Experimental Methods for Determining Virial Coefficients

A wide variety of experimental methods have been used to determine the virial coefficients of pure substances, and they are discussed in detail in previous reviews. These techniques can also be applied to mixtures, and interaction virial coefficients can be obtained from equations (4) and (5) if the virial coefficients of the pure components are known. To determine a second virial coefficient to 1 cm mol from volumetric studies at room temperature requires that measurements be made to a precision of a few parts in 10 . Under ideal conditions, this standard of precision on B(x), B, and B22 leads to an uncertainty more than twice as large in B12. 

High precision is especially hard to obtain in measurements on vapours. Pressure ranges in expansions are restricted by proximity to saturation conditions, and high-accuracy measurements of low pressures are difficult. Adsorption also complicates studies on vapours. For example, in measurements by the Burnett method on pure Ar and Kr near saturation, Weir et al. determined the effect of adsorption in their copper apparatus by performing experiments in vessels of different surface-to-volume ratios. Corrections for adsorption first became significant at temperatures at which the saturation pressure was just in excess of atmospheric, and rose rapidly at lower temperatures. For Ar at its triple point, where the saturation pressure is 70 kPa and B = — 280 cm mol, the correction due to adsorption was 8 cm mol. Hall and Eubank recommend that Burnett measurements be combined with isochoric data to avoid systematic errors due to adsorption. 

Measurements of adsorption isotherms of organic vapours on clean glass surfaces have been performed by Bottomley and co-workers. They find that, in general, monolayer coverage (based on liquid densities) is reached at approximately 75 per cent of the saturation pressure. For typical pK-apparatus they calculate that adsorption can produce errors in B as large as 30 to 100 cm mol when the adsorption occurs on a clean surface. Minute traces of grease 

Adsorption can be enhanced in vapour mixtures. Pavlychenko reports that the adsorption of mixtures of acetone and chloroform on graphitized carbon black is substantially greater than that of either of the pure components. Absorption of organic vapours can also be significant in elastomers. O-rings 

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