353s
ROBERT L. GRAHAM AND LOREN G. HEPLER [CONTRIBUTION FROM COBB CHEMICAL LABORATORY, UNIVERSITY O F
Vol. 80 I'IRGINIA]
Thermochemistry of Sodium Tungstate, Silver Tungstate and Tungstic Acid BY ROBERT L. GRAHAM AND LORENG. HEPLER RECEIVED FEBRUARY 21, 1955
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The heat of solution of Naan'O4 has been measured and the standard heat of solution calculated to be 1.6 kcal./mole. Because this heat of solution is very different from t h a t calculated from previously reported heats of formation, we have measured the heat of reaction of crystalline tungstic acid ( H z W O ~with ) excess aqueous sodium hydroxide and have found the standard heat of this reaction t o be -13.7 kcal./moie HZWO~.T h e heats of solution of AgzWO4 and AgNOI in aqueous ATH, and the heat of solution of AgNOv in water have been measured and the standard heats found t o be -12.39, -8.12 and 6.42 kcal./mole, respectively. Our heats of reaction and solution have been used to calculate the heats of formation of WO,'(aq), hTalW04(c)and AgzWO&(c)t o be -266.6, -379.6 and -230.7 kcal./mole, respectively. The thermodynamics of solution of AgzWO4 is discussed in terms of polytungstate formation.
Our earlier work' on the thermochemistry of hfo04-(aq) and NazMoOl(c) showed that the heats of formation of these substances as tabulated by the Bureau of Standards2are in considerable error. These incorrect heats of formation are based on the results of Calorimetric work done by Pissarjewsky, Pechard4 and Mixter.6 Because the heats of formation given by the Bureau of StandardsZ for WOr-(aq) and NazW04(c) are based on similar calorimetric work of Pissarjewsky3 and Mixtere and because the heat of solution of sodium tungstate calculated from tabulated heats of formation2 (+ 14 kcal./mole) seemed unreasonable in comparison with the directly determined heat of solution of sodium molybdate' (-2.4 kcal./mole), we decided to determine this heat of solution directly and also to investigate the heats of formation of W04-(aq) and NazW04(c). Pan7 recently reported the results of his investigations of the silver-silver tungstate electrode. From the temperature coefficient of the standard potential of this electrode and the standard potential a t 25", Pan has calculated the free energy, heat and entropy of solution of silver tungstate. Pan's calculated heats of solution of silver chromate* and silver m'olybdateg are in excellent agreement with the calorimetrically determined heats reported by Smith, Pitzer and Latimerlo and Muldrow and Hepler." Nevertheless, we felt it desirable to investigate silver tungstate calorimetrically because of the great tendency of tungstate ions to form more complicated species. Experimental Our solution calorimeter has been described i? detail Some minor improvements in our operation of the calorimeter have been made and several determinations of the heat of solution of KCl in water have been carried out. Our heats of solution of KCl are in good agreement with the results of earlier investigators and will be reported in detail in a subsequent paper. All of our calorimetric measure(1) R. L. Graham and L. G. Hepler, THISJ O U R N A L , 7 8 , 4846 (1956). (2) "Selected Values of Chemical Thermodynamic Properties," Circular 500, National Bureau of Standards, 1952. (3) L, Pissarjewsky, 2. alzo~g.Chem., 24, 108 (1900). (1) E. Pechard, A n n . chim. $ l a y s . , 28, 537 (1893). (.5) W. G , Mixter, A m . J . Sri., 29, 488 (1910). (I?) W. G. Rlixter, i b i d . , 26, 125 ( I D O R ) . (7) K . Pan, J . Chiirrse r h e m . S o r . , 1, 26 (1954). ( 8 ) K. P a n , ibid., 1, 1 (1954). (9) K . P a n , i b i d . . 1, 16 (1954). (10) W. V. Smith, K. S. Pitzer a n d W. M .Latimer, TBISJ O U R N A I . , 69, 2642 (1937). (11) C. N. hluldrow and L. G. Hepler, i b i d . , 78, 5989 (1956). ( 1 2 ) C. X Muldrow and L. 0 . Hepler, i b i d . , 79, 4045 (1957).
ments have been carried out using 950 ml. of solution in the calorimeter and all of the calorimetric measurements reported in this paper were carried out at 25.0 f 0.2'. Materials .-The AgNO, was Mallinckrodt analytical reagent and was analyzed by titration with I
