A known volume of air is drawn through a tube containing mercuric cyanide impregnated silica gel to trap the phosphine. The phosphorus is extracted and oxidized to phosphate using a hot, acidic permanganate solution. The extracted sample is analyzed for phosphate by formation of the phosphomolybdate complex, extraction into a mixture of isobutanol and toluene and reduction using stannous chloride. The absorbance of the reduced phosphomolybdate complex is measured at 625 nm.