r Kuril. i z EH* 27.5

ing to whether one, twn or three atoms of hydrogen are 2sul>-
stitiitecl for ruldmtL The Lust t w named phosphates are easily
soluble in water. \vhereus liicaldum phosphate is nearly insoluble
(0.136 gin in gm Of water at 20°) but soluble in soil acids.

In practice the reaction is never allowed to proceed as far as the
formation of phosphoric acid.

The possible reactions involved in the commercial process are
indicated as follows :

Can(Pl)4i, 4- H2SO4 — CaSO4 + 2CaHPO4, (1)

Diealeium phosphate

Cas(P04) 2 + 2H2SO4 — 2CaS04 + Ca(H2P04)2, (2)

» Monocaicium phosphate
("Superphosphate'*)

Ca3(P04)2 + 3H2SO4 — 3CaSO4 + 2H3PO4. (3)
Phosphoric acid
Sulphuric acid of 60-per cent concentration is most suitable
for making acid phosphates because this produces the maximum
quantity of monocaleium phosphate, the "water-soluble form.
"Reversion" may occur during storage if unchanged tricalcium
phosphate remains in the mixture. This is due to the inter-
action of rxionocalciuni phosphate with tricalcium phosphate, the
dicalcium salt being produced :
Ca.i(;P(>4>2 + Ca(H2P04)2 -^ 4CaHPO4.
Measure of Availability. — -Dicalcium phosphate Is soluble in
salt solutions, such as ammonium citrate, as well as In salt or
acid soil solutions. Hence both citrate-soluble and water-soluble
phosphorus are rated as available to plants. The phosphate
found in bone is in the form of the tricalcium phosphate but in
this case it is in a more porous condition and it is also inter-
mingled with organic matter. It is soluble to the extent of SO
to 40 per cent in ammonium citrate solution and it is somewhat
soluble in soil acids and salts.
The principles Involved in the determination of phosphorus in
phosphates are discussed on pages 87 to 92, Part I. This should
be reread before beginning the following determinations.
Determittttioa of Total' Phosphorus. — The choice of method for dissolving
the sample will depend upon the nature of the latter.