246 GROUP V
The solution obtained is evaporated somewhat, cooled in a vacuum
desiccator and the crystals of the tetraoxo-acid filtered off; too
drastic evaporation causes formation of the heptaoxodiphosphoric
acid by loss of water.
Industrially, phosphoric(V) acid is manufactured by two pro-
cesses. In one process phosphorus is burned in air and the phos-
phorus(V) oxide produced is dissolved in water. It is also manu-
factured by the action of dilute sulphuric acid on bone-ash or
phosphorite, i.e. calcium tetraoxophosphate(V), Ca 3 (PO 4 )2; the
insoluble calcium sulphate is filtered off and the remaining solution
concentrated. In this reaction, the calcium phosphate may be
treated to convert it to the more soluble dihydrogenphosphate,
Ca(H 2 PO 4 ) 2. When mixed with the calcium sulphate this is used as
a fertiliser under the name 'superphosphate'.
Tetraoxophosphoric acid is a colourless solid, very soluble in
water ; an 85 % solution is often used (ksyrupy phosphoric acid'). It is
tribasic, giving the ions :
H^OJ ^HPOr -PO^~
(tetrahedral)
decreasing hydrogen ion concentration
decreasing solubility of salts
In anhydrous phosphoric(V) acid, tetrahedral PQ^ groups are
connected by hydrogen bonds, a structure which can be representedV
P-- Q--H--**H—0 O— H
The dotted lines represent the hydrogen bonds and it is these bonds
which are responsible for the syrupy nature of the acid.
The tetraoxophosphates, except those of the alkali metals, sodium,
potassium, rubidium, caesium (and ammonium), are insoluble in
water but are brought into solution by the addition of acid which.
as shown, effects a change from the ion PO^ (with three negative
charges) to the ion H 2 PO4 (with one); this change increases the
solubility. Organic phosphatesfV) are of great importance in
biological processes, for example photosynthesis. The nucleic acids
have chains in which carbon atoms are linked through PO*~ groups,CONDENSED PHOSPHATES(v)
In addition to the above acids and anions which contain only one
phosphorus atom there are many other condensed phosphates(V)