Ferric potassium alum, preparation

Potassium ferri-sulphite, KFe(S03)S04, is prepared by the action of potassium hydrogen sulphite upon ferric ammonium alum. It crystallises in slender needles, which are sparingly soluble in cold water. With hot water, ferrous and potassium sulphates pass into solution, leaving an insoluble yellow residue. 

In 1863 R. C. Bottger of Frankfort-on-the Main found that thallium occurs in some spring waters. A certain salt mixture from Nauheim contained, in addition to the chlorides of sodium, potassium, and magnesium, those of cesium, rubidium, and thallium. Since he was able to prepare a thallium ferric alum exactly analogous to potassium ferric alum, he regarded thallium as an alkali metal (72, 73). Although it is sometimes univalent like sodium and potassium, it is now classified in Group III of the periodic system. 

Transfer the receiver and cork attachment to the neck of a 100-ml conical flask. Dissolve the mercury by allowing three 2-ml quantities of concentrated nitric acid to percolate through, wash with six 3-ml quantities of water, and remove the receiver. Place the flask on a water-bath for a few minutes, and add 5 per cent potassium permanganate solution until a permanent pink colour is obtained. Cool, and decolorise by adding a freshly prepared 2 per cent solution of ferrous sulphate. Cool to below 15°, add 0 5 ml of 10 per cent ferric alum solution, and titrate with 0 01, 0 02 or 0 05N ammonium thiocyanate until the solution has a distinct brown tinge (the titre should be between 5 and 10 ml). 

---