PREPARATION OF FERRIC OXALATE.
Ferric oxalate is an extremely unstable compound, and difficult to procure, so that the experimenter may have to prepare it. In any case, it must be tested from time to time by taking a small quantity in a test tube and adding a few drops of solution of ferricyanide. If this turns blue, the pre sence of ferrous salt is indicated, whilst if the oxalate is in proper condition it will turn brown. To prepare ferric oxalate, in 4 oz. of water dissolve 333 grs. of pure ferric chloride (this must first be tested for the presence of ferrous salts as just de cribed), and add about oz. of strongest liquid ammonia. There must be a. slight excess of ammonia to ensure the complete conversion of the ferric chloride into the red-brown insoluble ferric hydrate. Per form this operation in a beaker with a lip, stir vigorously, and allow to stand for a time. A good plan is to put the precipitate in a well-washed linen bag and dissolve in the same. It may then, after standing, be
pulled together and the water wrung out of the precipitate. Next wash the precipitate with boiling water by filling up and wring ing out repeatedly, until the solution is no longer alkaline when tested with red lit mus paper. Now add about 270 grs. of crystallised oxalic acid to dissolve the pre cipitate. The action takes place slowly, and the solution should be left to stand for a few days in the dark. The exact quantity of ferric oxalate produced will depend on the purity of the chemicals and the care with which the operations are carried out. To determine accurately the strength of the solution, which should be a yellowish green, involves some know ledge of quantitative analysis, but it will suffice if the solution is made up to 700 minims, when it will be practically the No. 1 solution given above.