This section is from the book "A Treatise On Beverages or The Complete Practical Bottler", by Charles Herman Sulz. Also available from Amazon: A Treatise On Beverages.
Citrate of iron, nine parts; pyrophosphate of sodium, ten parts; distilled water, eighteen parts. Dissolve the citrate of iron in the distilled water, by heating on a water-bath. To this solution add the pyrophosphate of sodium and stir constantly until it is dissolved. Evaporate the solution, at a temperature not exceeding 60° C. (140° F.), to the consistence of thick syrup, and spread it on plates of glass, so that, when dry, the salt may be obtained in scales. Keep the product in well-stopped bottles, in a dark place. Thin, apple-green, transparent scales, permanent in dry air when excluded from light, but turning dark on exposure to light, odorless, having an acidulous, slightly saline taste, and a slightly acid reaction. Freely and completely soluble in water, but insoluble in alcohol. Add the iron pyrophosphate in its scaly state to water in fountain.