The apparent molal volume, φV of boric acid, B(OH)3 and sodium borate, NaB(OH)4, have been determined in 35%. salinity seawater and 0·725 molal NaCl solutions at 0 and 25°C from precise density measurements. Similar to the behavior of nonelectrolytes and electrolytes in pure water, the φV of B(OH)3 is a linear function of added molality and the φV of NaB(OH)4 is a linear function of the square root of added molarity in seawater and NaCl solutions. The partial molal volumes, V ̄*, of B(OH)3 and NaB(OH)4 in seawater and NaCl were determined from the φV's by extrapolating to infinite dilution in the medium. The V ̄* of B(OH)3 is larger in NaCl and seawater than pure water apparently due to the ability of electrolytes to dehydrate the nonelectrolyte B(OH)3. The V ̄* of NaB(OH)4 in itself, NaCl and seawater is larger than the expected value at 0·725 molal ionic strength due to ion pair formation [Na+ + B(OH)4- → NaB(OH)40]. The volume change for the formation of NaB(OH)40 in itself and NaCl was found to be equal to 29·4 ml mol-1 at 25°C and 0·725 molal ionic strength. These large Δ V ̄*'s indicate that at least one water molecule is released when the ion pair is formed [Na+ + B(OH)4- → H2O + NaOB(OH)20]. The observed V ̄* in seawater and the Δ V ̄* (NaB0) in water and NaCl were used to estimate Δ V ̄* (MgB+) = Δ V ̄* (CaB+) = 38·4 ml mol-1 for the formation of MgB+ and CaB+. The volume change for the ionization of B(OH)3 in NaCl and seawater was determined from the molal volume data. Values of Δ V ̄* = -29·2 and -25·9 ml mol-1 were found in seawater and Δ V ̄* = -21·6 and -26·4 in NaCl, respectively, at 0 and 25°C. The effect of pressure on the ionization of B(OH)3 in NaCl and seawater at 0 and 25°C determined from the volume change is in excellent agreement with direct measurements in artificial seawater (culberson and Pytkowicz, 1968; Disteche and Disteche, 1967) and natural seawater (Culberson and Pytkowicz, 1968).
ASJC Scopus subject areas
- Geochemistry and Petrology