Determination of thorium by the arsenazo (III) method (spectrophotometric)

DETERMINATION OF THORIUM BY THE ARSENAZO(III) METHOD (PHOTO-METRIC)

Scope

This method covers the determination of up to 10 µg of thorium ( 2 to 20 µg Th/g U) in uranyl nitrate solution

Summary of method

Thorium is separated from uranium by anion exchange. The thorium-bearing effluent is treated directly with oxalic acid to complex any zirconium and with Arzenazo III to develop the blue thorium-Arsenazo III complex. Measurement is made at 665 nm. The method is nearly specific for thorium.

Reagents

• Anion Exchange Resin Cl – From (50 to 100 Mesh)-Prepare awater slurry of resin and pour it into the column to a height of 4 to 5 cm then rinse the column with 2 or 3 bed-volumes of 8 M HCl.

• Arsenazo (III) Solution o-(8-dihydroxy-3,6 disulfonaphthylene-2,7-bisazo)-bisbenzene-arsonic Acid , disodium Salt), (2 g/L)-Dissolve 0.2 g of Arsenazo (III) in water and add 1mL of sodium carbonat solution (Na₂CO₃, 5 g/L)in the solution. Adjust the volume to 100 mL with distilled water.

• Oxalic Acid (50 g/L)-Dissolve 5 g of oxalic acid (H₂C₂O₄) in distilled water and dilute to 100 mL

• Thorium Standard Solution A ( 1 mL = 100 µg Th)-Dissolve 0.2392 g of thorium nitrate (Th(NO₃)₄.4H₂Oin a 250 mL beaker and fume to dryness three times with 5 mL portions of HCl (sp.gr 1.18) Dissolve the residu with 0.1 M HCl and dilute to 1 L with 0.1 M HCl.

Procedure

• Weigh an aliquot of the solution containing no more than 0.5 g of uranium.

• Evaporate the solution to dryness four times with 10 mL portions of HCl (sp gr 1.18)

• Dissolve the residu in 2 to 3 mL of HCl using heat if necessary

• Dilute the solution to about 10 mL with 8 M HCl and transfer to the treated ion-exchange column.

• Rince the beaker with a minimum quantity of 8 M HCl (about 5 mL), and add the rinse solution to the column.

• Drain the column at a rate of 3 to 4 mL/min into a suitable graduated cylinder

• Discard the first 7 mL of effluent, and collect the remaining effluent in a 50 mL volumetric flask.

• Wash the resin with two 5 mL and two 10 mL portion of 8 M HCl, collect all the effluents in the 50 mL flask

• Add 1 mL of H₂C₂O₄ solution to the flask to complex any zirconium.

• Add 2 mL of a arsenazo (III) solution to the flask, and dilute the solution to volume with distilled water and mix thoroughly

• Measure at 665 nm in 10 cm cells using reagent blank as a reference.

• Prepare the standard curve by carrying 0 to 10 µg through step 2 to step 11.

• Elute the uranium from the column with 10 bed-volumes of 0.1 M HCl at the rate of 3.0 to 3.5 mL/min. Then regenerate as described in step 2 at reagent.

Calculations

Calculate the thorium content as follows

Thorium, µg/g = A/B

Where

A = Thorium found in the sample solution, µg
B = uranium contained in the sample solution, g.

Precision

The limit of error at the 95% confidence level for a single determination is ± 5%.

Literature

ASTM-C799, Standard Methods for “Chemical, Mass Spectrometric, Spectrochemical, Nuclear, and Radiochemical analysis of Nuclear-Grade Uranyl Nitrate solution”