FERRIC OXALATE (Fr., Oxalate ferrique; Ger., Ferrioxalat) Synonym, iron sesquioxalate. Fe, Molecular weight, 376. Solubilities, very soluble in water, insoluble in alcohol. It occurs in greenish, glistening scales, which are extremely sensitive to light, and it is therefore usually preferred to mix it in solution and preserve in the dark. It is the most light-sensitive of any of the iron salts. It is used in the kallitype process, but its chief use is as the sensitive salt in the platinotype process. The following is the best method of preparing the normal ferric oxalate solution : Powder some ammonia-iron alum, weigh out Soo grs. or 52o g., place in a tall cylindrical graduate, capable of holding 20 oz. or r,000 ccs., and add 192 minims or 200 ccs. of liquor ammonia? (•88o), and an equal quantity of distilled water. Stir well for about five minutes, and allow to stand for a further five minutes. Then fill up with distilled water,
stir well, and allow the precipitated ferric hydrate to settle down. Next decant or siphon off the clear supernatant liquid, and repeat the process until the wash water is no longer alkaline to litmus paper. Then allow the precipitate to settle till it occupies not more than 17 oz. or 85o ccs. Add 2,064 grs. or 215 g. of pure oxalic acid in powder, stir well, and allow to stand in the dark-room until the precipitate is completely dissolved. Now filter the solution, and wash the filter paper with distilled water so as to make the total bulk of the solution 20 oz. or 1,00o ccs. This forms the " normal iron solution " for platinotype, and contains 20 per cent. of ferric oxalate with about 1.2 per cent. of oxalic acid.