Measurement of p-V-T Properties of Gases and Gas Mixtures at Low Pressure
Low pressure p-V-T measurements on gases have a long history and date back to the work of Boyle and Charles in the seventeenth and eighteenth centuries when the foundations of what we now know as the perfect gas laws were laid down. It was not, however, until the nineteenth century that extensive use began to be made of such measurements when the need for the gas thermometer became apparent and likewise the need for determining atomic weights (or more correctly, molecular weights from which atomic weights could be derived). Both of these needs have continued through into the twentieth century and the gas thermometer remains to this day as the fundamental standard of temperature. The situation with regard to atomic weights has, however, changed considerably over the last thirty years. Mass spectrometry has improved to such an extent that it is unlikely that gas volumetric methods will be used again, in the foreseeable future, for the determination of atomic weights. Thus, for example, the ratio of the atomic masses 12C to 160 has been determined mass spectrometrically to better than one part in 107. This high precision has shown also that polynuclidic elements (elements with more than one naturally occurring isotope) have an overall atomic weight which varies with the source of the material. Thus in the 1969 IUP AC atomic weights table, oxygen is given as 15.9994 ± 0.0003 where the error bounds reflect variability of source and not the experimental uncertainty in determining the atomic weight of a particular sample of the gas.
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