4828 This result is unexpected from the reported pK2 values for the two acids, 1.716 for selenic acid and 1.98)' for sulfuric acid. Figure 2 is a plot of H o us. water activity (determined from aqueous vapor pressure) for sulfuric,'j perchloric, l 1 and selenic acids. All of these H ovalues have been obtained using primary aniline indicators. As can be seen the values for selenic acid fall on the curve described by the other two acids. Thus, all three acids at a particular water activity have the same proton donating ability. The values of Braun" for mixed (16) A. I
