[en] The composition, stability constants and molar absorption coefficient of the chelate of thorium ion with Semimethylthymol Blue, SMTB is determined spectrophotometrically in a medium of perchloric acid and sodium perchlorate solution (pH 2.0). In the thorium-SMTB system, a purple red Th(SMTB) chelate is formed with logarithmic overall stability constant of 20.76±0.01 and molar absorption coefficient of 1.44x10⁴ (560 nm) dm³ mol^-1 cm^-1. SMTB was proposed as a new reagent for the spectrophotometric determination of micro-molar amounts of thorium at λ_max 560 nm. The colour development depends on time, temperature, pH and buffer species. The interferences of different anions, cations and organic acids on thorium determination were also investigated. Beer's law was obeyed for 11.6-104.4 μg Th(IV)/25 ml(0.46-4.18 ppm). SMTB was used for the spectrophotometric determination of thorium in medium and low-grade minerals and ores and the results were reproducible and satisfactory. Comparative study including different statistical tests between the spectrophotometric and complexometric determinations of thorium in its minerals and ores using SMTB has been carried out

[en] The precision and accuracy attainable in direct complexometric titrations of Thsup(4+) 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. (Author)