Calculation of the Correction to be Applied to Gas Isotherms Measured in the Presence of Mercury

A calculation is made of the correction for the volatility of mercury which should be applied to observed pressures of compressed gases, when mercury is used as the confining fluid. The usual correction is simply the subtraction of the normal vapor pressure of mercury corrected for the effect of hydrostatic pressure (the Poynting effect). Here we show that this is an inadequate treatment of the problem, as the mixture of mercury atoms and compressed gas cannot behave as an ideal mixture. An estimation of the intermolecular forces between mercury atoms and the added gas leads to values of the virial coefficients from which a revised correction can be computed, assuming that the system is at equilibrium. This revised correction can be considerably larger than the usual correction and is often of opposite sign.

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