Multiparametric curve fitting—XIII Reliability of formation constants determined by analysis of potentiometric titration data

The thermodynamic dissociation constant of naphazoline by the regression analysis of potentiometric data

The mixed dissociation constant of naphazoline is determined at various ionic strengths I [mol dm−3] in the range of 0.01 to 0.26 and at temperatures of 25°C and 37°C using ESAB and HYPERQUAD regression analysis of the potentiometric titration data. A strategy of efficient experimentation is proposed in a protonation constant determination, followed by a computational strategy for the chemical model with a protonation constant determination. Two group parameters, L 0 and H T were ill-conditioned in the model and their determination is therefore uncertain. These group parameters, L 0 and H T, can significantly influence a systematic error in the estimated common parameter pKa and they always should be refined together with pK a. The thermodynamic dissociation constant pK aT was estimated by nonlinear regression of {pK a, I} data at 25°C and 37°C: for naphazoline pK alT = 10.41(1) and 10.13(2). Goodness-of-fit tests for various regression diagnostics enabled the reliability of the parameter estimates to be found.

The potentiometric and spectrophotometric determination of dissociation constants for same 2-mercapto-5-R-amino-1,3,4-thiadiazole derivatives

In order to establish the dissociation constants of organic compounds, the spectrophotometric and potentiometric methods are the most precise and useful ones. Comparing the results, we used both methods for three derivatives of 2-mercapto-5-R-amino-1,3,4-thiadiazole. For the pKa determination by the spectrophotometric method we measured the ratio between the concentration of dissociated and undissociated forms. The ratio was calculated from spectral data. The potentiometric method for pKa consists of measuring the pH values within a potentiometric titration with 0.1 M NaOH. The results proved a very low acid character of the derivatives. The pKa values were influenced by the properties of the amino group substituents, and the results confirm the theoretical considerations. The study confirms the thion-thiolic tautometry of the 2-mercapto-5-R-amino-1,3,4-thiadiazole derivates and their property of being ligands for the coordination of the cations of some representative and transitional metals, with application in the preconcentration, in the detection and the quantitative determination of polluting and toxic cations in environmental analysis.