GROUP V 247
which contain more than one phosphorus atom and P—O—P
bonds. Structures include both ring and chain forms. Separation of
these complex anions can be achieved by ion exchange and chroma -
tography.
Two examples of condensed phosphoric(V) acids are heptaoxo-
diphosphoric(V) (pyrophosphoric) and polytrioxophosphoric (meta-
phosphoric) acids.Heptaoxodiphosphoric acid, H 4 P 2 O 7 , as its old name suggests, is
formed as one product when phosphoric(V) acid is heated (loss of
water on heating leads to a mixture of acids). It forms two series of
salts, the sodium salts, for example, have the formulae Na 2 H 2 P2O 7
and Na 4 P2O 7.
In solution, both heptaoxodiphosphoric(V) acid and the hepta-
oxodiphosphates(V) (pyrophosphates) are slowly converted (more
rapidly on heating) to phosphoric(V) acid or its salts, for example
H 4 P 2 O 7 + H 2 O -» 2H 3 PO 4Polytrioxophosphoric(V) acid, (HPO 3 )n? is formed as a polymeric
glassy solid when phosphoric(V) acid is heated for a long period. It
may also be obtained in solution by passing sodium polytrioxo-
phosphate(V) through a cation-exchange column. It is a monobasic
acid, forming only one set of salts, but the simple formula, NaPO 3 ,
for the sodium salt, is misleading since there are many polytrioxo-
phosphates known of general formula (NaPO 3 )^ where n may be
3, 4 or a much larger number.
A salt originally called sodium hexametaphosphate, with n be-
lieved to be 6, is now thought to contain many much larger anion
aggregates. It has the important property that it "sequesters', i.e.
removes, calcium ions from solution. Hence it is much used as a
water-softener.
Arsenic
THE + 3 ACIDS
Arsenic(III) (arsenious) acid, H 3 AsO 3 .—When arsenic(III) oxide is
dissolved in water the corresponding acid is formed :
As 4 O 6 + 6H 2 O ^ 4H 3 AsO 3
It is an extremely weak acid but does form salts. Two kinds are
known, trioxoarsenates(III), for example Na 3 AsO 3 , and dioxo-
arsenates(III), for example Cu(AsO 2 ) 2 -