Phosphorous acid, in solid or liquid (aqueous) forms, also denominated phosphonic acid, usually falling under Chemical Abstracts Service ('CAS') numbers 13598-36-2 and 10294-56-1. The Customs and Statistics (‘CUS’) numbers usually corresponding to this product are 0021895-1 and 0043878-8.

1. Except where the context otherwise requires, the headings of this chapter apply only to: (a) separate chemical elements and separate chemically defined compounds, whether or not containing impurities; (b) the products mentioned in (a) above dissolved in water; (c) the products mentioned in (a) above dissolved in other solvents provided that the solution constitutes a normal and necessary method of putting up these products adopted solely for reasons of safety or for transport and that the solvent does not render the product particularly suitable for specific use rather than for general use; (d) the products mentioned in (a), (b) or (c) above with an added stabiliser (including an anti-caking agent) necessary for their preservation or transport; (e) the products mentioned in (a), (b), (c) or (d) above with an added anti-dusting agent or a colouring substance added to facilitate their identification or for safety reasons, provided that the additions do not render the product particularly suitable for specific use rather than for general use. 2. In addition to dithionites and sulphoxylates, stabilised with organic substances (heading 2831), carbonates and peroxocarbonates of inorganic bases (heading 2836), cyanides, cyanide oxides and complex cyanides of inorganic bases (headings 2837 to 2842), and other inorganic compounds, this chapter covers: fulminates, cyanates and thiocyanates, of inorganic bases; organic compounds of headings 2843 to 2846 and 2852; and hydrogen peroxide solidified with urea (heading 2847). 3. Subject to the provisions of Note 1 to Section VI, this chapter does not cover: (a) sodium chloride or magnesium oxide, whether or not pure, or other products of Section V; (b) organo-inorganic compounds other than those mentioned in Note 2 above; (c) products mentioned in Note 2, 3, 4 or 5 to Chapter 31; (d) inorganic products of a kind used as luminophores, of heading 3206; glass frit and other glass in the form of powder, granules or flakes, of heading 3207; (e) artificial graphite (heading 3801); products put up as charges for fire-extinguishers or put up in fire-extinguishing grenades, of heading 3813; ink removers put up in packings for retail sale, of heading 3824; cultured crystals (other than optical elements) weighing not less than 2.5 g each, of heading 3824.