Kiyama, Masao

Takada, Toshio

Torii, Hideo

Kyoto University Research Information Repository

Title Hydrolysis Products of TungstatesAuthor(s)Torii, Hideo; Kiyama, Masao; Takada, ToshioCitationBulletin of the Institute for Chemical Research, KyotoUniversity (1978), 56(5): 251-254Issue Date1978-12-20URL http://hdl.handle.net/2433/76793RightType Departmental Bulletin PaperTextversionpublisherKyoto UniversityBull. Inst. Chem. Res., Kyoto Univ., Vol. 56, No. 5, 1978            Hydrolysis Products of Tungstates              Hideo  TORII, Masao KIYAMA, and Toshio TAKADA*                              Received July 31, 1978      Aqueous suspensions, containing 0.2 M ammonium or sodium tungstate and different amounts of  one of HC1, HNO3, or H2SO4, were subjected to aging at room temperature. The resulting products  were examined by means of X-ray powder diffraction. The formation of 5(NH4)2O.12WO3.11H20  and hydrates of tungstic oxide takes place, depending mainly on the pH. The presence of sulfate  was found to hinder the crystal growth of hydrates of tungstic oxide except for that of H2W04•H20.                            INTRODUCTION    When alkaline solutions of normal tungstates (R2W04), such as Na2WO4 or (NH4)2W04, are made neutral or acidic, the tungsten (VI) ions are hydrolyzed to form precipitates. It is known that hydrates of paratungstates (3R20.7W03 or 5R20. 12W03),1-31 metatungstates (R20.4W03)3) and tungstic oxide (W03) 4) are obtained by selecting the pH, temperature and W(VI) ion concentration.    When aqueous suspensions, containing different amounts of HCl, HNO3 or H2SO4 and either 0.2 M ammonium or sodium tungstate, were subjected to aging at room temperature, various precipitates, such : as 5 (NH4) 20.12W03.11 H2O, and different hydrates of oxide, were formed as a crystalline one. The chemical compositions and crystal structures of the aging products formed at various pH values will be reported.                           EXPERIMENTAL    Two stock solutions were prepared by dissolving 1.0 mol of H2W04 in 1.0 l of 0.5 M NH4OH or 1.0 mol of Na2WO4 (each analytical grade) in 1 l water. To 20.0 ml portions of each solution, one of 4 M HC1, 4 M HNO3, or 2 M H2SO4 was added in different quantities. The total volume of each suspension was made 100.0 ml with water in a polyethylene bottle (150 ml). A number of such suspensions, each contain-ing 0.2 M W(VI) ions, were subjected to aging at 25°C for 50 h. The details of the procedure of aging were previously described.5)    After the pH of suspensions had been determined with a Horiba-Hitachi electrode pH meter, the aging products were separated from the solution by filtration, washed with water and then dried at room temperature on absorbent paper.    The samples thus obtained were examined by means of X-ray powder diffraction using CuKa or FeKa radiation. * ,f,,4 , * tt : Laboratory of Solid State Chemistry, Institute for Chemical Research,   Kyoto University, Uji, Kyoto 611. (251 )                                 H.  TORII, M. KIYAMA, and T. TAKADA                             RESULTS AND DISCUSSION         When an aqueous solution of HNO3 was added to a solution of ammonium tung-     state containing excess NH4OH, a whitish precipitate was formed at pH 8, the pH of      the suspension being decreased as shown in Fig. 1. The titration curve was obtained      by slowly adding 4 M HNO3 to 200 ml aqueous solution containing 100.0 ml of the      stock solution at 25°C with stirring. The pH decreases suddenly with addition of 65-     75 ml of HNO3 to a point N of discontinuity. As the pH decreased with addition of HNO3, the dissolution of most precipitate occurred in the 4>pH>2 range, and at pH      2 a precipitate again began to be formed.         The point N is considered to be the point of neutralization of NH4OH, which has      been present in excess amount, with 4 M HNO3. The stock solution is estimated from      Fig. 1 to contain 1.4 M excess NH4OH. The pH value of the point N slightly increased      with the time palse as a result of the dissolution of a slight amount of the precipitate.         When the suspensions, containing 0.2 M W(VI) ions and different amounts of      acids, were subjected to aging, the formation of a precipitate took place in the      suspensions with 4<pH<8 containing NH4+ ions. As the pH of the suspension was      decreased with the increase in the amount of acid, the amount of products increased.      