Potentiometric titrations in methanol/water medium: intertitration variablity

Homogeneous sets of data from strong acid-strong base potentiometric titrns. in a mixed solvent medium (9:1 vol./vol., methanol/water), performed by means of the glass electrode, at various const. ionic strengths and with different ref. electrodes, were analyzed by statistical criteria. A standardized procedure was established to obtain reliable potentiometric data in mixed solvents. It has been demonstrated how, with the aid of a proper linearized model, anal. of variance (ANOVA) applied to the standardization titrns. can be used to test the reliability of a potentiometric chain in a medium other than pure water. The error expected in the stability consts. thus evaluated is related to the intertitration error of the operational pKw*, for the same medium and the same chain. The results obtained by applying ANOVA to the mixed solvent data substantially confirm the trend found for aq. media, the intertitration error being larger than the intratitration error for all the parameters investigated (E'0, pKw*', mean equivalence vol.). The stochastic error thus obtained depends on the ionic medium used and on the kind of ref. electrode employed in the electrochem. chain.