A REVIEW OF SOME ASPECTS OF ELECTROMOTIVE-FORCE MEASUREMENTS FOR THE CELLS - L-BAR-PBO2(S)VERTICAL-BAR-PBSO4(S)VERTICAL-BAR-PT, AND AL-BAR-HG2SO4(S)VERTICAL-BAR-HG(L)VERTICAL-BAR-PT, WITH P-DEGREES=0.1 MPA

In 1935 Hamer (J. Am. Chem. Sec. 1935, 57, 9) and Harned and Hamer (J. Am. Chem. Sec. 1935, 57, 27) published the results of extensive e.m.f . measurements for the activities of H2SO4 (ag) from low to high molal ities over the temperature range 273.15 K to 333.15 K, using the elect rochemical cells: Pt\H-2(g, p = 0.101325 MPa)\H2SO4(aq)\PbO2(s)\PbSO4( s)\Pt, and Pt\H-2(g, p = 0.101325 MPa)\H2SO4(aq)\Hg2SO4(s)\Hg(1)\Pt. T he e.m.f.s were reported only as least-squares functions of temperatur e at rounded molalities. These results were criticized severely in the ensuing years as yielding inaccurate values of water activities and m ean activity coefficients at molalities above 3 mol.kg(-1), values of the standard potentials and standard enthalpies of the cell reactions that depend on the molality of H2SO4 (aq), and incorrect entropy chang es for the cell reactions. However, there are more recent claims that these e.m.f.s are accurate. The original measured potentials for the f irst cell from (0.0004985 to 7.093) mol.kg(-1) at the temperature 273. 15 K were located (Harned, H. S.; Hamer, W. J. U.S. Naval Research Lab oratory Report No. P-1071, 1934), and the available evidence was criti cally examined. We conclude that their experimental e.m.f.s at this te mperature are accurate to at worst (0.1 to 0.2) mV up to 2.2 mol.kg(-1 ) or higher, except for a large bias potential as is typical for this cell. At higher temperatures, the sums of the reported smoothed e.m.f. s for these two cells are consistent to a few hundredths of a mV up to (2 to 3) mol.kg(-1). For the individual cells in this molality region , however, the smoothed e.m.f.s yield standard potentials that show mo lality-dependent variations of(0.9 to 1.6) mV, with the size of this v ariation increasing with temperature. We attribute this variation part ly to their graphical smoothing of e.m.f.s to round molalities. It is likely that the cells no longer behaved reversibly above (2 to 3) mol. kg(-1).