Laboratory Methods of Inorganic Chemistry, 2nd English Ed. 1928

PLATINOTYPES 153

bottom layer into a dry beaker, free it from drops of water by pour-

ing it back and forth into dry beakers, and remove the ether by

warming it for some time upon the water bath. Dissolve the resi-

due in a little water, filter the solution, concentrate it to a small

volume, and bring it to crystallization by surrounding the beaker

with ice. If no crystals separate, the solution must be concen-

trated still further. Drain the crystals quickly from the liquid; do

not wash this product, but free it from mother-liquor by spreading

it on an unglazed plate. After the crystals have become white on

the porous plate, dry them completely by letting them remain in

a vacuum desiccator over sulphuric acid. Yield, 13 to 14 g.

When working with larger amounts of silicotungstic acid, large

characteristic crystals can be obtained by slow evaporation in a

vacuum desiccator.

ORGANOCOMPLEX COMPOUNDS.

118. Potassium Ferric Oxalate, K 3 [Fe(C 2 O 4 )3]; Platinotypes.
     Oxidize a solution of 35 g. of crystallized ferrous sulphate in
     100 c.c. of water by boiling it with just the necessary amount of
     nitric acid (test with potassium ferricyanide). Dilute the liquid
     to 2 liters, add ammonia and wash the precipitate of hydrated ferric
     oxide by decantation for several days, then collect it on a large,
     plaited filter and wash it with hot water.
     To a hot solution of 44 g. of crystallized acid potassium oxalate
     in 100 c.c. of water add the ferric oxide hydrogel a little at a time
     until no more will dissolve. Such a solution is sensitive to direct
     sunlight. Filter and concentrate the filtrate to crystallization,
     wash the emerald-green crystals with water and alcohol, and dry
     them in a vacuum desiccator over sulphuric acid.

Platinotypes.
Potassium ferric oxalate is changed by the action of light to potassium
ferrous oxalate, and the latter reduces platinum salts to metallic platinum.

Soak a piece of filter paper, the size of a photographic plate,

with potassium ferric oxalate solution and dry it in the hot closet.

Then place the paper in a shallow glass tray; wet it uniformly with

about 2 c.c. of a 5% chloroplatinic acid solution, whereupon it

turns yellowish red; and dry it again, avoiding any strong illumi-

nation. Then expose it for about an hour to a medium light under