Rigorous solution to the problem of interfering dissociation steps in the titration of polybasic acids

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.

Confocal Raman Microscopy for pH-Gradient Preconcentration and Quantitative Analyte Detection in Optically Trapped Phospholipid Vesicles

“…12,16 To account for the pH change due to accumulation of analyte, the acid−base equilibria of the buffer must be considered. For the citric acid buffer within the vesicle, three acid dissociation equilibria are relevant: where pK 1 = 3.13, pK 2 = 4.76, and pK 3 = 6.40 37,38 and the total buffer concentration is given by (8) In total, there are 10 variable concentrations constrained by conservation of charge, five equilibrium constants (three buffer dissociation constants, the analyte K A , and the autoprotolysis constant of water) and four parameters set by conditions of the experiment (concentration of the citrate buffer, the initial internal pH which establishes the internal sodium ion concentration, the source-phase pH, and concentration of analyte in the source phase). Because accumulation is measured on a single vesicle in the confocal Raman experiment, the vesicle volume fraction is infinitesimal (∼10 −11 ) and depletion of analyte from the source phase can be neglected.…”

“…In order to employ pH-gradient preconcentration as a quantitative method for detection of analytes in a source-phase solution, the enrichment factors predicted by eq must be corrected for the finite capacity of the buffer within the vesicle, because the interior pH may change significantly from its initial value as the accumulated analyte concentration approaches that of the buffer. , To account for the pH change due to accumulation of analyte, the acid–base equilibria of the buffer must be considered. For the citric acid buffer within the vesicle, three acid dissociation equilibria are relevant: where p K 1 = 3.13, p K 2 = 4.76, and p K 3 = 6.40 , and the total buffer concentration is given by The citric acid used to hydrate the vesicles was titrated with sodium hydroxide to set the initial pH of the interior buffer. From the equilibria in eqs 4a –4c, the total concentration of citrate, [C] in total , and the initial pH giving [H + ] 0 , the sodium ion concentration in the internal buffer can be determined as follows: where β = [H + ] 3 + [H + ] 2 K 1 + [H + ] K 1 K 2 + K 1 K 2 K 3 .…”

“…where pK 1 = 3.13, pK 2 = 4.76, and pK 3 = 6.40 37,38 and the total buffer concentration is given by…”