258

QUANTITATIVE AGRICULTURAL ANALYSIS

Afl,<>,r solution appears to be complete, cool and place the crucible on its side
in 50 cc of hot water in a beaker or casserole. Warm to hasten solution of
the mass of sulphates. Remove and rinse the crucible, heat to boiling and
add 1 cc of 5-per cent stannous chloride solution to reduce the iron. Cool
rapidly in running water and add, all at once, 50 cc of 5-per cent mercuric
chloride solution. This must produce a precipitate of pure white mercurous
chloride. If no precipitate is produced, not enough stannous chloride was
added. If the precipitate is gray, instead of white, too much stannous
chloride was used.
Titrate at once with standard potassium dichromate, following the
details of the method described on page 74, Part I. Or the determination
may be made with standard potassium permanganate as directed on page
72, Part I. Calculate the per cent of the total oxide and phosphate residue
and of ferric oxide in the soil sample. The latter, together with the per cent
of phosphorus pentoxide (as determined in a separate sample), subtracted
from the per cent of total residue, gives the per cent of aluminium
oxide and titanium oxide. The titanium is usually ignored, as
already stated.
Determination of Aluminium: Direct Method.—Make the double pre-
cipitation of hydroxides and wash free from chlorides, as directed above,
saving the filtrates and washings for the determination of calcium and
magnesium.
Place a 500-cc beaker under the filter and redissolve the precipitate with
warm dilute hydrochloric acid. Pierce the filter and wash the paper well
with hot water. Dilute the solution to about 400 cc. Add 30 cc of a 10-per
cent solution of ammonium phosphate and then stir and add dilute ammo-
nium hydroxide until a precipitate appears. Add 1.5 cc of concentrated
hydrochloric acid and 50 cc of a 20-per cent solution of sodium thiosul-
phate and boil for a few minutes. Now add 15 cc of a 20-per cent solution
of ammonium acetate and 8 cc of 30-per cent acetic acid and boil for 15
minutes. The colloidal aluminium phosphate becomes granular and it is
then easily filtered and washed. Save the filtrate and washings for the
iron determination unless the original precipitate of hydroxides was per-
fectly white, indicating the presence of no more than a trace of iron.
Redissolve the phosphate on the filter with concentrated hydrochloric
acid, wash through with hot water and reprecipitate aluminium phosphate
exactly as before. Wash with hot water, ignite and weigh as aluminium
phosphate, A1PO4. Calculate the per cent of aluminium oxide in the
sample.
Determination of Calcium.—Evaporate the combined filtrates from.
aluminium and iron hydroxides to about 50 cc, cool, add ammonium sul-
phide to precipitate the manganese, filter and wash with hot water. Discard
the precipitate. Again evaporate the solution to about 50 cc, make slightly
basic with ammonium hydroxide and add, while still hot, 4-per cent ammo-
nium oxalate solution, dropwise and with stirring, so long as any precipitate
is produced. Heat to boiling, allow to stand one hour or longer, decant
the clear solution on a filter, pour about 20 cc of hot water on the precipitate