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A purpose of this paper is to present some of the experimental electrochemical properties of the palladium hydride electrode that have been found between 25 and 200°C. The electrode potentials of palladium hydride were studied in aqueous solutions of sodium hydroxide and hydrochloric acid, in the presence and absence of carbon dioxide and nitrogen. All potentials are measured with respect to a platinum-hydrogen electrode. Use of relative resistance is made to establish the hydrogenpalladium composition and potential dependency as a function of time, temperature and pressure. Some simple polarization studies are also reported on fully equilibrated palladium hydride electrodes.

The e.m.f. values of cells with respect to time, involving the platinum-hydrogen and (ol+p) palladium hydride electrode in hydrogen-saturated aqueous electrolytes are reported between 25 and 195°C and 1-27 atm hydrogen pressure. Relative resistance measurements of the (a+B) palladium hydride electrode as a function of time are also reported under these conditions.

E.ni.f. measurements on cells containing aqueous sulpliuric acid at molalities less than 0.1 using hydrogen, lead dioxide/lead sulphate, and mercury/mnercurous sulphate electrodes have been used to derive stoichiometric activity coefficients of sulphuric acid. The new results and the previously reported measurements from 0-1 -8 m are consistent with activity coefficient ratios derived from vapour pressure measurements, but differ by 10 % from those reported in standard texts since these latter are based either directly or indirectly on the e.m.f. measurements of Hamer which are again shown to be incorrect. There is a systematic uncertainty in the determination of the activity coefficients due to uncertainty in the correct choice of value for the second ionization constant and to a lesser extent in the ion size parameter required for extrapolation to obtain the standard e.m.f.s.In previous communications 1 9 2 new measurements were reported for the cells :(1) (11) Pt, H2 I &SO4 (nz) I PbSO4 1 PbO2, Pt Pt, H2 I H2s04 (fn) I Hg2S04 1 Hg where m = 0.1 to 8 m. These two cells had been studied by Hamer 3 9 4 whose results are inconsistent with the vapour pressure data of Shankman and Gordon 5 and 6 with the isopiestic data of Stokes. By substituting values for the stoichiometric activities of sulyhuric acid solutions (derived through the Gibbs-Duhem equation from Stokes' vapour pressure measurements) into the Nernst equations corresponding to cells I and 11, it was shown,l, 2 that with the new measurements but not with Hamer's results, that constant values were obtained for the standard e.m.f.s of cells I and 11.Hamer 3 evaluated the standard e.m.f. of cell I by an extrapolation procedure but owing to the greater solubility of mercurous sulphate compared with lead sulphate this had not been possible 4 with cell 11. Harned and Hamer stated that below In = 0.05 the results for cell 11 became invalid because of the enhanced solubility of mercurous sulphate at low sulphate molalities giving rise to the possibility of an appreciable liquid-junction potential. In view of our interest 79 8 in other electrodes of the second kind where enhanced solubility at low molalities or other reasons may vitiate their use, new measurements were made on cell I1 to check Marned and Harner's statements which had not been supported by their experimental evidence. Parallel measurements at the same sulphuric acid molalities were made on cell I as a check and also on cell 111, since it was to be expected that, because of the higher solubility of silver sulphate compared with mercurous sulphate, cell I11 would fail at low sulphate molalities. 2050 Pt, H2 I H2S04 (m) I AgZS04 I Ag,

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