Acidity constants in different media ( I=0 and I=0.1 M KCl) from the uncertainty perspective.

Koort, Eve; Gans, Peter; Herodes, Koit; Pihl, Viljar; Leito, Ivo

Procedures for estimating the measurement uncertainty for the acidity constant K a (or the p K a value) in different media ( I=0 and I=0.1 mol L−1 KCl), as determined by potentiometric titration, are presented. The uncertainty budgets (the relative contributions of the different input quantities to the uncertainty in the result) of the p K a( I=0) and p K a( I=0.1 mol L−1 KCl) values are compared. Unlike the values themselves, the uncertainties and uncertainty budgets of the values are comparable. The uncertainty estimation procedures are based on mathematical models of p K a measurement and involve the identification and quantification of individual uncertainty sources according to the ISO GUM approach. The mathematical model involves 52 and 48 input parameters for p K a( I=0) and p K a( I=0.1 mol L−1 KCl), respectively. The relative importance of each source of uncertainty is discussed. In both cases, the main contributors to the uncertainty budget are the uncertainty components due to the hydrogen ion concentration/activity measurement, which provide 63.7% (for p K a( I=0)) and 89.3% (for p K a( I=0.1 mol L−1 KCl)) of the uncertainty. The remaining uncertainty contributions arise mostly from the limited purity of the acid. From this work, it is clear that the uncertainties of the p K a( I=0.1 mol L−1 KCl) values tend to be lower than those of the p K a( I=0) values. The main reasons for this are that: (1) the uncertainty due to the residual liquid junction potential is nominally absent in the case of p K a( I=0.1 mol L−1 KCl) due to the similarly high concentrations of background electrolyte in the calibration solutions and measured solution; (2) the electrode system is more stable in solutions containing the 0.1 mol L−1 KCl background electrolyte and so the readings obtained in these solutions are more stable.

ACIDITY function; MATHEMATICAL functions; MATHEMATICAL models; MATHEMATICAL statistics; POTENTIOMETRY; VOLUMETRIC analysis; HYDROGEN ions; MONOVALENT cations

Analytical & Bioanalytical Chemistry, 2006, Vol 385, Issue 6, p1124

1618-2642

Academic Journal

10.1007/s00216-006-0519-x