A study of mercury (I) phosphate electrodes

Abstract

This thesis describes some research on mercury (I) phosphate electrodes. Two electrode systems have been investigated involving Hg3PO4 and Hg2HPO4, which are stable at low and high phosphoric acid concentrations respectively. Hg2HPO4 is prepared by adding 0.05 m HgNO3 solution to 0.5 m H3PO4 solution, and Hg3PO4 is prepared by adding 0.05 m HgNO3 solution to primary and secondary phosphate solutions (NaH2PO4 and NaH2PO4). These compounds are well characterised by X-ray powder diffraction and chemical analysis. Two platinized platinum hydrogen electrodes were used and agreed with each other to within 0.1 mV. In both electrode systems, the equilibrium state was reached within a few hours. The reproducibility was within 0.2 mV in the concentration range between 0.1 and 0.02 molal for the Hg2HPO4 electrode. In the case of Hg3PO4, the reproducibility was less than 1 mV for acid concentration higher than 2 x 10-3 molal and within 2 mV for the more dilute solutions. The evaluation of E° values, depends on the way of expressing aH+ aH2PO4-. By letting either: (a) aH+aH2PO4- = m±2 γ±2 = (αm)2γ±2 or (b) aH+aH2PO4- = K1au =K1m(1 - α) γu and either assuming γu = 1 (satisfactory for Hg3PO4 but not for Hg2HPO4) or expressing γu by: lnγn = Bmu = B(1 - α)m or lnγu = B'αm The α values required are calculated from conductance data down to 0.003 molal then from K1, the first dissociation constant of H3PO4. The α values are in good agreement in the overlap region. Calculations and graphical extrapolations are in good accord to give: Hg/Hg2HPO4, E° = 0.6470 V ± 0.3 mV Hg/Hg3PO4, E° = 0.6909 V ± 0.5 mV From those E° values, the activities, au, of H3PO4 obtained show good agreement with previous work. The equilibrium concentration, mH3PO4' = (1.06 ± 0.13)x10-2 (total acid concentration) is obtained for Hg2HPO4 and Hg3PO4 in aqueous H3PO4 solution. The standard free energy of formation, ΔG°f, and solubility product, Ksp, of Hg2HPO4, and Hg3PO4 are calculated as follows: ΔG°f/Hg2HPO4(s)/ = - 240.329±0.014 kcal mol-1 ΔG°f/Hg3P04(s)/ = - 222,371±0.035 kcal mol-1 Hg2HPO4 = - Hg2 2+ + HP04 2- K sp = (7.478±0.702)xl0-13 H+ + Hg2HPO4 = Hg2 2+ + H2PO4- K sP = (1.204±0.116 )x10-5 Hg3P04 = 3/2Hg2 2+ + PO4 3- K sp = (1.985±0.22)x10-25 2H+ + Hg3PO4 = 3/2Hg2 2+ + H2PO4- K sp = (7.039±1.130)x10-6