Partial Molal Volumes and Compressibilities of Phosphoric Acid and Sodium Phosphates in 0.725 Molal NaCl at 25 °C

The apparent molal volume V(phi) and adiabatic compressibility kappa(phi) of H(3)PO(4), NaH(2)PO(4), Na(2)HPO(4), and Na(3)PO(4) have been determined from density and sound speed measurements in 0.725 m NaCl solutions at 25 degrees C. The partial molal volumes (V) over bar*(i) and compressibilities (kappa) over bar*(i) have been determined by extrapolating the values of V(phi) and kappa(phi) to infinite dilution in the NaCl media. The results have been compared to earlier measurements made on a number of electrolytes in NaCl media. The phosphate results showed correlations between the values of (V) over bar*(i) and (k) over bar*(i) as found for other electrolytes. The values for the volume Delta V(i) and compressibility Delta kappa(i) changes for the dissociation of H(3)PO(4) in 0.725 m NaCl have been calculated from the measurements. The calculated values of Delta V(i) and compressibility Delta kappa(i) have been used to estimate the effect of pressure K(p)/K(0) on the dissociation of H(3)PO(4) in the oceans using RT ln[K(P)/K(0)] = Delta V(i)P + 0.5 Delta kappa(i)P(2), where P is the applied pressure in bars. The results are in reasonable agreement with directly measured values.