Synthetic Inorganic Ion-exchange Materials. XXXVIII. Acid-base Properties of a Cryptomelane-type Hydrous Manganese(IV) Oxide and Some Chromatographic Applications

Abstract

Abstract The pH titration curves of a cryptomelane-type hydrous manganese(IV) oxide(CRYMO) apparently showed a dibasic Brönsted-acid towards Li+, Na+, K+, and Rb+ ions and a monobasic Brönsted-acid towards Cs+ ions, instead of an amphoteric character, using a mixed solution of alkali metal nitrate and nitric acid or each hydroxide with the ionic strength of 0.1. The apparent ion-exchange capacities(meq/g) of alkali metal ions increased in this order: Li+(0.50)<Na+(0.55)<Cs+(0.85)<Rb+(1.00)<K+(1.10) at pH 2 and Cs+(1.20)<Rb+(1.72)<Na+(1.78)<Li+(1.80)<K+(1.81) at pH 10. The point of zero charge(PZC) of CRYMO was estimated to be much lower than 2. The unusual selectivity for K+ and Rb+ ions can be successfully applied to some ion-exchange separations: e.g., the removal of K+ ions from NaNO3 and NaMnO4 solutions, the removal of Rb+ ions from a CsNO3 solution, and the selective uptake of K+ ions from seawater.