ISO 787-19:2020,General methods of test for pigments – Part 19: Determination of water-soluble nitrates (Salicylic acid method).
ISO 787-19:2020 specifies a general method of test for determining the water-soluble nitrates in a sample of pigments by a spectrophotometric method using salicylic acid.
ISO 787-13 specifies a method for determining the water-soluble nitrates in a sample of pigments using Nessler’s method.
The nitrate present in the extract of the pigment sample is used to nitrate salicylic acid in sulphuric acid medium. The nitro-compound formed is of an intensive yellow colour in alkaline solution and the colour is measured spectrometrically at a wavelength of 410 nm.
Pipette into a 250 ml one-mark volumetric flask 50 ml of the clear aqueous extract obtained, as appropriate for the pigment under test, by the hot extraction method (see ISO 787-3)， or the cold extraction method (see ISO 787-8).Make up to the mark with water and mix.
ISO 787-19:2020 pdf can be downloaded here and the size of the file is 1.81M.