Ferric alums double salts

After adding (NH4)2S04 to the solution so obtained, the double salt, ammonium ferric sulphate, or ferric alum, may be crystallized out. In order to obtain a satisfactory product it is very important to adjust the amount of add very precisely. Too little acid will allow a brown basic salt to form (mFe(0H)3-nFe2(S04)3), which will discolor the preparation. Too much acid has the obvious disadvantage that it will cling to the crystals of the product from which it cannot be removed by evaporation or by washing (because of the great solubility of the product). 

A complex salt is to be distinguished from a double salt such as ferric alum (ferric ammonium sulfate). Double salts exist only in the sohd state, and then only because the different constituent ions happen to fit into a common crystal lattice. In solution, double salts give aU the reactions of their constituent simple ions (Experiment 4). 

Mitscherlich also measured the angles of the crystals of double sulphates of potassium and ammonium with magnesium, ferrous iron, manganese, zinc, copper, cobalt and nickel. This established the isomorphism of potassium and ammonium sulphates. He found that the double potassium sulphates had one proportion of water more than the double ammonium sulphates, and also obtained a salt crystallising in beautiful and large octahedra with exactly the same composition as alum but containing ferric oxide instead of alumina (ferric alum). He says he hoped to show that a study of crystallisations would give the compositions of bodies as certainly and definitely as chemical analysis . A letter from Humboldt to Mitscherlich said this beautiful work had exposed the incorrectness of Haiiy s view. ... 

The Preparation of Alums.—Since aluminum sulfate is a commercial chemical, the aluminum alums can be prepared by crystallizing the salt along with the double molecular weight of an alkali sulfate. Chromium and ferric sulfates are not well crystallized and have limited uses, so that they are not readily obtained. The preparation of their alums involves the preliminary preparation of the sulfates themselves. 

The bronze age did not arrive in China until around 1500 bce, and iron appeared only about 500 bce, but by the beginning of their alchemical age, around 100 ce, the Chinese had knowledge of zinc and brass (a copper-zinc alloy), mercury, sulfur, and several of the common salts, such as niter. In the 400s bce an administrator made a list of known materials that included lead oxide, lead carbonate, sulfur, calcium carbonate, hydrated calcium sulfate, ferric oxide, niter, talc or soapstone, hydrated magnesium silicate, alum (the double sul-... 

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