Comparative studies of phosphate adsorption at the mercury-solution interface

Abstract

Summary Large differences between directly measured interfacial tensions and those derived from capacitance curves were observed at anodic rational potentials in solutions of Na 2 HPO 4 and Na 3 PO 4 . These discrepancies associated with sticking of the mercury meniscus in the Lippmann electrometer at extreme anodic potentials were attributed to the probable formation of highly insoluble compounds such as (Hg 2 ) 3 (PO 4 ) 2 , Hg 2 HPO 4 and HgO. The charge due to specifically adsorbed HPO 4 2− anions was calculated by comparing experimental relative surface excesses with those of the GCS diffuse layer theory. The p.z.c. for Na 2 HPO 4 and Na 3 PO 4 solutions over a wide range of concentrations was found to vary within about only 1 mV. No significant anion specific adsorption was detected for Na 2 HPO 4 until negative or positive q m values were attained. Results of q − ′ calculations were compared with previously published work on NaH 2 PO 4 (ref. 1) and Na 2 SO 4 . The presence of an ion-free layer of adsorbed water molecules was considered as being able to exert an appreciable influence on the calculated values of q − ′, and this concept was employed to obtain “corrected data” for anion specific adsorption according to the method reported for NaH 2 PO 4 by Parsons and Zobel 1 . Capacitance results in Na 3 PO 4 solutions were not thermodynamically analysed because of the different nature and charge of the anions in solution. The adsorbability of the three phosphate anions on positively charged mercury was empirically related to the high degree of structure-ordering of the solvent as indicated by their large positive B coefficients for the Jones and Dole equation. Considerations of molar fluidity persuaded us to qualitatively suggest that increasing specific adsorption may occur at anodically polarized Hg in the order: PO 4 3− 4 2− 2 PO 4 − . Ancillary information on homogeneous ion-solvent interactions led us to speculate on a possible simplified molecular model of the interface to account for the observed double layer properties of HPO 4 2− . Structural effects at the metal-solution interface due to the combined influences of ionic and electrode fields appear to be consistent with the suggested modelistic interpretation despite its crude nature and the obvious pitfalls of extending macroscopic thermodynamic measurements to speculate on the micro-physical entities involved in the double layer.