Hydrogen ion titration of oleic acid in aqueous media: further examinations on sodium and potassium oleate systems

Abstract.

Hydrogen ion titration profiles of the aqueous potassium oleate/oleic acid systems were examined as a function of the degree of neutralization, β, in corresponding metal chloride solutions. At temperatures higher than 5 °C, the titration curves were almost identical to those of sodium oleate systems at temperatures higher than 20 °C characterized by a single constant-pH region due to the multiple-phase equilibrium including metal oleate micelles. At a low temperature, 0 °C, on the other hand, different titration curves characterized by two constant-pH regions were observed. The ranges of β of the high constant-pH region were 0.6<β<0.95 in contrast to the sodium oleate system (0.5<β<0.95). These results suggested that the acid soap compositions, oleate acid/metal oleate ratios, were 2:3 for the potassium oleate and 1:1 for the sodium oleate systems. Differential scanning calorimetry analysis for the potassium oleate/oleic acid and the sodium oleate/oleic acid two-component systems detected the formation of an acid soap with equivalent compositions in aqueous media. To inspect further the changes in the temperature profiles of the titration curves, the temperature dependence of the solution properties of potassium and sodium oleates was examined.