No abstract

solving ammonium hexanitrato cerate in sulfuric acid. The fact is, ceric sulfate solution and ceric ammonium nitrate solution are not equivalent, when standardized against arsenious oxide.The evidence compiled in this laboratory clearly indicates that Zielen did not use ceric sulfate in his experiments. Most likely, Zielen used ceric ammonium nitrate in sulfuric acid.Because there was a mix-up in reagents Zielen correctly ob-served directional dependence but incorrectly attributed the phenomena to ceric sulfate and osmium tetroxide.The authors agree with Zielen's conclusion that it is better to standardize ceric solutions potentiometrically rather than use colored indicators.A general equation for determining thermodynamic proton dissociation constants is described. The treatment is applicable to the simplest case, that of a monobasic acid, and to polybasic acids where the successive dissociation steps may or may not interfere. Its versatility is most rewarding in the solution of the problem of overlapping dissociation steps by the analysis of potentiometric data collected from a straight-forward titration of polybasic acids with a strong monoacid base. The interpretation of the general solution is specifically illustrated for citric acid and adipic acid, and the results, when compared with published data, are, in both cases, satisfactory. A third example on another acid, whose dissociation constants are not in the literature, illustrates the brief modification required in dealing with an acid salt.