2 M HCl is added, the solution de-chlorinated and deaerated with nitrogen (30 min), and titrated potentiometrically with standard chromium(I1) sulfate, using the wax-impregnated carbon electrode. A single end point (dark green solution) corresponds to the combined reduction of tungsten(V1) to tungsten(\’) and vanadium(1V) to vanadium(II1) and is reached in about 60 min. In the absence of vanadium, the digestion step is omitted and the titration is much more rapid (15 min). Blank titrations, on solutions carried through the entire procedure, must be carried out for each set of titrations and usually amount to 0.2ml of 0.1M CrS04. (b) SPECTROPHOTOMETRIC DETERMINATION OF VANADIUM IN PRESENCE OF TUNGSTEN AND PHOSPHORUS. A sample containing 4-5 mg of vanadium(V) is dissolved in 5 ml of water. After dissolution, 4 ml of 6 M NaOH is added and the mixture warmed until colorless. Exactly 2.00 ml of the citric acid solution is added, and the pH of the solution adjusted to between 4 and 5 with 1M H2S04. The solution is then covered and boiled gently for 10 min. The resulting deep
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(16) A. I. Vogel, “A Textbook of QuantitativeInorganic Analysis,” 2nd ed., Longmans, Green and Co., London, 1951, p 574.
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Table I. Determination of W and W V by Titration with CrS04 Tungsten alone, mg-atoms taken: 1 ,030 mg-atoms found: 1.031, 1.029, 1.029, 1.046 Tungsten vanadium, mg-atoms taken: 1.001 mg-atoms found: 1.006, 1 . OOO, 0.999
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Table 11. Comparison of Vanadium Determinations Sample Sample Method Stock soh, m M A,a % Ra Citrate complex 97.0, 97.2 7.67 3.86 ... 7.61, 7.76 3.88 Rao and Rao Titn. w. Cr(I1) 96.6, 97.7, 97.9 ... ... KMn04titn.* 97.7, 96.9 7.65, 7.73 3.84, 3.90
l z
0-
-200
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a Samples A and B were two heteropoly tungstovanadophosphates. * Titration performed at 80 “C on material eluted from column with excess 1M H2SOa, following Procedure (c) (Experimental Section).
-400-
1
-6000
0.2
0.4
06
08
1.0
1.2
Chromium (11). Meq.
Figure 1. Titration of vanadium(V) and/or tungsten(V1) in 12M HCl with 0.153N chromium(I1) using a wax-impregnated carbon electrode Room temperature. Solutions contained 0.97 meq V; 1.03 meq W ; 1.00meq V W.
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blue solution is cooled, the pH carefully adjusted to 7.0 i 0.1 with NaOH, and the solution quantitatively diluted to 50 ml with water. The absorbance of this solution is measured at 302 mp against a blank containing the same amount of citric acid. (c) SPECTROPHOTOMETRIC DETERMINATION OF TUNGSTEN IN PRESENCE OF VANADIUM AND PHOSPHORUS. The solution of the sample, containing 150-200 mg of tungsten(VI), is prepared as in (b). Following addition of the citric acid solution, the pH is adjusted to 3 with 1M H2S04and the solution covered and boiled gently for 10-15 min. The final volume of the deep blue solution should not exceed 25 ml. After cooling, the solution is transferred quantitatively to the ion-exchange column with the aid of 25 ml of water, acidified to pH 3 with sulfuric acid. The column is then washed successively with 50 ml of water, 2 ml of 1 M H2S04, and 75 ml of water. During this process the blue band of absorbed V 0 2 +moves about two thirds of the way down the column, while all phosphate, tungstate, and citric acid is removed. The combined eluate is diluted to 250 ml with water. To a 5-ml aliquot of this solution (3-4 mg W) is added 2.00 ml of citric acid solution, the pH adjusted to 2, and the solution diluted to 50 ml with water (see Discussion). The absorbance of this solution is measured within 30 min at 234 mp against a blank which had undergone the entire procedure. RESULTS AND DISCUSSION Potentiometric Titration. Figure 1 illustrates the sharp end points obtainable by this method. The titration of vanadium alone is shown for the purposes of comparison only, as other redox methods (14) are more convenient for this element. Because the vanadium(IV)/vanadium(III) couple is irreversible (17), the potential of a mixture of tungsten and (17) J. J . Lingane and J. H. Kennedy, Anal. Chim. Acta, 15, 465 (1956).
Table 111. Comparison of Tungsten Determinations in Tungstovanadophosphates Sample no. Citrate complex, Cr(I1) titration, % 1 62,29 61.96 2 58.99 58.57 3 58.43 58.43 4 58.43 58.46 5 53.84 53.73, 53.76
vanadium is determined by the tungsten(VI)/tungsten(V) couple, and separate end points are not observed. Some typical results of this method of analysis are given in Table I. The average deviation for a set of 6-7 ml titers was
