» Ferric Subsulfate Solution contains, in each 100 mL, basic ferric sulfate equivalent to not less than 20 g and not more than 22 g of iron (Fe). Ferric Subsulfate Solution may be prepared as follows. Add 55 mL of Sulfuric Acid to 800 mL of water in a porcelain dish and heat to nearly 100, then add 75 mL of nitric acid, and mix. Divide 1045 g of Ferrous Sulfate, coarsely powdered, into 4 portions, and add these portions one at a time to the hot liquid, stirring after each addition until effervescence ceases. If, after the Ferrous Sulfate has dissolved, the solution has a black color, add nitric acid, a few drops at a time, with heating and stirring, until red fumes cease to be evolved. Boil the solution until it assumes a red color and is free from nitrate, as indicated by the test for Limit of nitrate below, maintaining the volume at about 1000 mL by the addition of water as needed. Cool, and add enough water to make the solution measure 1000 mL. Filter, if necessary, until the Solution is clear.