Quantitative determination of thorium in iron and steel by the spectrophotometric method using arsenazo III has been investigated.
Thorium-arsenazo III complex showed maximum absorbance at 665 mμ and the concentration of HCl did not affect the determination at 4∼7 N. The interference by zirconium complex with arsenazo III could be eliminated by adding oxalic acid and was masked most effectively in a 2.5 N HCl medium. The coloration followed Beer’s law over the range of 0∼30 μg thorium at 2.5 N HCl in 50 mL for 30 mm cell. Up to 10 mg Al³⁺, 20 mg Fe³⁺, 3 mg Ni²⁺ and 1 mg Pb²⁺ didn’t interfere the determination of thorium.
The method of determination is as follows:
The sample, after decomposition with mixed acid and perchloric acid, can be treated for the separation of iron and other metals by magnetic electrolysis with the use of a mercury cathode. The solution is diluted exactly to 250 mL and an aliquot is taken. After evaporating, dissolve the residue in 10 mL of HCl(1+1) and transfer to a 50 mL volumetric flask. Add 14 mL of saturated oxalic acid in HCl(1+1), 15 mL of 4% oxalic acid solution, ascorbic acid to reduce the iron (III) and 5 mL of 0.05% solution of arsenazo III and dilute to the mark with water. The photometric determination is carried out at 665 mμ in 30 mm cell. The thorium content is found via a calibration curve.
By this means, 0.010% thorium in iron and steels could be determined with satisfactory results.