246 QUANTITATIVE AGRICULTURAL ANALYSIS
glazed paper, and then replace the paper in the funnel. Place the crucible
under the funnel and dissolve the remainder of the precipitate in the smallest
amount of nearly boiling water, allowing the solution to run into the crucible.
Evaporate to dryness on the steam bath, carefully wipe the outside of the
crucible with a clean towel and dry for 30 minutes at 105°. Weigh and
calculate the per cent of potassium in the soil.
Use of a Gooch or Alundum Crucible.—Proceed as above until ready to filter
out the potassium chlorplatinate. Prepare two Gooch niters as directed
on page 50, paying attention to the precautions suggested, and using strong
suction in forming the asbestos felt; or wash alundum crucibles with hot
water, using suction. Finally rinse the crucibles with alcohol, remove, wipe
the outside and dry at 100° to 105° for 30 minutes or until the weight is
constant. Weigh and replace in the holder. If Gooch crucibles were used,
moisten the asbestos with one or two drops of alcohol before the suction
pump is again turned on. Start the pump, then filter and wash the precipi-
tate exactly as above directed. Remove the crucible, dry in the oven and
weigh. Calculate potassium as before.
Recovery of Platinum from Waste and Preparation of Chlorplatinic
Acid.1—Place the waste solutions in an evaporating dish having a capacity
of 2 liters for each 100 gm of platinum and evaporate until most of the water
has been expelled. Make basic with sodium hydroxide and add, stirring,
sodium formate, either solid or in concentrated solution. A quantity of
sodium formate equal to about half the weight of platinum to be reduced
will be required. If foaming occurs, add more sodium hydroxide. Heat
on the steam bath for one hour, stirring occasionally, then acidify with
hydrochloric acid (25-per cent solution) stirring during the addition of acid.
Filter off the precipitated platinum on a soft paper, using suction. Wash
twice with hot 2-per cent hydrochloric acid, then with hot water until free
from acid. Separate the platinum from the paper, dry, ignite and weigh.
Pour over the plabinum in a porcelain dish five times its weight of 25-per cent
hydrochloric acid, heat on the steam bath and add slowly 50-per cent nitric
acid until no more gas is evolved. About 1 cc of nitric acid will be required
for each gram of platinum.
After the platinum is in solution, add 10 cc of 25-per cent hydrochloric
acid, evaporate to small volume and repeat this process twice. This
reduces and eliminates nitric acid. Dilute with water and evaporate, two or
three times, to expel hydrochloric acid. Finally dilute, cool and filter on a
soft paper whose approximate weight is known. If the filtrate is not per-
fectly clear, refilter. Wash the paper free from any platinum stain and if
any appreciable residue remains on the paper, dry and weigh it on the filter.
Correct the weight of platinum for this weight of carbon, or other residue,
then make the solution to the desired concentration. For potassium
determinations the solution should contain 10 per cent of the element
platinum.
1 BELONG, Chem. Weekblad, 10, 833 (1914).