Spectrophotometric determination of small amounts of phosphorus in iron and steel by solvent extraction method

Abstract

Colorimetric determination of phosphorus in iron and steel by molybdenum blue method has been investigated. A sample (0.11.0 g) is dissolved in 30 ml HNO₃(1 : 1.2) with gentle heating and it is diluted with 30 ml water. Two per cent potassium permanganate solution is added to it drop by drop with gentle heating to convert phosphorus completely to phosphate ion until a small amount of manganese dioxide appears. A few drops of 10 per cent sodium sulfite solution is added to dissolve manganese dioxide, and the solution is heated to expel sulfur dioxide. After cooling the solution is neutralized with ammonia solution(1 : 1) until a slight amount of ferric hydroxide precipitate appears. Five to ten ml of nitric acid (1 : 1.2) is added to dissolve the precipitate again, and the solution istransfered into a 100 ml measuring flask. It is made up to the mark with water after adding 23 g am monium nitrate. One ml of the sample solution, 0.35 ml perchloric acid (60%) and 0.5 ml ammonium molybdade (5%) are put into a 10 ml centrifuge tube with a stopper and are diluted with water exactly to 5 ml. Two ml of n-butylacetate is added, the tube isshaken vigorously for 30 seconds and centrifuged for 30 min. with 2, 500 rpm. One ml of the organic layer is transfered to a 5 ml Nessler tube. To it is added 0.3 ml stannous chloride solution (5%) and the whole is diluted exactly to the mark with ethanol. The ab sorbance is measured at 725 mμ.