Resumen de Swift Measurement of Densities of Low Melting Point Compounds
Recently I needed volume fractions of organic compounds dissolved in mixed solvents, to carry out (spectral) absorbance subtractions at room temperature. In the approximation of linear additivity of absorbance spectra (1, 2), such volume fractions should be evaluated from the partial molar volume in solution. The partial molar volume of a chemical species at a fixed temperature depends on the nature of the species or that of the other components, and on the composition of the solution. Thus it is rarely available. An accessible approximation is the molar volume, the ratio of the molar weight and density. For 2,4,6-trimethylphenol (I, mp 71-72 °C) and 2,6-di-tert-butylphenol (II, mp 35-38 °C) I was unable to find the densities in the literature in a reasonable time. An estimate was obtained following the indications given by G. S. Girolami (3), which gave 1.02 and 0.95 g cm-3, respectively, for I and II.
But a more appealing number would have come from an even approximate experimental determination. Such a measurement was performed by weighing a precision 100-mL gas-tight syringe (Hamilton, which by mercury filling was found to stay within the stated 1% accuracy). It was weighed first fully filled with the liquid or molten compound, and then with the piston tip on zero - that is, with only the needle filled. The maximum temperature allowed for such syringes, 50 °C, practically coincides with the highest melting point of compounds for which the method presented is applicable conveniently. Thus I found 1.01 g cm-3 for I (with some difficulty to keep the temperature everywhere higher than 75 °C), and 0.92 g cm-3 for II.
For compounds that are solid at room temperature, the above procedure gives the density of the liquid near its melting point (similar to that quoted in the Aldrich catalog), and not that of the phase stable at room temperature. But since both dissolution and fusion substitute the long-range ordered structure of the crystalline solids with the substantially random structure in the liquid, it appears that the molar volume of the pure liquid is a better approximation.
Literature Cited 1. DiFoggio, R. Appl. Spectrosc. 1995, 49, 67. Lunelli, B. Rev. Sci. Instr. 1990, 61, 2696.
2. Rodger, P. M. Mol. Phys. 1996, 89, 1157. Jiao, H.; von Rague Schleier, P. J. Am. Chem. Soc. 1994, 116, 7429.
3. Girolami, G. S. J. Chem. Educ. 1994, 71, 962.
