PEATINGEY Ey 275 
supernatant liquid. Repeat these operations until the liquid 
no longer turns red litmus paper blue. Finally, allow the 
precipitate to settle, and syphon off the water until the total 
bulk measures no more than 850ccm (1314 0z.). Add the 
oxalic acid dry, stir well for a few minutes until the solution 
clears up, then filter, and wash the filter with enough water 
to make the total bulk 1000 ccm (160z.). The result will be 
a 20 per cent solution of ferric oxalate with an excess of 1.2 
per cent of oxalic acid. The addition of the oxalic acid and 
the subsequent filtration must be effected by artificial light, 
not daylight, and the solution must be kept in the dark. Dry 
ferric oxalate, as obtainable commercially, is rarely suitable. 
Another stock solution required is a mixture of lead-iron 
oxalate. This is made as follows: 
Lead acetate, pure 10g 154 gr. 
Water 100ccm 30z., 183 min. 
Dissolve by the aid of heat, and add: 
Oxalic acid, pure 4¢ OZ. or. 
Water 50 cem)) oz) 330) min 
A white precipitate of lead oxalate is formed, which should 
be collected on a filter, well washed with water, and dried. 
Add 1 part of the dry precipitate to 100 parts of the normal 
ferric oxalate solution, as given above. 
A stock solution of sodium ferric oxalate will also be 
required: 
Sodium ferric oxalate 500 g 3840 er. 
Water ~ 1000 ccm 16 oz. 
Shake until dissolved and filter. A stock solution of oxalic- 
gelatine will be required, but this will not keep more than 
3 or 4 days: 
Gelatine 10¢ 96 gr. 
Water 100 ccm 2 OZ. 
