Determination of Phosphorus by the Spectrophotometric Method (molybdenum blue)

Phosporus by the Molybdenum blue (photometric) method


Scope
This test method covers determination of phosphorus in nuclear grade uranyl nitrate solutions. Appropriate dilution may be made to facilitate obtaining samples containing 0 to 60 µg P.

Summary of test method
Phosphorus is determined by the formation of heteropoly molybdopghosphoric acid and its subsequent reduction to molybdenum blue. Sodium molybdate is used to comlex the P in an acid solution containing the sample. The yellow complex is then extracted into isobutanol. After the excess molybdate is washed out with water, the organic phase is contacted with an acid solution of stannous chloride to reduce the complex. The resulting molybdenum blue is read at 725 nm using a spectrophotometer.

Interferences
· The molybdenum blue reaction is not specific for phosphorus, however adjustment of the acidity to above 0.9 N avoids the formation of molybdosilicic acid.
· Fluoride and chloride must be fumed off before the heteropoly acid is formed.

Equipment
Spectrophotometer

Reagents
· Isobutanol
· Phosphorus Standard Solution A (1 mL = 100 µg P). Dissolve 0.4393 g of KH2PO4, which has been dried at 110 oC for 1 h, in 1 L of water.
· Phosphorus Standard Solution B (1 mL = 10 µg P). Dilute 10 mL of Solution A to 100 mL with water.
· Sodium Molybdate Solution (10 g/L). Dissolve 25 g of Na2MoO4.2H2O in 250 mL of water. Filter if turbid and store in a polyethylene bottle.
· Stannous Chloride Solution (2 N in HCl). Dissolve 2.38 g of SnCl2.2H2O in 170 mL of HCl (sp gr 1.19), and dilute to 1 L with water. Store in a polyethylene bottle containing 1 to 2 g of tin metal pellets or granules.

Procedure
· Transfer a weighed portion of sample solution containing 0.1 g of uranium or an appropriate dilution to a 150 mL beaker.
· Add 3 mL of HClO4 (72%) or H2SO4 (sp gr 1.84) to the beaker and heat to strong fumes.
· Add 40 mL of water and 5 mL of Na2MoO4.2H2O solution and let stand 5 min.
· Transfer to a 125 mL separatory funnel.
· Add 40 mL of isobutanol and extract for 1 min.
· Discard the aqueous layer.
· Wash the organic layer with two 25 mL portion of water and discard the aqueous layer.
· Add 25 mL of SnCl2 solution and shake for 15 s
· Discard the aqueous layer
· Drain the organic layer into a 50 mL volumetric flask. Wash the funnel with isobutanol and add the washings to the flask.
· Make to volume with with isobutanol and read at 725 nm in 1 cm cells with isobutanol as the reference.
· Prepare a standard curve by carrying 0 to 60 µg P through the procedure step 2 to step 11.

Calculation
Calculate the phosphorus content as follows :

Phosphorus = A/B , µg/g

Where
A = phosphorus 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 ± 3%.

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