Diffusion Coefficients in Aqueous Solutions of Potassium Chloride at High and Low Concentrations

Mutual differential isothermal diffusion coefficients of electrolytes in aqueous solutions have been measured in our laboratory using an open-ended capillary cell [112], Fig 1, developed some years ago. Diffusion may be followed by measuring the ratio of electrical resistances of the electrolyte solution in both capillaries as time proceeds, using an alternating current transformer bridge and an oscilloscope where an ellipse is manually reduced to an horizontal line. Attempts to use an automatic system to continuously measure the resistance ratios have, until now, been successful only for capillary resistances of the order of 5 kQ, or lower, that is, electrolyte concentrations of the order of 0.1 M or higher. In fact, whereas with the manual system it is possible to measure the pure ohmic resistance, by manually balancing the capacity effects, and therefore go to concentration of 0.001 M, the automatic system measures impedance ratios. The present paper reports the experimental alterations which enabled us to successfully use the automatic system, in conditions of very high electrical resistance of the solutions inside the capillaries (which will facilitate research on this field), presents data obtained at high and low KCI concentations, and discusses them on the basis of the Robinson and Stokes [13], Onsager-Fuoss [14] and Pikal [15] models.