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).
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.