ISO 6382:1981

Specifies a method based on the depolymerization of any polymerized silica by treatment with sodium fluoride in the presence of hydrochloride acid resulting in the formation of the oxidized molycdosilicate, followed by the selective reduction of this complex in the presence of oxalic acid and in a sulphuric acid medium, and spectrophotometric measurement of the blue-coloured complex at the wavelength corresponding to maximum absorption (about 800 nm).