Microsoft Word - WaterChemistry

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Phosphorus (V) Compounds
Oxides
The most prevalent compounds of phosphorus are derivatives of phosphate (PO 4 3-), a
tetrahedral anion. Phosphate is the conjugate base of phosphoric acid, which is produced
on a massive scale for use in fertilizers. Being triprotic, phosphoric acid converts stepwise
to three conjugate bases:

H 3 PO 4 + H 2 O H 3 O+ + H 2 PO 4 − Ka1= 7.25×10−3

H 2 PO 4 − + H 2 O H 3 O+ + HPO 4 2− Ka2= 6.31×10−8

HPO 4 2− + H 2 O H 3 O+ + PO 4 3− Ka3= 3.98×10−13

Phosphate exhibits the tendency to form chains and rings with P-O-P bonds. Many
polyphosphates are known, including ATP. Polyphosphates arise by dehydration of
hydrogen phosphates such as HPO 4 2- and H 2 PO 4 -. For example, the industrially important
trisodium triphosphate (also known as sodium tripolyphosphate, STPP) is produced
industrially on a megaton scale via this condensation reaction:

2 Na 2 [(HO)PO 3 ] + Na[(HO) 2 PO 2 ] → Na 5 [O 3 P-O-P(O) 2 -O-PO 3 ] + 2 H 2 O

Phosphorus pentoxide (P 4 O 10 ) is the acid anhydride of phosphoric acid, but several
intermediates are known between the two. This waxy white solid reacts vigorously with
water.

With metal cations, phosphate forms a variety of salts. These solids are polymeric,
featuring P-O-M linkages. When the metal cation has a charge of 2+ or 3+, the salts are
generally insoluble; hence they exist as common minerals. Many phosphate salts are
derived from hydrogen phosphate (HPO 4 2-).

PCl 5 and PF 5 are common compounds. Both are volatile and pale or colorless. The other
two halides, PBr 5 and PI 5 PI 5 are unstable. The pentachloride and pentafluoride adopt
trigonal bipyramidal molecular geometry and are Lewis acids.

With fluoride, PF 5 forms PF 6 – , an anion that is isoelectronic with SF 6. The most important
oxyhalide is phosphorus oxychloride (POCl 3 ), which is tetrahedral.

Before extensive computer calculations were feasible, it was proposed that bonding in
phosphorus(V) compounds involved d orbitals. It is now accepted that the bonding can be
better explained by molecular orbital theory and involves only s- and p-orbitals on
phosphorus.

Nitrides
Compounds of the formula (PNCl 2 )n exist mainly as rings such as the trimer
hexachlorophosphazene. The phosphazenes arise by treatment of phosphorus
pentachloride with ammonium chloride: PCl 5 + NH 4 Cl → 1/n (NPCl 2 )n + 4 HCl The chloride
groups can be replaced by alkoxide (RO-) to give rise to a family of polymers with
potentially useful properties.