WCO Explanatory Notes reproduced for reference. © World Customs Organization. Verify against official WCO publication.
28.09 - Diphosphorus pentaoxide; phosphoric acid; polyphosphoric acids, whether or not chemically defined. - Diphosphorus pentaoxide 2809.20 - Phosphoric acid and polyphosphoric acids 2809.10 This heading covers diphosphorus pentaoxide, phosphoric acid (orthophosphoric acid or common hosphoric acid), pyrophosphoric (diphosphoric) acid, metaphosphoric and other polyphosp oric acids. R (A) DIPHOSPHORUS PENTAoXI.DE Diphosphorus pentaoxide (phosphorus (V) oxide, phosphorus pentoxide, phosphoric anhydride) (P205)is obtained by combustion, in air, of phos horns extracted fiom natural hosphates. It is a very corrosive white powder, absor s water avi y, and is transported in airtig t packings. It is used for drying gases, and in organic synthesis. R di' Diphosphorus pentaoxide exists in crystalline, amorphous or vitreous form. These three varieties mixed together give " phosphonc snow ",classified here. (B) PHOSPHORIC ACID Phosphoric acid (orthophosphoric acid or common phosphoric acid) (&Po4) is obtained by the action of sulphuric acid on natural tricalciurn hosphate. The commercial acid thus pre ared contains as mpurities di hosphorus pentaoxi e, calcium dihydrogenorthophos hate, su phur trioxide, sul huric acid, uorosilicic acid, etc. Pure phosphoric acld is obtaine by controlled hydration o diphosphorus pentaoxide. ? R B a P Phosphoric acid may be in the form of deliquescent prismatic crystals, but, as it is dificult to presewe in the solid state, it is usually put up in aqueous solutions (e.g., 65 %, 90 % The concentrated solution, which remains supersaturated at room temperature, 1s sometimes &own as " syrupy phosphoric acid ". Phosphoric acid is used to prepare concentrated (triple) superphosphates; it is also employed in the textile industries and as a pickling (rust-removing) agent. Phosphoric acid by condensation at high temperature ives rise to several polymeric acids : pyrophosphoric (diphosphoric) acid, metaphosphoric aci s and other polyphosphoric acids. f (C) POLYPHOSPHORIC ACIDS (I) Acids characterised by alternating P-0-P atoms are classified here. They may be derived formally by condensin two or more molecules of orthophosphoric acid with elimination of the elements of water. In t is way, a series of linear acids of general formula Hn+2Pn03n+l, where n is 2 or more, and a cyclic series of general formula (HP03),, where n is 3 or more, can be generated. % (I) Pyrophosphoric acid (diphosphoric acid, H4P207)is formed by controlled heating of orthophosphoric acid. It is unstable in moist air and readily reconverts to the orthoacid. (2) Meta hosphoric acids. These are the cyclic acids exemplified by cycIo-triphosphoric acid (HI'&), and cyclo-tetraphosphoric ac~d(HP03)&which occur as minor components of mixed pol hosphoric acids containing more than 86 % PZ05.Glacial pol hosphoric acid ? I metaphosphoric acid) is a non-chemically defined mixture o polyphosphoric (commercta acids mainly linear), whlch may also contain sodium salts thereof. Such mixtures, which are c assified here, occur as vitreous masses, volatilising at red heat and are uncrystallisable. YP I They are highly absorbent of water and are used for dryxng gases. (3) Other polyphosphoric acids of the P-0-P type. These are normally mixtures, marketed under the names " polyphosphoric " or " superphos horic " acids, which contain higher members of the series such as triphos horic aci (H5P3Ol0)and tetraphosphoric acid (W4oL3). These mixtures are also classi ed here. 2' a (11) Other polyphosphoric acids. acid) @I&Os). This This part includes, inter alia, hypo hos horic acid (diphosphoric corn ound is in the form of a crystal me hydrate which must be kept in a dry place. It IS more stab e in weak solutions. P P 'f The heading does not include : (a) Other phosphorus acids and anhydrides (phosphonic acid and its anhydrides, phosphinic acid) (heading 28.11). (b) Hydrogen phosphides (headiog 28.48).
