superphosphate after its
manufacture is what is known as reversion of the soluble phosphate. Thus
it is found that on keeping superphosphate for a long time the
percentage of soluble phosphate becomes less than it was at first. The
rate at which this deterioration of the superphosphate goes on varies in
different samples. In a well-made article it is practically
inappreciable, whereas in some superphosphates, made from unsuitable
materials, it may amount to a considerable percentage. The causes of
this reversion are twofold. For one thing, the presence of undecomposed
phosphate of lime may cause it. This source of reversion, however, is
very much less important than the other, which is the presence of iron
and alumina in the raw material. When a soluble phosphate reverts, what
takes place is the conversion of the monocalcic phosphate into the
dicalcic. Now in the first case, where reversion is due to the presence
of undecomposed phosphate, the action taking place may be represented as
follows:--

Lime } } { lime } }
Lime } phosphoric acid } { water} phosphoric acid }
Lime } } + { water} } =
(One molecule of insoluble } { (One molecule of soluble}
phosphate) } { phosphate) }

Lime } } { lime }
Lime } phosphoric acid } { lime } phosphoric acid.
Water} } + { water}
(One molecule of reverted } { (One molecule of reverted
phosphate) } { phosphate.)

It may be mentioned, however, that reversion from this cause probably
takes place to a very slight extent in practice.[230] Where reversion is
due to the presence of iron and alumina in the raw material, the nature
of the reaction is not well understood, and is consequently not so
easily demonstrated as in the former case. Where iron is present in the
form of pyrites, or ferrous silicate, it does not seem to cause
reversion. It is only when it is present in the form of oxide--and in
most raw phosphatic materials it is generally in this latter
form[231]--that it causes reversion in the phosphate.

_Value of reverted Phosphate._

The value of reverted phosphate is a subject which has given rise to
much dispute among chemists. That it has a higher value than the
ordinary insoluble phosphate is now admitted; but in this country, in
the manure trade, this is not as yet