CHANGE OF SOLUBILITY OF POTASSIUM-SULFATE IN WATER CAUSED BY TRACES OF CHROMIUM(III)

The solubility of potassium sulfate in water is shown to be decreased by a trace (up to 150 ppm) of chromium(III) added as green sulfate (Cr//2(SO//4)//3 multiplied by (times) 4H//2O). The proposed mechanism for the impurity action is that an adsorbed layer of chromium(III) on the surface of the dissolving potassium sulfate crystals stops their dissolution and prevents the solution concentration from reaching the true equilibrium value. The measured solubility in the presence of chromium(III), therefore, is only an 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(III) and dissolution of potassium sulfate crystals. This mechanism offers a reasonable explanation of the experimental results on the 'solubility'. (Edited author abstact)