The determination of atomic weights by means of the microbalance, and the values obtained for carbon, nitrogen, and fluorine

Abstract

The chemical determination of atomic weights possesses one advantage over physical methods in that it gives the average atomic weight of an element as found naturally, whether that element be a simple one or a mixture of a number of isotopes. Thus, whilst the mass spectrograph method is capable of determining the mass of any isotope to a high degree of accuracy, the evaluation of the abundance ratios of each isotope present in a natural element is often a matter of considerable difficulty. The method which we have used, namely, the microbalance method of limiting pressures, is not new, nor does it involve any new considerations or properties of the gas in question. The fundamental principle involved in the method is a variation of that originally put forward by BERTHELOT in 1898, and known as the method of limiting densities. BERTHELOT showed that gases only obey AVOGADRO’s Hypothesis at zero pressure, and consequently the extrapolation of the ratio of the densities of two gases, measured at a number of pressures down to zero pressure, would lead to the accurate ratio of their molecular weights. Hence, if the molecular weight of one of the gases was known, that of the other could be calculated with ease. In the microbalance method, the pressures of two gases are measured at which they have equal densities as shown by the deflexion of the balance. The ratio of the pressures so obtained would be equal to the inverse ratio of the molecular weights, if both the gases were perfect. In practice, however, owing to the deviations of gases from BOYLE’s LAW, this is only true at zero pressure, and consequently a number of ratios are taken, corresponding to different densities. These, when plotted against the measured pressures of one of the gases, enable the extrapolation to zero to be carried out.