ISO 3709:1976

Phosphoric acid for industrial use (including foodstuffs) — Determination of oxides of nitrogen content — 3,4- Xylenol spectrophotometric method

Applicable to products having a content, expressed as N, equal to or more than 0.2 mg/kg. Consists in oxidation of nitrous nitrogen to nitric nitrogen by potassium permanganate and reaction of nitrogen(V) with 3,4-xylenol under specified conditions to form a nitrate derivative. Subsequent destillation of the nitrate derivative and absorption in sodium hydroxide solution. Spectrophotometric measurement of the yellow nitrophenol at a wavelength of about 435 nm.