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.
Commercial citric acid frequently contains small quantities of calcium salts, sulphuric acid, due to imperfect manufacture, also traces of iron, lead and copper, these last being derived from the vessels used for the crystallization and evaporation of the acids. The presence of all these impurities is indicated by igniting a small quantity of the sample in a porcelain crucible; the ash usually varying from 0.05 to 0.25 per cent. When the proportion of ash does not exceed the latter amount, it is rarely of importance to examine it further, except for poisonous metals.
The presence of lead or copper will be readily indicated by dissolving the ash in a few drops of nitric acid, diluting largely, and passing sulphuretted hydrogen, or by testing a solution of citric acid with hydrosulphuric acid; a black coloration or precipitate indicates the presence of lead or copper.
Sulphuric acid is readily detected in the aqueous solution of citric acid by the white precipitate occurring on the addition of some chloride of barium
Iron is detected by the blue color occasioned by fer-rocyanide of potassium.
Calcium is detected by neutralizing the citric acid solution with ammonia, then to acidulate with acetic acid, and add oxalate of ammonium, when a white precipitate of oxalate of calcium will appear.