I n d . Eng. Chem. Res. 1988,27,930-934
930
Change of Solubility of Potassium Sulfate in Water Caused by Traces of Chromium(II1) Noriaki Kubota,* Isao Uchiyama, Kyoichi Nakai, and Kenji Shimizu Department of Applied Chemistry, Iwate University, 4-3-5 Ueda, Morioka 020, Japan
John W. Mullin Department of Chemical and Biochemical Engineering, University College, London, Torrington Place, London WClE 7JE, U.K.
T h e solubility of potassium sulfate in water is shown t o be decreased by a trace (up t o 150 ppm) of chromium(II1) added as green sulfate (Cr2(S04)3.4H20).The proposed mechanism for the impurity action is that a n adsorbed layer of chromium(II1) on the surface of the dissolving potassium sulfate crystals stops their dissolution and prevents the solution concentration from reaching the true equilibrium value. T h e measured solubility in the presence of chromium(III), therefore, is only a n apparent solubility and is always lower than the true equilibrium solubility. Its actual value is probably determined by two competing rate processes: adsorption of chromium(II1) and dissolution of potassium sulfate crystals. This mechanism offers a reasonable explanation of the experimental results on the "solubility". Chromium(II1) is suggested t o act in solution as a kind of molecular complex compound. 1. Introduction Crystal growth and nucleation kinetics are frequently changed by traces of impurities or additives (Mullin, 1972a). Since these kinetics are essentially surface- or structure-sensitive phenomena, it is understandable that the impurities can be active in very small amounts (say up to 100 ppm). On the other hand, the solubility of a moderately soluble substance such as potassium sulfate in water is hardly expected to be changed by traces of impurities, since solubility, a phase equilibrium condition, is a typical structure-insensitive property. However, if the impurity concentration is high enough to change the common ion concentration or the ion activities, the solubility could be changed by the common ion effect, salting-in or salting-out effects (Mullin, 197213). Halstead (1970) reported that the solubility of potaasium sulfate in water was decreased by adding small quantities of chromium(III), iron(II), and iron(II1) as sulfates, and he simply assumed that this was due to the common ion (S042-) effect. In the present study, however, chromium(II1) is shown to be active in decreasing the potassium sulfate solubility in water even a t concentrations lower than 100 ppm. Another mechanism of the impurity action must therefore be sought. The mechanism proposed here is that the decrease in solubility is caused by an adsorbed layer of chromium(II1) being established on the crystal surface, thus inhibiting the dissolution process of potassium sulfate crystals. In other words, the solubility measured is an apparent solubility, or a pseudosolubility, which is determined by two competing rate processes, i.e., chromium(II1) adsorption and potassium sulfate crystal dissolution. This proposed mechanism is not contradictory with the retarding action of chromium(II1) on the growth or crystallization of potassium sulfate (Halstead, 1970; Yamamoto, 1939). 2. Experimental Section Three kinds of measurements have been made to study the chromium(II1) effect on the solubility of potassium sulfate in water: (1)conventional solubility measurements, (2) determination of adsorption isotherms of chromium(111)on potassium sulfate crystals, and (3) dissolution and growth rate measurements of single crystals of potassium sulfate. 0888-5885/88/2627-0930$01.50/0
In all the experiments, chromium(II1) was added as green sulfate (Cr2(S04)3.4H20). 2.1. Materials. Potassium sulfate and green sulfate were of extra pure reagent grade (Kanto Chemical Co. Inc., Japan), which were used without any further purification. According to the manufacturer data, possible impurities in the potassium sulfate are acid (C0.006 mass % as H2S04),alkali (C0.006 mass % as KOH), chloride (C0.002 mass TO),heavy metals (C0.0006 mass % as Pb), iron (qO.001 mass %), sodium (
