Binary mutual diffusion coefficients of aqueous ammonium and potassium sulfates at 25°C

The diffusion coefficients of aqueous ammonium and potassium ions at infinite dilution differ by only 0.01% at 25°C, so, according to Nernst's law, the binary mutual diffusion coefficients D of pairs of dilute aqueous ammonium and potassium salts should be nearly identical. Yet precise optical interferometric data for ammonium and potassium sulfates contradict this rule: the value of D reported previously for 0.05 mol-L⁻¹ aqueous ammonium sulfate is 0.80×10⁻⁵ cm²-s⁻¹, while the corresponding value for potassium sulfate is 1.24×10⁻⁵ cm²-s⁻¹ about 50% higher. To confirm this surprising result, the diffusion coefficients of the two salts have been measured by the Taylor dispersion technique. The diffusion coefficients of the salts are indeed nearly identical, and the earlier ammonium sulfate data are found to be incorrect.