Complexation equilibria of some sulphoazoxines. Part VII. Formation of oligomers by naphthylazoxine 6S, SNAZOXS and 2-, 3-and 4-CAPAZOXS evaluated by regression analysis of potentiometric data

Abstract

The formation of oligomers by the five sulphoazoxines Napthylazoxine 6S, SNAZOXS and 2-, 3-and 4-CAPAZOXS was studied potentiometrically. Two pH scales were used. Either proton activity was measured and the system was calibrated with standard buffers, or the hydrogen ion concentration was measured and the system was calibrated externally in terms of h=[H⁺]. At concentrations higher than 1 mmol dm^–3 dimers and trimers are formed, for which a reaction scheme is proposed. The concentration scale (h) gives more precise and more accurate values for the formation constants because external calibration of the glass electrode cell in terms of log h is more accurate than calibration with different NBS buffers. The main reason is that calibration with buffers is carried out in a medium very different from that used in the actual titration. The algorithm used has almost no influence on the precision of the formation constants in comparison with the reproducibility of the titration.