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The determination of the purity of crystalline materials by measurement of melting point depression is a well established calorimetric technique. Purity analysis by both dynamic and isothermal step differential scanning calorimetry is considered. The limitations in applying the dilute solution van't Hoff theory to the results of dynamic heat flow measurements are discussed. The two widely used procedures for linearizing the sample temperature-reciprocal liquid fraction data relationship, (T − 1/F), namely, the trial and error, and the Sondack methods are compared. Analytical procedures for materials which decompose or exhibit phase changes during or subsequent to melting are discussed. The precision and accuracy of the dynamic method is demonstrated using data obtained with a three component model system, namely, a pure phenacetin host doped with known amounts of benzamide and p-aminobenzoic acid. The advantages and disadvantages of the equilibrium melting isothermal step method are presented. Providing due allowance is made for the sample heat capacity contribution to the step-wise fusion enthalpy, the T − 1/F relationship is linear over a wide range of 1/F-values, and impurity levels up to 10 mol % may be determined. Examples using the benzamide doped phenacetin system are presented to demonstrate this contention.
purity analysis, crystalline materials, fusion, differential scanning calorimetry, dynamic scan, isothermal step, van't Hoff, Sondack correction, phenacetin, benzamide, p, -aminobenzoic acid
Section head, Institute for Mining and Minerals Research, University of Kentucky, Lexington, KY