Molal Volume of Aqueous Boric Acid-Sodium Chloride Solutions

The apparent molal volume phi(v) of boric acid has been determined in various sodium chloride solutions at 0 and 25 degrees C from precise density measurements. Similar to its behavior in pure water, the phi(v) of boric acid in NaCl solutions is a linear function of the concentration. The infinite dilution phi(V)degrees and the slope S(V)* of B(OH)(3) are larger in NaCl solutions than in pure water. NaCl appears to be able to dehydrate B(OH)(3) and cause an increase in B(OH)(3)-B(OH)(3) interactions. The mean apparent molal volumes Phi(V) of the B(OH)(3)-NaCl solutions are predicted from pure water data using a modification of Young's rule for electrolyte-nonelectrolyte mixtures and are compared to the directly measured values. A similar treatment was carried out on the density data of acetic acid-sodium chloride solutions. The modified Young's rule was found to give a good first approximation of the mean apparent molal volumes of nonelectrolyte-electrolyte systems. The deviations from the Young's rule approximation are studied as excess volumes of mixing boric acid and NaCl solutions.