Equations for testing Onsager's reciprocal relations for isothermal diffusion depend on the frame of reference chosen for the flows. This subject is considered for certain frames of reference, as is the problem of measuring diffusion coefficients when there is a change of volume on mixing. Frames of reference discussed are those moving with the local center of mass, the local center of volume, the local velocity of the solvent (or of any single component), and that fixed on the diffusion cell. Multicomponent systems, both of strong electrolytes and nonelectrolytes, are considered. An expression is derived which relates the flow of a component in the cell-fixed frame of reference to the flows in any other frame of reference when there is a change of volume on mixing. This relation is used to show that a flow relative to the cell becomes identical to that in the volume-fixed frame as the initial differences in concentration within the diffusion cell are made sufficiently small. Throughout this article a special effort has been made to present derivations and final equations in a form well adapted for use in experimental work.

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