Water and solute activities in the aqueous H 2 SO 4 -(NH 4 ) 2 SO 4 solutions

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I.

INTRODUCTION

THE aqueous

solution system, H 2 S O 4 - ( N H 4 ) 2 5 0 4 , is important in ammonium sulfate fertilizer production. Although studies of this solution system have important implications, no available data have been reported in the literature. Thermodynamic data of this solution system are necessary to understand fully the ammonium sulfate producing reaction. Among many thermodynamical factors, water and solute activities are the most basic parameters in characterizing the solution. In this study, the activities of water in the H 2 S O 4( N H a ) 2 S O 4 - H 2 0 solution system were determined by an isopiestic method at 298 K. The experimental results were compared to those calculated from the Zdanovskii rule t~l and the Robinson-Bower empirical equation.t2] The applicability of these calculation methods for estimating the water activities of this solution system was determined. In a previous study, the authors showed experimentally that a thermodynamic equation derived by McKay and Perring is useful in calculating the mean activity coefficients of solutes using water activity data. ]3J Therefore, the mean activity coefficients of solutes in the solution system, HzSO4-(NH4)2SOa-H20, were determined by the McKayPerring method r41 using the water activities determined in these solutions. This paper presents the activity data of water and solutes determined for the solution system, H 2 S O 4 - ( N H 4 ) 2 S O 4H20, at 298 K.

SUNG-KOOK PARK, Graduate Student, YASUHIRO AWAKURA, Lecturer, and HIROSHI MAJIMA, Professor, are with the Department of Metallurgy, Kyoto University, Sakyo-ku, Kyoto, Japan 606. Manuscript submitted March 7, 1988.

METALLURGICAL TRANSACTIONS B

II.

EXPERIMENTAL PROCEDURES AND CALCULATION METHODS

A. Determination of Water Activity by the Isopiestic Method The water activities in aqueous solutions of H 2 S O 4(NH4)2SO 4 w e r e determined by an isopiestic method since both sulfuric acid and ammonium sulfate are nonvolatile. The apparatus and the experimental procedures are the same as those reported previously, tSj The determinations of the water activity were made at 298 K • K. All chemicals were reagent grade. Solutions were prepared with deionized water (resistivity above 5 • 106 ohm cm) provided by an ion-exchange column. Molalities of the stock solutions of sulfuric acid and ammonium sulfate were determined by density measurements. Each test solution was prepared by mixing the deionized water with the stock solutions using a buret just before each experiment.

B. Determination of the Mean Activity Coefficient of H2S04 by Emf Measurement The electromotive force of a cell consisting of a Pt-Pt black electrode and a PbO2-PbSO4 electrode was measured at 298 K to determine the activity values of H 2 S O 4 in some typical solutions of H2SOa-(NH4)2SO 4. These studies were conducted to determine whether or not the calculation method proposed by McKay and Perring could be applied to the aqueous solution system under investigation. The mean activity coefficient of n 2 s o 4 w a s cal

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