Bulk flow in diffusion coefficient studies

The viewpoint is taken that bulk flow terms in the general equation of molecular diffusion must be measured in apparatus which offers no constraint to the molecular flux in either direction. It is proposed to carry this out in a miniaturized experiment operating with one side of the diffusion gradient as an infinite sink. Dimethyl acetamide–water diffusivities computed from such an experiment were found to be 7.02 × 10−6 sq.cm./sec. over a wide range of compositions. These are compared with values showing considerably wider variation with composition, namely, 2.45 to 7.00 × 10−6 sq.cm./sec. by estimating methods and 11.45 to 14.05 × 10−6 sq.cm./sec. by large cell experiments. Arguments are presented in support of the values obtained by the proposed method operating on a nonideal binary N, N‐dimethyl acetamide and water.