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What chemical compounds might be present in drinking water?
What chemical compounds might be present in drinking water?

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3.3.1 Polyphosphates

Phosphate tetrahedra, PO43−, can link up by oxygen-sharing to give polyphosphates in the form of rings and chains (Figures 12 and 13). The phosphates, unlike the silicates, contain a central atom with a maximum valency of five. Thus, the maximum number of oxygen atoms that each tetrahedron can share is three, since there must always be one vertex of the tetrahedron that is a terminal oxygen, bound as P=O.

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Figure 12 (a) Phosphorous(III) and (b) phosphorous(V) oxide.
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Figure 12 A key identifies a small red ball as meaning 'O' and a slightly larger orange ball as meaning 'P'.

At the top of the diagram in part (a) there is an orange ball. Three lines radiate out and down from the orange ball, each ending at a red ball. Next to one of these lines it says 165.6 pm. From each red ball a line goes roughly vertically down. The angle between the lines above and below the red balls is shown as 127.0 degrees. Each vertical line ends with an orange ball. Between each of these three orange balls there is a red ball, a little lower down and a little further out. Each orange ball has two lines which go down and out to the red ball on either side of it, and the angle between these lines is shown as 99.5 degrees.

At the top of the diagram in part (b) there is a red ball. A line of length 143 pm goes vertically down to an orange ball. Three lines radiate out and down from the orange ball, each ending at a red ball. The angle between two of these lines is shown as 102 degrees. From each red ball a line of length 160 pm goes roughly vertically down. The angle between the lines above and below the red balls is shown as 123.0 degrees. Each vertical line ends with an orange ball. Three lines radiate out and down from each of these orange balls, each ending at a red ball. The three red balls which are in between two orange balls, have lines to the orange balls on both sides of them, while the remaining three red balls are connected to just the nearest orange ball.

Figure 12 (a) Phosphorous(III) and (b) phosphorous(V) oxide.

  • Consider phosphoric oxide, P4O10 (Figure 12b), how many P-O-P linkages does each phosphorus exhibit?

  • Each phosphorus atom forms the maximum three P-O-P linkages.

Condensed phosphates can be formed if phosphoric oxide is treated with limited amounts of water; or by dehydrating phosphorus oxoacids or their salts by heating.

Two molecules of phosphoric acid can condense by loss of a water molecule; the two tetrahedra share one oxygen, giving an acid of formula H4P2O7, known as 'pyrophosphoric', or (more correctly) diphosphoric acid. In practice, this can be obtained by heating phosphoric acid ('pyro' comes from the Greek word for fire). Continuation of this process gives triphosphoric acid, H5P3O10, and, eventually, chain polymers called metaphosphoric acids, which contain repeating [HPO3] units. In Figure 13 the ring polymers which are also shown are also called metaphosphoric acids.

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Figure 13 Metaphosphoric acid chains and rings.
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Figure 13 There are four structures composed of P, O and H.

The first structure goes across the top of the figure. It is a long line of alternating Ps and Os, connected by alternating single and then dashed lines. Above each P is a double vertical line to an O, while below and to one side of each P is an 'O H', so each P is tetrahedral.

In more details, starting from the left, a line goes down and to the right to an O, then a dashed line goes up and to the right to a P. From here a triangular wedge spreads down and to the left to 'H O', while a double line goes vertically up to an O. From the P a line goes down and to the right to an O, then up and to the right to another P. From here a triangular wedge spreads down and to the right to 'O H', while a double line goes vertically up to an O. From the P a dashed line goes down and to the right to an O, then up and to the right to another P. From here a triangular wedge spreads down and to the left to 'H O', while a double line goes vertically up to an O. From the P a line goes down and to the right to an O, then up and to the right to another P. From here a triangular wedge spreads down and to the right to 'O H', while a double line goes vertically up to an O. From the P a dashed line goes down and to the right to an O, then up and to the right to another P. From the P a dashed line goes down and to the right to an O, then up and to the right to another P. From here a triangular wedge spreads down and to the left to 'H O', while a double line goes vertically up to an O. From the P a line goes down and to the right to an O, then up and to the right.

The second structure, on the left of the figure, is a ring of three Ps, with an O between adjacent Ps. Above each P is a double vertical line going to an O, while sticking out from each P is an OH, so each P is tetrahedral. In more detail, starting at the rear left, there is a P. A dashed line goes down and to the left to an 'H O', while a double vertical line goes up to an O. From the P a line goes down and to the right to an O, then a dashed line goes up and to the right to another P. From the P a line goes down and to the right to an 'O H' while a double line goes up to an O. From the P a triangular wedge spreads down and to the left to an O, then a line goes up and to the left to another P. From the P a triangular wedge spreads down and to the left to an 'H O' while a double line goes up to an O. From the P a dashed line goes down and left to an O, then a triangular wedge points up and to the right to the first P described.

