UC Davis

We study the deprotonation of MAl12 Keggin ions in monovalent electrolyte solutions of varying composition and concentration by potentiometric titration. The structures exhibit very steep deprotonation, where the singly coordinated aquo groups lose protons within a narrow pH range. Once the deprotonation is substantial, the Keggin ions start to aggregate by dehydration and linkage of terminal functional groups into hydroxo-bridges. In the present study, we address three aspects with our experiments. We test the cation-specificity, the anion-specificity and the overall effect of electrolyte concentration with respect to the deprotonation behavior. Our results show that the cation series in chloride systems does not show any ion-specificity and all the curves coincide for the 100 mM solutions, whereas the anion series in sodium systems does. The most structure-making anion (bromate) used in this study causes aggregation of the Keggins prior to the onset of aggregation in the presence of border-line and structure-breaking anions (chloride, nitrate and perchlorate). For the latter, no significant difference is observed. The fluoride ion causes a completely different behavior. No significant deprotonation pH effect is observed where deprotonation occurs in the absence of fluoride. At even higher pH, massive consumption of hydroxide occurs. Some possible scenarios for the behavior of the fluoride-containing systems are discussed.