Estimatinig Impurities by Means of the Melting Point Curve

W. P. White
1919 The Journal of Physical Chemistry  
Fourteen years ago Hiittner and Tammannl mentioned that it was possible, from the behavior of the temperature during crystallization, to draw a conclusion as t o the amount of impurity. Other writers have since discussed the melting or freezing curve more a t length, but little practical use has yet been made of Hiittner and Tammann's suggestion. The present paper considers under what conditions this method is most likely to be useful, and also discusses the technic. Theoretical Basis The
more » ... ying laws are familiar but may deserve to be summarized. Impurities may form solid solutions, or only eutectics, and the crystallization phenomena have significant differences in the two cases. Where mix crystals only are formed, the temperatures and concentrations change as in Figs. I and 2. (More complicated freezing curves than these are known, but if the amount of impurity is small, so that the extreme right or left end only of the phase rule diagram is used, that end will rarely differ essentially from the end of I or 2 .) As the liquid mixture cools for either figure its concentration and temperature follow the line UV. At temperature V mix crystals of composition W begin to form. The relation of V to W depends on the form of the double pointed figure CD, which is a characteristic of the combination of substances in question. Since the composition W is different from the original melt, the separation of crystals W changes the composition of the liquid, which moves toward composition X as solidification proceeds. A t the same time, if complete equilibrium persists, the composition of the crystals, including those already deposited, changes along WY. K. Hiittner and G. Tammann: "Ueber die Schmelzpunkte und Unwandlungspunkte einiger Salze," 2. anorg. Chem., 43, 218 (1905).
doi:10.1021/j150203a005 fatcat:32ieclzw5vgaxm4cj3evjdryqm