FERTILIZERS 289
to the bottle which has been provided for waste platinum solutions and wash
the precipitate five times with 10-cc portions of the ammonium chloride
solution. Wash again thoroughly with SO-per cent alcohol, exercising par-
ticular care to remove ammonium chloride from the upper part of the filter.
Dry the precipitate for 30 minutes at 100°, cool and weigh. The weight of
potassium chlorplatinate is given without further treatment. The precipi-
tate should be completely soluble in warm water.
(6) In Commercial Potassium Chloride ("Muriate of Potash").—To 50 cc
of the solution already prepared add a few drops of hydrochloric acid and
10 cc of chlorplatinic acid solution, Evaporate over a steam bath to a thick
paste and .treat the residue as in the case of mixed fertilizers.
(c) In Potassium Sulphate, Potassium Magnesium Sulphate and Kainite.—
Acidify 50 cc of the solution with a few drops of hydrochloric acid, add 15 cc
of chlorplatinic acid solution and evaporate on the steam bath to a thick
paste. From this point proceed as with mixed fertilizers, except that 25 cc
portions of the ammonium chloride solution should be used in the washing
process.
The potassium is reported as per cent of potassium oxide (often called
"potash") instead of as the element.
Perchlorate Method.—In the discussion of methods for
the determination of potassium in soils, page 244, attention
was called to the fact that the increasing price of platinum has
greatly handicapped laboratory work of this character and that
methods not requiring the use of platinum solutions are rapidly
increasing in importance. The perchlorate method as described
for soil work is adapted also to fertilizer investigations. The
solutions of potassium, obtained by extraction of the fertilizer
for determinations by the chlorplatinate method, may be used for
this purpose, the determination of potassium in these being
performed exactly as directed for potassium in soils.
Centrifugal Method.—There is need for a short approximate
method for determining potassium which will fill somewhat the
same place as the Babcock method for determining fat in cream
and milk. A method has been devised by Sherrill1 which is
based upon a comparison of the volumes of precipitates of potas-
sium cobaltic nitrite formed from two solutions—the potassium
concentration of one being known. The precipitates are sep-
arated into graduated tubes by centrifugal action and the
volumes noted. The method seems to be fairly accurate and it is
useful when a rapid determination for factory control is necessary.
1 J. IncL Eng. Chem.} 13, 227 (1921).
19