ISO 5930:1979

Specifies a method applicable to products having phosphorus contents equal to or greater than 0,002 % (m/m). The principle is based on alkaline fusion of a test portion with a mixture of sodium carbonate and boric acid, dissolution of the fused mass in nitric acid, neutralization of suitable aliquot portion with sodium hydroxide solution, reaction with acid ammonium molybdate solution to form a molybdophosphate complex, reduction by a mixture of sodium disulphite and 4-amino-3-hydroxynaphtalene-1-sulphonic acid, and photometric measurement of the reduced complex.