Isothermal binary and ternary diffusion studies of strong electrolytes from low to high concentrations

Abstract

Binary mutual diffusion coefficients are reported for NaHCO$\sb3$-H$\sb2$O and KHCO$\sb3$-H$\sb2$O at 25$\sp\circ$C for concentration ranges of 0.015 to 0.9 M. Experimental results are compared with those diffusion coefficients calculated from the Onsager-Fuoss theory. Thermodynamic diffusion coefficients are calculated. Ternary diffusion coefficients for NaHCO$\sb3$(0.25 M)-KHCO$\sb3$(0.25 M)-H$\sb2$O at 25$\sp\circ$C are reported. The results suggest that up to 0.5 ionic strength for this system, ternary diffusion coefficients may be estimated within 1-2% accuracy from the experimental binary diffusion coefficients and the limiting ternary diffusion coefficients which are calculated from limiting ionic conductances. A few important partial molar properties of the ternary system are calculated using Young's rule. Density measurements are reported for the KHCO$\sb3$-H$\sb2$O system up to 1.0 M. As part of an extensive investigation of transport properties of NaCl-MgCl$\sb2$-H$\sb2$O system, precise diffusion coefficients (D$\sb{\rm ik}$) data have been obtained over concentrations ranging from dilute to near saturation using Gouy interferometric method at 25$\sp\circ$C. Experiments have been performed at fixed molarity ratios of 1/3, 1/1, and 3/1 for NaCl to MgCl$\sb2$, at total concentrations of 0.5, 1.0, 2.0, 3.0 and 3.79 M. Also a series of experiments where NaCl or MgCl$\sb2$ was in "trace" amounts in a concentrated solution of the other solute have been conducted, the total concentrations being the same as in previous experiments. The off-diagonal diffusion coefficients are positive and in some cases are large, and are indicative of the largest solute-solute interactions in electrolyte solutions known so far. The cross-term D$\sb{12}$ (NaCl - 1, MgCl$\sb2$ - 2) becomes larger than all the other diffusion coefficients at higher concentrations. This shows that a gradient of MgCl$\sb2$ is more effective in producing a flow of NaCl than an equal gradient of NaCl itself. It is also causing greater flow of NaCl than MgCl$\sb2$ itself. Thermodynamic diffusion coefficients (M$\sb{\rm ik}$) have been defined and calculated for the ternary case. The results indicate the importance of interpreting diffusion processes in terms of thermodynamic driving forces. Densities have been measured for all the solutions and used to calculate the partial molar volumes (V$\sb{\rm i}$) of all components in the solutions. These show disruptions in the hydration structure of the solutes. Finally the diffusion coefficients together with the activity data have been used to verify the Onsager reciprocal relations of irreversible thermodynamics for isothermal diffusion.