Notes. 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 28.31), carbonates and peroxocarbonates of inorganic bases (heading 28.36), cyanides, cyanide oxides and complex cyanides of inorganic bases (heading 28.37), fulminates, cyanates and thiocyanates, of inorganic bases (heading 28.42), organic products included in heading 28.43 to 28.46 and 28.52 and carbides (heading 28.49), only the following compounds of carbon are to be classified in this Chapter : (a) Oxides of carbon, hydrogen cyanide and fulminic, isocyanic, thiocyanic and other simple or complex cyanogen acids (heading 28.11); (b) Halide oxides of carbon (heading 28.12); (c) Carbon disulphide (heading 28.13); (d) Thiocarbonates, selenocarbonates, tellurocarbonates, selenocyanates, tellurocyanates, tetrathiocyanatodiamminochromates (reineckates) and other complex cyanates, of inorganic bases (heading 28.42); (e) Hydrogen peroxide, solidified with urea (heading 28.47), carbon oxysulphide, thiocarbonyl halides, cyanogen, cyanogen halides and cyanamide and its metal derivatives (heading 28.53) other than calcium cyanamide, whether or not pure (Chapter 31). 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 32.06; glass frit and other glass in the form of powder, granules or flakes, of heading 32.07; (e) Artificial graphite (heading 38.01); products put up as charges for fire-extinguishers or put up in fire- extinguishing grenades, of heading 38.13; ink removers put up in packings for retail sale, of heading 38.24; cultured crystals (other than optical elements) weighing not less than 2.5 g each, of the halides of the alkali or alkaline-earth metals, of heading 38.24; (f) Precious or semi-precious stones (natural, synthetic or reconstructed) or dust or powder of such stones (headings 71.02 to 71.05), or precious metals or precious metal alloys of Chapter 71; (g) The metals, whether or not pure, metal alloys or cermets, including sintered metal carbides (metal carbides sintered with a metal), of Section XV; or (h) Optical elements, for example, of the halides of the alkali or alkaline-earth metals (heading 90.01). 4.- Chemically defined complex acids consisting of a non-metal acid of sub-Chapter II and a metal acid of sub-Chapter IV are to be classified in heading 28.11. 5.- Headings 28.26 to 28.42 apply only to metal or ammonium salts or peroxysalts. Except where the context otherwise requires, double or complex salts are to be classified in heading 28.42. 6.- Heading 28.44 applies only to : (a) Technetium (atomic No. 43), promethium (atomic No. 61), polonium (atomic No. 84) and all elements with an atomic number greater than 84; (b) Natural or artificial radioactive isotopes (including those of the precious metals or of the base metals of Sections XIV and XV), whether or not mixed together; (c) Compounds, inorganic or organic, of these elements or isotopes, whether or not chemically defined, whether or not mixed together; (d) Alloys, dispersions (including cermets), ceramic products and mixtures containing these elements or isotopes or inorganic or organic compounds thereof and having a specific radioactivity exceeding 74 Bq/g (0.002 μCi/g); (e) Spent (irradiated) fuel elements (cartridges) of nuclear reactors; (f) Radioactive residues whether or not usable. The term “isotopes”, for the purposes of this Note and of the wording of headings 28.44 and 28.45, refers to : - individual nuclides, excluding, however, those existing in nature in the monoisotopic state; - mixtures of isotopes of one and the same element, enriched in one or several of the said isotopes, that is, elements of which the natural isotopic composition has been artificially modified. 7.- Heading 28.53 includes copper phosphide (phosphor copper) containing more than 15 % by weight of phosphorus. 8.- Chemical elements (for example, silicon and selenium) doped for use in electronics are to be classified in this Chapter, provided that they are in forms unworked as drawn, or in the form of cylinders or rods. When cut in the form of discs, wafers or similar forms, they fall in heading 38.18. Subheading Note. 1.- For the purposes of subheading 2852.10, the expression “chemically defined” means all organic or inorganic compounds of mercury meeting the requirements of paragraphs (a) to (e) of Note 1 to Chapter 28 or paragraphs (a) to (h) of Note 1 to Chapter 29.