However, it began to decrease at pH 6, becoming negligible at pH 4. Optical—      microscopic and X-ray diffraction examinations showed that the aging products,      obtained in the 8>pH>4 range, consist of needle-like particles (5-10 ,am in length)      with the same crystal structure as that of 5(NH4)2.12W03.11H20,6) irrespective of the      kind of the acid used. In the present experiments, sodium paratungstates were not      obtained because of the small W(VI) ion concentration(=0.2 M).          The formation of whitish or yellowish precipitates took place in the suspensions      with pH<2, containing excess HC1 and ammonium or sodium tungstate. Increasing      the concentration of excess HC1 accelerated the formation of the yellowish precipitate      and increased its yield.  14--------------------------------------------------------------------------------------- 12 - 10 - 8-  _ -/N 6- 4- 2- 0 20 4060 80 100                                    4M-HNO3 (ml)                Fig. 1. Titration curve of 0.1 M ammonium tungstate with 4 M HNO3 at 25°C. (252)                           Hydrolysis Products of Tungstates    The aging products were found by X-ray diffraction examination to consist of one of phase B (Fig.  2(b)) or D(e), and mixtures of phases A, B, C, and D. The pH value required for the formation of phases A, B, C, and D decreases in this order as seen in Fig. 2. In the presence of NH4+ ions, an amorphous precipitate was formed besides phases B, C, and D in the pH range for the formation of phase A described above. X- ray diffraction data of phase A in the mixture (Fig. 2 (a)) are similar to those of Na20-(W03.1/2H20)zo-4o,7) whereas X-ray diffraction data of phases C and D were identi-cal with those of H2W048) and H2W04•H20,9) respectively. The product of phase A is estimated to be H2W207 (or WO3.1/2H20) having a cubic crystal structure with ac, 6.41 A as shown below (Fig. 3) and the product of phase B might be W0311H2O (0.5<n<1).    When H2SO4 was used in place of HC1 or HNO3, the precipitate of H2W04•H20 was formed in the strongly acidic suspensions containing NH4+ ions, whereas the pre-cipitates, formed in the pH range for the formation of phases A, B, and C, were amor-phous-like.    In order to study the effect of aging temperature on the products, similar experi-ments were carried out at 50 and 70°C with suspensions containing ammonium tungstate and different amounts of an acid-HNO3, HC1, or H2SO4. In the 8>pH>4 range, 5(NH4)20.12W03.11H20 and 5(NH4)2.12W03.5H2010) were formed at 50 and 70°C,respectively, irrespective of the kind of the acid used. In the pH<0.05 range, H2W04 was formed at 50 and 70°C. In the intervening pH range, the products of phase A (Fig. 3) were obtained at 50 and 70°C in the presence of SO42- ions, whereas A(d)  yc (a)     (b)    kopiohowidi \41)i*ilisA14410404404$0440 (e)   C~      (c) C 515 25 35 45 55 5 15 25 35 45 55 26 (CuKa)26 (CuKa)            Fig. 2. X-ray diffraction patterns of the aging products obtained at pH                  1.43 (a), 1.30 (b), 0.29 (c), 0.16 (d), and 0.05 (e) in the absence                   of NH4+ ions. Arrows in (a), (c), and (d) indicate the main peaks                    caused by the presence of each phase. (253 )                            H.  'TORII, M. KIYAMA, and T. TAKADA                    (200)                (111)       (100) I1I   10203040 50 2 6 (Cu Ka)          Fig. 3. X-ray diffraction pattern of the product of phase A obtained at 50°C                 in the suspension with pH 2 containing SO42+ ions. in the presence of Cl- or NO3- ions the products of phase B were obtained in spite of the rise of aging temperature.                        ACKNOWLEDGMENT    The authors are indebted to Mr. T. Kusaka for his help in some experiments.                            REFERENCES (1) T. M. Taylor, J. Amer. Chem. Soc., 24, 629 (1902). (2) K. Saddington and R. W. Cahn, J. Chem. Soc., 1950, 3526. (3) 0. Glemser, W. Holznagel, W. Holtje, and E. Schwarzmann, Z. Naturforschg., 20b, 725 (1965). (4) A. M. Morley, J. Chem. Soc., 1930, 1987. (5) T. Takada, M. Kiyama, H. Torii, T. Asai, M. Takano, and N. Nakanishi, Bull. Inst. Chem. Res.,       Kyoto Univ., 56, 18 (1978). (6) X-ray powder data, Card 18-127, A.S.T.M. (7) Ibid. Card 17-616. (8) Ibid. Card 6-0242. (9) Ibid. Card 16-166.   (10) Ibid. Card 18-128. (254 )

Hydrolysis Products of Tungstates

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Hydrolysis Products of Tungstates

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