Silicon by the molybdenum blue (photometric) method

Silicon by the molybdenum blue (photometric) method

Scope
This test method covers the determination of up to 20 µg Si ( 2 to 20 µg Si/g U) in uranyl nitrate solutions.

Summary test method
Silicon is determined by the formation of B-silicomolybdic acid and subsequent reduction to molybdenum blue. Ammonium molybdate is used to complex the silicon in an acid solution of the sample. Oxalic acid and sodium sulfite are added to prevent phosphorus interference and to stabilize the complex. Reduction to molybdenum blue is carried out with stannous chloride. Measurement is made at 820 nm using a spectrophotometer.

Interferences
Phosphorus also forms a heteropoly acid with molybdate, however, oxalic acid is used to decompose it.

Equipments
· Plastic beakers
· Polyethylene pipets
· Spectrophotometer

Reagents
· Ammonium Mmolybdate Solution (65 g/L). Dissolve 65 g of (NH4)6Mo7O24.4H2O in water, dilute to 1 L, and store in a plastic bottle.
· Hydrochloric Aacid (1+9). Mix 100 mL of HCl (sp gr 1.19) with 900 mL water and store in a platic bottle.
· Oxalic Acid Solution (100 g/L). Dissolve 25 g of HO2CCO2H in 250 mL of water and store in a plastic bottle.
· Silicon Standard Solution A ( 1 mL = 100 µg Si ) Fuse 0.0214 g of silicon dioxide (SiO2) with 1 g of Na2CO3 until clear melt is obtened. Dissolve the cooled melt in water and dilute to volume in a 100 mL polyethylene volumetric flask.
· Silicon, standard solution B ( 1 mL = 2 µg Si ) Dilute 2 mL of standard silicon solution A to 100 mL in a polyethylene volumetric flask.
· Sodiun Carbonat ( Na2CO3 ). Anhydrous powder.
· Sodium Sulfite Solution (150 g/L) Dissolve 30 g of Na2SO3 in 200 mL of waterand store in a plastic bottle.
· Stannous Chloride solution A(100 g/L) Dissolve 50 g of SnCl2.2H2O in 100 mL of HCl (sp gr 1.19} and 50 mL of water. Heat, if necessary, to dissolve. Dilute to 500 mL with water and store in a plastic bottle containing 1 g of metallic tin (Sn).
· Stannous Chloride Solution B. Dilute 10 ml of stannous chl;oride solution A to 100 mL with water. Make fresh daily.
· Sulfuric Acid (1+3). Add 125 mL of H2SO4 (sp gr 1.84) slowly to 375 mL of water. Cool and store in a plastic bottle.

Procedure
· Pipet 5 mL of (NH4)6Mo7O24.4H2O solution and 4 mL of HCl (1+9) into a 100 mL plastic beaker.
· Transfer a weighed portion of sample solution containing 2 to 3 drops of HF (48%) or an appropriate dilution into the beaker and swirl to mix.
· Dilute to 35 mL with water and let stand 15 to 30 min.
· Add, in order and swirling after each addition, 2 mL of oxalic acid solution, 2 mL of sodium sulfite solution, and 5 mL of sulfuric acid (1+3), and let stand 3 min.
· Add 1 mL of stannous chloride solution B, dilute to 100 mL volume with water, and let stand 5 min.
· Read at 820 nm in 5 cm cells with water as a reference.
· Prepare a standard curve by carrying 0 to 20 µg Si through the procedure step 1 to step 6.

Calculation
Calculate the silicon content as follows :

Silicon, µg/g = A/B

Where :
A = Silicon 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”