The dissociation of phosphoric acid in NaCl and NaMgCl solutions at 25�C

Abstract

The pK 1 * , pK 1 * and pK 3 * for the dissociation of H3PO4 have been measured in NaCl solutions from 0.5 to 6m at 25°C. The results have been used to evaluate Pitzer interaction parameters ψ(NaClH2PO4)=−0.028±0.005, λ(NaH3PO4)=−0.075±0.025, θ(HPO4Cl)=0.105±0.009, θ(PO4Cl)=−0.59±0.02 and ψ(NaClHPO4)=−0.003±0.004, ψ(PO4NaClH)=0.110±0.008. These parameters yield values of pK 1 * , pK 2 * and pK 3 * in NaCl that agree with the measured values with average deviations of ±0.04, ±0.03 and ±0.05 in pK 1 * . Measurements of pK 1 * and pK 2 * were also made in NaMgCl solutions. These results have been used to evaluate β(O)(MgH 2 PO 4)=−3.55±0.07,β(1)(MgH 2 PO 4=−16.9±0.03, β(O)(MgH 2 PO 4=−17.5±0.03 and β(1)(MgH 2 PO 4)=27.4±0.8 at 25°C. The results for pK 2 * in NaMg−Cl solutions were also used to calculate log K MX * =3.2±0.1 for the formation of the ion pair MgHPO 4 o .