DIFFERENTIAL SCANNING CALORIMETRY AS A GENERAL METHOD FOR DETERMINING PURITY AND HEAT OF FUSION OF HIGH-PURITY ORGANIC CHEMICALS . APPLICATION TO 95 COMPOUNDS

A method for determining the purity of organic compounds by differential scanning calorimetry is presented. The heat of fusion (ΔHf) and temperature during melting are measured and a curve is plotted, the slope of which is the melting point depression (ΔT) due to impurities in the sample, and whose zero intercept represents the melting point of a theoretical sample with zero impurity (T0). The total purity is computed by inserting these values into the equation: mol % impurity = 100 × (ΔHf/RT02) × ΔT. Organophosphates, amides, ureas, carbamates, heterocyclics, chlorophenoxy acids, esters, and halogenated compounds were analyzed. Because the concentration of the contaminants, rather than the pure substance, is determined, the effect of experimental error on the resultant purity value is minimized. The method is accurate only for samples over 98% pure, and does not measure impurities which are soluble in the solid phase or insoluble in the melt. It is also inapplicable to chemicals which decompose at their melting points or have inordinately high vapor pressures. © 1969, American Chemical Society. All rights reserved.