244 QUANTITATIVE AGRICULTURAL ANALYSIS
In the solution of salts finally obtained potassium may be
separated from sodium by the chlorplatinate or the perchlorate
method or it may be precipitated as potassium sodium cobalti-
nitrite and a volumetric method used for its determination. The
accuracy of the various methods is in the order named, although
the high cost of platinum is a great
obstacle to the continued use of the
chlorplatinate method and its re-
covery from residues involves con-
siderable expense and loss of metal in
each operation.
Chlorplatinate Method.—This con-
sists in the precipitation of potassium
chlorplatinate from an alcoholic solu-
tion by chlorplatinic acid:
2KC1 + H2PtCl6 -> K2PtCl6 + 2HCL
Sodium chlorplatinate is soluble in
alcohol and this fact is used in its
separation from potassium. Ammo-
nium chlorplatinate, also, is only
slightly soluble in alcohol. It is
therefore necessary that ammonium
salts be expelled by heating, before the
„„ T _ „ . . ... reagent is added, and that the work be
FIG. 56.—,T. L. Smith crucible. , . . .
done in a room tree from ammonia.
Decomposition of Soil Sample.—Grind in an agate mortar 0.5 gm of soil,
accurately weighed, with 0.5 gm of ammonium chloride. When thoroughly
mixed, add 4 gm of precipitated calcium carbonate and mix well by grinding.
Place about 2 gm of calcium carbonate in the bottom of the crucible then
add the ground mixture from the mortar and rinse the latter with about
0.5 gm of calcium carbonate. Brush the mortar well and add any traces
of material to the charge in the crucible. Settle the mixture well by tapping
gently, place the crucible in a hole in an asbestos board and heat in such a
way that only the lower portion is reddened. After ammonia ceases to
escape, turn on the full heat of the burner to all but the upper portion of
the crucible and continue the heating for 45 minutes. The crucible should
be red hot. The entire heating may be done more conveniently by placing
the asbestos carrying the crucible over the top of a small electric furnace of
crucible type, the lower portion of the crucible being in the furnace. The