SOILS 253 |!H
particles and the length of heating. It is therefore obvious that tj H
such an extraction constitutes only a conventional division into j ] |
somewhat arbitrary classes of materials. "^
Other Inorganic Constituents.—The methods outlined in the jj^
following pages may be applied to the analysis of the original ||| I i
soil or of an acid extract, as explained above. As the soil always -j| j j
contains a large proportion of materials insoluble in acids the jj<jf]
principal analysis must be preceded by a decomposition such as
will bring the sample into complete solution, with the exception
of silica, which is separated and weighed. Two methods for the
decomposition will be discussed.
Decomposition of Soil Sample: (a) Hydrofluoric Acid Method.—
The soil is treated with hydrofluoric acid and a small amount
of sulphuric acid. Silica of silicates is volatilized as silicon
tetrafluoride:
Si02 + 4HF -» SiF4 + 2H20.
Metals are left as sulphates, which decompose by ignition, leaving j$»,
oxides. These are, for the most part, soluble in hydrochloric if II1
acid. This method of soil decomposition is often used where a \if
determination of silica is not required. It obviates, to some ,j'
extent, the difficulties which are encountered when silica is [M
determined in the same sample with the other constituents but j
the residue of oxides is often difficult to dissolve and the fusion 1J | |
method is to be preferred. [ jj
(6) Fusion with Alkali Carbonates.—The decomposition of a
soil by fusing with sodium carbonate is of greatest use when total
silica is to be determined. The silica is usually chiefly com-
bined with sodium and aluminium in albite (NaAlSi3O8), with
potassium and aluminium in orthoclase (KAlSi3Og), or with
aluminium in clay (Al2Si2O7^H20). When orthoclase, for |j
example, is fused with sodium carbonate, the reaction which
takes place may be represented as follows:
2KAlSi808 + 6Na2C03~*K2Si03 + 5Na2Si03 + 2NaA102 +
6C02. (1)
The silicates formed in the above reaction may be decomposed
easily by hydrochloric acid, forming soluble alkali chlorides and
silicic acid:
Na2Si03 + 2HC1 -> H2Si03 + 2NaCL (2)