Phosphorus can be detei-mirted in iron and steels by forming phosphovanadomolybdate in perchloric-nitric acid solution and subsequently measuring the extracted yellow colour in isoamyl alcohoP or isobutyl methyl ketone 2. The latter method is limited to steels which do not contain tungsten, niobium or tantalum, and colour development time must be increased to I h for samples containing up to 2% of titanium and up to o.5% of zirconium. A lengthy modification must be made to determine phosphorus when tungsten, niobium or tantalum are present. The B.I.S.R.A. Chemical Analysis Committee 3 has recommended an improved method which allows the determination of phosphorus in steels containing either 7% tungsten or 0.8% niobium, provided that the samples ~dll dissolve in a mixture of concentrated hydroehloric and nitric acids.
This paper describes a method for the spectrophotometric determination of phosphorus in all types of alloy steels. The only interferences are chromium, which must be volatilized as chromyl chloride when more than 2% is present, and tungsten above xo%. The interference of tungsten can be overcome by selecting a smaller sample size. Hydrofluoric acid is used to complex tungsten, niobium, tantalum, titanium and zirconium and prevent their interference in the phosphorus determination, thus removing the need for lengthy modifications required by earlier methods.
EXPERIMENTAL
Apparatus and eeagents
A Unicam SP 600 spectrophotometer with 2-cm cells was used.
Standard lbhosphorus solution.
Prepare a stock solution containing o.I mg P/nil by dissolving o.4393 g of potassium dihydrogen phosphate in i 1 of water. Prepare a working solution (o.o 4 mg P/ml) by diluting IOO ml of stock solution to 250 ml.
A~monium ~vnolybdate solution (z5%). Prepare from (NHa)~ MoTO2a'4H20 and store in a polythene bottle.
A,nmonium vanadate solution (0.25%). Dissolve 2.5 g of ammonium meta-va2nadate in 50o ml of warm water. Cool and dilute to I 1.
Recommended #roced, tre .[or saml~les containing less tha,~ zo% W and less than 0.08% P Weigh 0.5 g of sample into a x25-rrd conical beaker and add 7 ml of water, 7 ml .Anal. Chim. Acta, 49 (x97o) 5xx-5x7