Cation Exchange Properties of Tantalum Phosphate

Abstract

Tantalum phosphate was prepared by adding a conc. hydrochloric acid solution of tantalum pentachloride to an aqueous solution of phosphoric acid. After the gelatinous precipitate was allowed to stand for 25 hr in contact with the solution, it was separated, washed and dried. The tantalum phosphate which possessed the composition TaO₂, (H₂PO₄)_0.88, 6H₂0 is amorphous and chemically stable.
From acid-base titration curves, it was found that tantalum phosphate is a typical weak acid cation exchanger having the capacity up to 3.8 meq/g in an alkaline solution. Affinities of tantalum phosphate to alkali metal ions, increased in order Li⁺<Na⁺<<K⁺<Rb⁺<Cs⁺ similar to that in organic cation exchangers or zirconium phosphate. The value of separation factor between Na⁺ and K⁺ was greater than that of strongly acid organic cation exchanger under the same condition.
A tentative explanation for the cation exchange mechanism of tantalum phosphate was based on acid-base titration curves and infra red spectra: hydrogen ion produced by the dissociation of dibasic phosphoric acid group is exchanged by alkali metal ions in a relatively low pH solution.