The third structure, on the right of the figure, is a small structure within brackets and labelled 'repeat unit'. At the centre is a P. From the P a double line goes vertically up to an O, while a triangular wedge spreads down and to the left to an O, from where a line goes down and left to an H. From the P, a dashed line goes down and left to an O, from where a triangular wedge spreads up and left to the left bracket. Also from the P a line goes down and to the right to an O, then up and to the right to the right bracket.

The final structure, at the bottom of the figure, is a ring of four Ps, with an O between adjacent Ps. Above each P is a double vertical line going to an O, while sticking out from each P is an OH so each P is tetrahedral. In more detail starting at the rear left, there is a P. A dashed line goes down and to the left to an 'H O', while a double vertical line goes up to an O. From the P a line goes down and to the right to an O, then up and to the right to another P. From the P a dashed line goes down and to the right to an 'O H' while a double line goes up to an O. From the P a triangular wedge spreads down and right to an O, then a dashed line goes up and left to another P. From the P a triangular wedge spreads down and right to an 'O H' while a double line goes up to an O. From the P a line goes down and left to an O, then up and left to another P. From here a triangular wedge spreads down and left to an 'H O' while a double line goes up to an O. From the P a dashed line goes down and left to an O, then a triangular wedge points up and right to the first P described.

Figure 13 Metaphosphoric acid chains and rings.

The P-O-P link in polyphosphates is readily hydrolysed; in excess water, metaphosphates revert to orthophosphate. So, unlike the condensed silicates, polyphosphates are never found as minerals. The hydrolysis of polyphosphates is an important source of energy within the body (Section 3.3.2).

Sodium polyphosphates are the best known; the general reaction for their formation is the dehydration of sodium dihydrogen phosphate by heating. The temperature of dehydration controls the nature of the product (Figure 14).

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Figure 14 The dehydration of sodium dihydrogen phosphate at various temperatures.
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Figure 14 On the left is the chemical formula 'Na H subscript 2 P O subscript 4'. Four arrows, each labelled with a temperature, point to the right.

The first arrow, which is labelled '250 degrees C', points to the formula 'Na subscript 2 H subscript 2 P subscript 2 O subscript 7'. To the right of this, in brackets, is a structure which starts with Na superscript + then superscript minus O, horizontal line P. Above the P a double vertical line goes to an O, while below the P a single vertical line goes down to an 'O H'. From the P, a horizontal line goes to an O, then on to another P. Once again, above the P a double vertical line goes to an O, while below the P a single vertical line goes down to an 'O H'. From the P, a horizontal line goes to an 'O superscript minus, then superscript + Na.

The second arrow, which is labelled 'greater than 250 degrees C', points to '( Na P O subscript 3 ) subscript n'. In brackets after this it says 'insoluble metaphosphate; n very large'.

The third arrow is labelled '630 degrees C' and points to '(Na P O subscript 3 ) subscript 3'. To the right of this is a structure: left square bracket, a structure containing 3 Ps and 9 Os, right square bracket, superscript '3 minus', 3 Na superscript +. The structure within the square brackets is a ring of three Ps, with an O between adjacent Ps. (The Os are all a little lower than the Ps.) Each P also has a double vertical line up to an O, and another O outside the ring, and once again a little lower than the P.

The final arrow is labelled '800 degrees C' and points to 'molten liquid (Na P O subscript 3 ) subscript n'.

Figure 14 The dehydration of sodium dihydrogen phosphate at various temperatures.
  • Partial dehydration at low temperature (c. 250 °C) produces disodium dihydrogen diphosphate, Na2H2P2O7, a substance used in baking powders as a slow-acting acid for the controlled release of CO2 gas, which produces the aerated texture of cakes.
  • Long-chain polyphosphates are produced by heating NaH2PO4 above 250 °C.
  • The sodium salt Na3P3O9, containing the cyclic P3O93− anion, is formed when NaH2PO4 is heated to 600-640 °C and the melt then maintained at 500 °C.
  • High-temperature dehydration (800 °C) above the melting temperature of NaH2PO4 (628 °C) yields a liquid with very high relative molecular mass, which gives slightly soluble solids of formula (NaPO3)n:
    nNaH2PO4 = (NaPO3)n + nH2O
    (Equation 29)
  • Heating together a 2 : 1 mixture of Na2HPO4 and NaH2PO4 at 450 °C produces a chain of only three phosphate units:
    2Na2HPO4 + NaH2PO4 = Na5P3O10 + 2H2O
    (Equation 30)

This triphosphate ion, P3O105−, is illustrated in Structure 9.

Structure 9