Freezing point depression in the ternary ZnSO 4 -H 2 SO 4 -H 2 O system Vielma T, Salminen J, Lassi U Abstract Freezing points in the ternary ZnSO 4 -H 2 SO 4 -H 2 O system were measured from 0.25 to 2.0 mol kg -1 ZnSO 4 and 0.25 to 2.0 mol kg -1 H 2 SO 4 . The experimental data was compared with predictions made with a multi-component Pitzer model and Zdanovskii's mixing rule. Good agreement between measured and predicted results was achieved. Introduction Freezing point depression is a common phenomenon both in nature and in industry, having also found a wide range of applications from determination of molar mass to anti-freeze and road salt. Measurement of freezing point depression of an aqueous solution is in essence studying the solubility of ice in the solution, and can be used to derive activity of solvent water at the measurement temperature. It is thus a practical tool for probing both solute and solvent properties at low temperatures, although with knowledge of heat of dilution of the solute, conversion to, say 298.15 K can be made. In this work, we have measured freezing point depressions of the ternary ZnSO 4 -H 2 SO 4 -H 2 O system. This is a direct continuation to our previous work [1] in studying physico-chemical properties of this industrially imporant system. Although freezing point depression in the binary subsystems has been studied [1-4], previously reported measurements for the ternary system are scarse. Agde and Schimmel [5] studied the three-phase equilibrium ZnSO 4 ·7 H 2 O(s) + ice + solution down to 263.15 K. Their results, however, differ greatly from the results of our thermodynamic model [1], which partly motivated this short study. In addition to information of the activity of water that can be used to test or fit a thermodynamic solution model, our results find a more direct practical application in the hydrometallurgical zinc processing. Harsh winters in the northern hemisphere, for example in the Nordic countries and Canada, can subject the zinc and sulfuric acid bearing process solutions to subzero temperatures. Our freezing point depression data can be directly used to estimate the risk of ice formation in outdoor storage vessels, pipelines and settling basins. Experimental H 2 SO 4 stock solution was prepared from concentrated H 2 SO 4 (≥95 %, Sigma-Aldrich). Concentration of the stock solutions was determined by BaSO 4 precipitation. Working solutions