Determination of Molybdenum by the Thiocyanate method (spectrophotometric)

DETERMINATION OF MOLYBDENUM BY THE THIOCYANATE METHOD (PHOTOMETRIC)

Scope

This method covers the determination of up to 100 µg ( 5 to 500 µg Mo/g U ) molybdenum in nuclear grade uranyl nitrate solutions.

Summary method

After conversion of the solution to the sulfate by fuming with H2SO4, the molybdenum thiocyanate complexis formed and extracted with 1-hexanol. Spectrophotometric measurement is made on the axtract at 470 nm.

Equipment

Spectrophotometer.

Raegents

• Ammonium Thiocyanate Solution(200 g/L)-Dissolve 20 g of NH4SCN in water and dilute to 100 mL.

• I-Hexanol – Saturate technical grade !-Hexanol with NH4SCN and SnCl2 by shaking in a separatory funnel. Store in a brown bottle.

• Iron Solution (1 mg Fe/mL)-Dissolve 3.6 g of ferric sulfate Fe₂(SO₄)₃ in water and dilute to 1 L.

• Ethanol absolute

• Molybdenum Standard Solution (100 µg/mL) –Dissolve 0.184 g of ammonium molibdat (NH₄)₆Mo₇O₂₄.4H₂O in water, add 50 mL of sulfuric acid (1+9) and dilute to 1 L.

• Stannous Chloride Solution (350 g/L)-Heat 350 g of SnCl2.2H2O in 200 mL of HCl (1+1) on a steam bath until dissolution is complete. Filter and dilute to 1 L with freshly boiled water.

• Wash Solution-Mix 50 mL of H2SO4 (1+9), 2 mL of NH4SCN solution and 2 mL of SnCl2

Procedure

• Dilute 10 mL of sample solution to 100 mL in a volumetric flask.

• Pipet 5 mL of this solution into a 100 mL beaker and add 20 mL 0f H2SO4 (1+1) and 1 mL of iron solutions

• Evaporate to fumes of H2SO4, cool and dilute with 25 mL of water. Transfer to a 250 mL separatory funnel with water and dilute to about 100 Ml.

• Add 4 mL of NH₄SCN solution and mix.

• Add 10 mL of SnCl2 solution, mix again and allow to stand about 5 min

• Pipet 10 mL of 1-Hexanol into the funnel and shake for 1.5 min.

• Allow the layers to separate and discard the aqueous Layer.

• Add 40 mL of wash solution and shake for 30 s allow to separate and discard the aqueous layer.

• Transwer the organic layer to a 50 mL graduated flask And dilute to volume with methanol.

• Measure at 470 nm in 2 cm cell using 10 mL 1-Hexanol diluted to 50 mL with methanol as a reference.

• Prapare the standard curve by carrying 0 to 100 µg of molybdenum through step 2 to step 10 in the presence of 200 mg of uranium (4 mL of uranium solution).

Calculation

Calculate the molybdenum content as follows

Molybdenum, µg/g = A/B

Where

A = molybdenum 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”