62

QUANTITATIVE AGRICULTURAL ANALYSIS

I I

by adding the reagent drop-wise and stirring vigorously. This
method serves not only to minimize occlusion of the reagent but
also to prevent the formation of a very finely divided precipitate.
Determination of the Sulphate Radical.—Weigh duplicate samples of
0.25 gm of the sulphate into beakers and dissolve in 75 cc of distilled water.
Add 1 cc of dilute hydrochloric acid, heat to boiling and add, drop-wise and
with constant stirring, a clear 5-per cent solution of barium chloride until
the sulphate is completely precipitated. Digest on the steam bath until
the precipitate settles and the solution clears, then filter and wash with
hot distilled water, testing the washings finally with dilute sulphuric acid to
insure removal of barium chloride.
While the digestion of the precipitate is proceeding the crucibles should
be prepared. New crucibles generally lose weight slightly during the first
heating. Clean two porcelain crucibles and mark them with small symbols,
I and II (small dots are best), using an ordinary pen and ink. Allow the ink
to dry, then place the covered crucibles over a blast lamp or a No. 4 Me"ker
burner and heat with the full flame for 30 minutes. Remove the flames and
allow the crucibles to cool to below redness, then place them in the desic-
cators and, after 15 minutes standing, weigh accurately, handling only with
the tongs. The rubber tipped tongs are conveniently used for the cold
crucibles.
After the paper and precipitate has been washed free from soluble salts,
drain thoroughly and then slip the paper up the side of the funnel and fold
as shown in Fig. 9. Place- the folded paper in the weighed crucible. The
crucible is then inclined on the triangle, as indicated in Figs. 10 and 11, and
the flame of the ordinary burner is applied, gently at first to avoid loss of
precipitate by spattering. After the paper has become dry the temperature
is raised, the burner being placed under the bottom of the crucible so that
warm air, and not products of combustion, pass through the crucible.
Proceed in this way until all carbon has been oxidized and the precipitate is
white, but without allowing the crucible to become more than a dull red.
When the precipitate is quite white the covered crucible is cooled in the
desiccator for 15 minutes and weighed. The difference between this and
the first weight represents barium sulphate, from which the per cent of
the sulphate radical, of sulphur trioxide or of sulphur is calculated.
In order to confirm, the accuracy of the work the covered crucible is
heated for additional periods of 10 minutes and cooled and weighed after
each heating. The weight should not change more than about 0.2 mg after
such heating, unless the temperature has been carried too high. If any
trouble has been experienced in obtaining constant weight it may be well
to add a drop of dilute sulphuric acid to the cooled material, then to evaporate
carefully over a flame, and finally to heat gently and reweigh. This will
correct for the formation of barium sulphide or oxide, as already explained.
Volumetric, by Titration with Standard Base or Carbonate.—
Just as the chloride of hydrogen (hydrochloric acid) may be