Titration of thorium and rare earths with ethylenediaminetetraacetic acid using Semimethylthymol Blue by visual end-point indication

Abstract

The precision and accuracy attainable in direct complexometric titrations of Th⁴⁺ consecutively with either lighter (La³⁺, Nd³⁺, Sm³⁺, Eu³⁺ or Gd³⁺) or heavier lanthanides (Dy³⁺) in different proportions using Semimethylthymol Blue (SMTB) as a metallochromic indicator and disodium dihydrogen ethylenediaminetetraacetate were studied. Thorium(IV) was titrated at pH 2, the pH was adjusted to 5.5–6.0 by adding hexamethylenetetramine (hexamine) buffer and acetylacetone–acetone solution and La³⁺(or Nd³⁺, Sm³⁺, Eu³⁺, Gd³⁺ or Dy³⁺) was then titrated. A comparison of the indicators SMTB and Methylthymol Blue (MTB) for successive titrations of Th⁴⁺ and any of the rare earth ions was carried out. The proposed titration method was applied successfully to some naturally occurring ores and minerals containing thorium and some lanthanides and the results were satisfactory.