The Effect of Pressure on the Ionization of Boric Acid in Aqueous Solutions from Molal-Volume Data

The apparent, phi(v), and partial, (V) over bar (2), molal volumes of dilute (0.02 to 0.7 m) aqueous boric acid, B( OH) 3, and sodium borate, NaB(OH)(4), solutions have been determined from 0 to 50 degrees C from precision density measurements. The phi(v) of boric acid is a linear function of the first power of concentration. For sodium borate, the Debye-Huckel theoretical limiting law slope for the phi(v) as a function of the square root of molar concentration is approached at all temperatures studied. The positive deviations at higher concentrations increase with decreasing temperature. The infinite-dilution volume properties of both B(OH)(3) and NaB(OH)(4) indicate that these solutes behave like "structure breakers" between 0 and 50 degrees C (that is, partial derivative(2)phi(v)degrees/partial derivative T(2) is negative). The volume change for the ionization of boric acid in water, A Delta(V) over bar degrees is calculated from the volume data at various temperatures. This volume change has been used to estimate the effect of pressure on the ionization of boric acid solutions from 0 to 50 degrees C and 0 to 1000 bars. The calculated effect of pressure on the ionization is in good agreement with direct measurements.