Molal volume of aqueous boric acid-sodium chloride solutions

Abstract

The apparent molal volume φ v of boric acid has been determined in various sodium chloride solutions at 0 and 25°C from precise density measurements. Similar to its behavior in pure water, the φ v of boric acid in NaCl solutions is a linear function of the concentration. The infinite dilution φ ° 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 Φ 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.