Partial molal volumes and compressibilities of phosphoric acid and sodium phosphates in 0.725 molal NaCl at 25 °C

Frank J Millero; Fen Huang; Antonio Lo Surdo; Faina Vinokurova

doi:10.1021/jp101766v

Partial molal volumes and compressibilities of phosphoric acid and sodium phosphates in 0.725 molal NaCl at 25 °C

J Phys Chem B. 2010 Dec 16;114(49):16099-104. doi: 10.1021/jp101766v. Epub 2010 Jul 27.

Authors

Frank J Millero¹, Fen Huang, Antonio Lo Surdo, Faina Vinokurova

Affiliation

¹ Rosenstiel School of Marine and Atmospheric Chemistry, University of Miami, Miami, Florida 33149, USA.

PMID: 21141930
DOI: 10.1021/jp101766v

Abstract

The apparent molal volume V(ϕ) and adiabatic compressibility κ(ϕ) 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 °C. The partial molal volumes V(i)* and compressibilities κ(i)* have been determined by extrapolating the values of V(ϕ) and κ(ϕ) 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(i)* and κ(i)* as found for other electrolytes. The values for the volume ΔV(i) and compressibility Δκ(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 ΔV(i) and compressibility Δκ(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)] = -ΔV(i)P + 0.5Δκ(i)P(2), where P is the applied pressure in bars. The results are in reasonable agreement with directly measured values.