Comparative study of various polyols as complexing agents for the acidimetric titration of tungstate

Abstract

Most polyols (L = alditol or carbohydrate) from dinuclear tungstate complexes according to the over-all equilibrium 2WO₄^2–+ 2H⁺+ L ⇌[W₂O₇L]^2–+ H₂O. When the reaction is fast and complete, it allows the acidimetric titration of tungstate. The formation constants of the complexes of a series of polyols were determined by potentiometry. Their values were higher at low ionic strengths. Opposite structural factors govern the stabilities and the formation rates of complexes: alditols of threo configuration react with tungstate faster than those of erythro configuration, but the stability order is erythro > threo. Of the polyols investigated, only xylitol and D-glucitol (sorbitol) allowed a fast and accurate potentiometric titration. Using 0.02 M HCl as titrand, 0.04 mmol of tungstate was determined in a volume of 100 cm³. The interference of molybdate is discussed in detail.