A study was made of the calculation methods which give an accurate purity value when the purity is measured with a mixture which forms an eutectic mixture.It was revealed by a comparison of the purity values obtained by various measurement methods using DSC that those obtained by the stepwise method are more accurate than those obtained by other methods.Aiming to apply the advantages of the stepwise method to the dynamic method, the following improvements were made.The total heat of fusion was measured inclusive of the eutectic peak.To calculate the melt fraction, the total heat of fusion was regarded as extending up to the inflexion point at the end of the peak.Furthermore, the melt fraction was calculated assuming the heat up to each temperature to be the partial heat of fusion.A van't Hoff plot was assumed starting with a point 5% below the principal component peak following the end of the eutectic mixture's peak and ending at the inflexion point. An analysis using the improved method excludes any bias, and the obtained value was highly reliable.A 95mo1% pure sample gave a measurement with an error within 0.5mol% or less.Keywords Differential scanning calorimetry, purity measurement, eutectic impurity, eutectic mixture, total heat, fusionOrganic chemists are used to estimating the purity of chemicals by measuring the melting point. This method has been developed into a system where a DSC measurement and a melt-fraction calculation are used to estimate the concentration of impurities in the sample.Thermal methods are easy to use, can be performed quickly, and are effective in measuring unknown impurities. However, its use has been limited to a substance of rather high purity. The minimum purity required for this method is approximately 98mol%. For this reason, HPLC and GC are used for generalpurpose purity measurements.In the present work, the authors expanded the scope of applications of thermal measurements, proposing a practical method which gives satisfactory results for samples of greater impurity content than before. DSC measurements were made on the temperatureraising mode. Many organic compounds contain impurities having similar chemical structures which are not well separable from a synthetic mixture. These compounds and impurities of often combine to form an eutectic mixture which has a minimum melting point. Our previous study confirmed that in two-component systems which form an eutectic mixture, fusing starts at the eutectic temperature and the major component, which remains as a solid, gradually dissolves into the melt phase. Based on this observation, we have reported that the heat of fusion of the eutectic mixture has to be so that the purity can be calculated from the DSC' data.In this report, the method of purity calculation was where To is the melting point of pure substance (K), Tm, the melting point (K), R, the gas constant (8.314 J moL'), ~Hfus, the heat of fusion (J moL') and X2, mol fraction of impurity.The melting point (Tm) and heat of fusion (zHfus) are the observed values for a part...
