288

QUANTITATIVE AGRICULTURAL ANALYMlti

filtrate measures about 200 cc. Add to the filtrate 2 cc of concentrated
hydrochloric acid, heat to boiling, transfer to a 250-cc volumetric flask and
add to the hot solution a slight excess of ammonium hydroxide and sufficient
ammonium oxalate to precipitate all of the calcium. Cool, dilute to 250 cc,
mix and pass through a dry filter. Reject the first 25 cc of the filtrate.
(6) Simple Potassium Salts, Potassium Magnesium Sulphate and Kainite.—
Dissolve 2.5 gm of sample in a 250-cc volumetric flask and dilute to the
mark without the addition of ammonium hydroxide or ammonium oxalate.
(c) Organic Compounds: Cotton Seed Meal, Tobacco tite.mx, /<^c.—Saturate
10 gm of sample with concentrated sulphuric acid, them evaporate and
ignite at a temperature not above that of dull redness to destroy organic
matter. A muffle furnace will be found to be convenient for thih ration.
Add a little concentrated hydrochloric acid and warm slightly in order to
loosen the mass from the dish. Wash into a 500-cc volumetric flask, add
ammonium hydroxide and ammonium oxalate to precipitate calcium, dilute
to the mark and mix well. Filter through a dry paper and reject the first
25 cc of the filtrate.
(d) Ashes from Wood or Cotton Hulls.—Digest 10 gm with 300 cc, of boiling
water for 30 minutes in a covered flask. Precipitate calcium with ammo-
nium hydroxide and ammonium oxalate, as directed under (a), above, rinse
into a 500-cc flask, dilute to the mark and mix well. Filter through a dry
paper and reject the first 25 cc of the filtrate.
Determination of Potassium: (a) In Mixed Fertilizers and A«fos.—The
principles underlying the determination of potassium have been diHCusncd
under the head of soil analysis, page 244. Directions for the preparation of
chlorplatinic acid solution and of 80-per cent alcohol also have been given.
Prepare, in addition, a 20-per cent ammonium chloride solution, saturated
with potassium chlorplatinate by agitating occasionally for several hours,
after having added about 10 gm of the salt for each 500 cc of solution.
Allow to settle and then filter.
Evaporate 50 cc of the prepared solution nearly to dry ness in a dish, add
1 cc of sulphuric acid (1 to 1), evaporate to dryncss and ignite at a dull red
heat until organic matter is removed and the residue is white. Dissolve the
residue in hot water, using at least 20 ec for each decigram of potassium
oxide present, add a few drops of concentrated hydrochloric acid and
enough chlorplatinic acid to precipitate all of the potassium and to leave
about 1 cc of platinum solution in excess. If the per cent of potassium is
approximately known the quantity of platinum solution that is necessary
should be calculated. Contamination with ammonia vapor must be avoided.
Evaporate the solution on a steam bath to a thick paste, cool and add to
the residue 25 cc of 80-per cent alcohol. Stir thoroughly and allow to stand
for 15 minutes. Filter through a weighed Gooeh crucible. If the nitrate is
not colored, sufficient chlorplatinic acid solution is not present and the
analysis must be begun again with another portion of the; solution, increasing
the amount of platinum solution.
Wash the precipitate with 80-per cent alcohol, continuing the washing
after the filtrate has become colorless. Remove the filtrate and washings