Calorimetric Determination of Purity Design and Operation of a Small Adiabatic Calorimeter D. D. TUNNICLIFF and HENRY STONE Shell Development Co., Fmeryville, Calif.
for most substances directly proportional to the mole fraction of impurity and thus is an ideal basis for a method of determining purity. Rossini and coworkers ( 6 ) have developed a precise method of measuring melting points for this purpose. Unfortunately, the melting point of the sample is useful only as it can be compared to the melting point of the pure compound to give the melting point depression. In addition, the value of the cryoscopic constant or the heat of fusion is required for calculating the purity from the melting point depression. ;ilthough the method described by Rossini includes a procedure for extrapolating the observed data to obtain a value for the melting point for zero impurity and for obtaining the heat of fusion, this complicates the method considerably. Also methods based on a comparison of the melting point of the sample with the previously determined melting point of the pure compound have the disadvantage of requiring a high degree of absolute accuracy in the temperature measurement. The 40-ml. sample required for the Rossini method further limits its applicability
An adiabatic calorimeter has been developed for determination of the purity of the reference compounds required for spectroscopic investigations. Two interchangeable calorimeters with volumes of 0.5 and 5 ml. are used. A complete determination, including calculations, usually requires less than 4 hours. Although the purity determination is based on the measurement of the melting point depression, this method, unlike conventional melting point methods, does not require a previous determination of the heat of fusion and the melting point of the pure compound. Instead, the method determines these quantities as part of the analysis. The calorimeter is used to measure the equilibrium temperature of the sample as a function of heat input as it passes from the solid state to the liquid state. Analysis of the data gives the heat of fusion, the melting point of the sample, and the melting point of the pure compound. The error in the determination of the purity of samples with a purity above 99.8 mole % is less than 0.05 mole %. For less pure samples the error increases with the amount of impurity.
T
HE reliability of an observed physical property of a compound is frequently questionable because of the lack of any information regarding the purity of the particular sample used for the measurement. The present paper is concerned 1% ith the design and operation of an apparatus for determining the purity of small samples of reference compounds such as are required for spectroscopic and mass spectrometric methods. The purity of a sample may be used as a basis for deciding that the sample is adequately pure for some particular purpose, for applying a correction to the observed measurements for the effect of the impurities, or as a basis for planning further purification. Methods for determining the purity may be based on a direct determination of the percentage of either the major component or the sum of the impurities. For either approach the errors are apt to be roughly proportional to the magnitude of the quantity being measured. Consequently, in a direct determination of the major component, the error will tend to be independent of the purity and a very accurate method ail1 be required for proper evaluation of a pure sample. On the other hand, a direct determination of the sum of the impurities tends to give a very accurate measure of the purity of a pure sample and less accurate results for impure samples. The latter approach seems to be the most consistent with the purposes of a purity determination as stated, except, possibly, for applying a correction for the effects of the impurities. This case is of limited interest, as there is often considerable question as to the validity of the corrected value obtained from an impure sample regardless of the accuracy of the purity determination. If the errors are approximately proportional to the magnitude of the quantity being measured, then the error in the determination of the sum of the impurities can represent a relatively large fraction of the total impurity without significantly affecting the usefulness of the determination. Considerable advantage has been taken of this fact in designing the apparatus described in this paper. The melting point depression (the lowering of the melting point of a pure substance due to the presence of the impurity) is
20 % k + 7 h T k
0
Heat Input in C a l o r i e s
Figure 1.
Calculated Melting Curve
p-Xylene, 5 grams, 99 mole
yo pure
Nost of the disadvantages of the usual melting point method can be avoided by determining the equilibrium temperature of the sample in a calorimeter as a function of heat input as the sample is melted. Analysis of these data yields the desired melting point depression and the heat of fusion without reference to any other measurement. THEORY
A41thouglithe calculation of the purity of a sample from its melting curve hm been described in varying degrees of detail by a number of workers ( 2 ) 4,6-11), the principles of the method do not seem to be generally known; consequently, it seems pertinent to discuss the theory briefly.
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ANALYTICAL CHEMISTRY
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The usual simplified equatioii ( S ) relating the niole fraction of impurity in a sample to the melting point is as follows:
whew = mole fraction of inipurit!. AH, = molar heat of fusion of the nixjor c~ompoiirntin thr pure state a t T O To = melt,ing point of the purc cwnil)nuiitl, I
