Improving the accuracy of gas pycnometry measurements on small samples

Density is a fundamental structural property of matter, and gas pycnometry provides an advantageous means for measuring the density of solids, especially powders. A report is given of an investigation of the use of a commercial constant volume gas pycnometer, with emphasis on the difficulties associated with small samples. Measurements were made on various numbers of high purity aluminium pellets, and on a series of barium tellurite glasses, with compositions from 10 to 25 mol% BaO. As the sample gets smaller, there is an increase in the statistical error, but more importantly the systematic error due to inadequate calibration becomes larger. We advocate the approach where calibration is performed using several standards of varying volume, and the calibration measurements are performed using exactly the same measurement procedure as for the samples of interest. The measurements on the aluminium pellets were used, with reference to the crystallographic density of aluminium, to refine the calibration of the pycnometer. The refined calibration was used to correct the glass density measurements, with notable improvement in the behaviour of the results and their consistency with results from the literature.