[503] Zeitsch. anorg. Chem. 1910, 67, 266.
The solubility-curve of thorium sulphate was examined by Demarçay and by Roozeboom. Three important hydrates are known, viz. Th(SO₄)₂,9H₂O, Th(SO₄)₂,8H₂O, and Th(SO₄)₂,4H₂O, other unstable intermediate compounds being said to exist. From a study of the diagram it will be seen that the hydrate with 8 molecules of water is labile, whilst the 9-hydrate and the 4-hydrate have a transition temperature at 43°C., the transition temperature of the 8-hydrate and the 4-hydrate being just below this.
Fig. 10
Since the 8-hydrate is labile with regard to the 9-hydrate, and the transition temperatures are so near, the former will be formed first as a solution cools, and by reason of the great similarity of the solubility-curves for the 9- and 8-hydrates the rate of change of this to the 9-hydrate will be very slow. In practice, therefore, it is always the 8-hydrate which is formed, and it is on the separation of this compound that the success of the process depends. The anhydro-compound, Th(SO₄)₂, which can be obtained by heating any of the hydrates to 300°-400°C., is very soluble at 0°, but slowly hydrates itself and separates from the solution as the 8-hydrate, which has a very low solubility. The sulphates of the cerium metals, compounds of which form the chief impurities to be removed, are considerably more soluble, and can be separated by repeated crystallisations.
The thorium hydroxide to be purified is dissolved in sulphuric acid, and in the first form in which the method was employed, the thorium sulphate obtained by evaporation of the solvent was heated until it became anhydrous. This was dissolved to saturation at 0°, and the solution raised to the boiling-point, the 4-hydrate being precipitated; this treatment was repeated several times. It was pointed out by Bunsen, from theoretical grounds, that this method could never yield a pure thorium salt, and Krüss and Nilson accordingly introduced a modification. The impure sulphate, after dehydration, as before, is dissolved at 0°, and allowed to come to ordinary room temperature, 20°; the hydrate which separates (the 8-hydrate) is collected and dried at high temperature and the crystallisation repeated. This method gives a fairly pure salt after three recrystallisations, but the process is very tedious, owing to the time required for drying and heating the hydrate. For this reason the method was further modified by Cleve and Witt. The crude sulphate is boiled with ammonia, and the hydroxide obtained dissolved in hydrochloric acid; addition of sulphuric acid to the concentrated solution in the cold transforms the chloride into the sulphate, which separates as the 8-hydrate at ordinary temperatures. Three repetitions give a satisfactory product, and in this form the method is now much used.
The work of Koppel and Holtkamp referred to above has placed the process on a sound basis. These authors have examined the solubilities of the various hydrates in presence of hydrochloric, nitric, and sulphuric acids, and mixtures of these, at different temperatures. They find that hydrochloric acid is to be preferred to nitric acid, in the process of Cleve and Witt, as besides its lower price, its use involves less loss than that of the latter acid; excess of hydrochloric acid is not harmful within wide limits, whilst a slight excess of sulphuric acid over the quantity required to form the sulphate is desirable, to secure the greatest yield. Finally, the temperature at the addition of the sulphuric acid must not be allowed to rise above 25°, for in the presence of so much acid the transition temperature to the 4-hydrate, normally 42°, is considerably lowered; it is necessary to avoid separation of the 4-hydrate, which is a flocculent unworkable precipitate.
Recently it has been proposed to carry out the purification by use of alkyl hydrogen sulphates,[504] as it is stated that the differences of solubilities of the alkyl sulphates of thorium and the cerium metals are greater than in the case of the sulphates themselves. It is also claimed that the presence of a small quantity of the alkyl sulphate in the thorium nitrate which forms the final product has a good effect on the quality of the mantles made from it.
[504] Kreidl u. Heller, D. R. P. 233023, March, 1911; F. 414463, June, 1910.
Another process of purification which has found considerable commercial application is the acetate crystallisation, thorium acetate being considerably less soluble than the acetates of the cerium elements. The impure hydroxide is dissolved in acetic acid and the solution evaporated to dryness; repeated washing with small quantities of water removes the cerium acetates, and a fairly pure salt is obtained. This is repeatedly damped with nitric acid and heated to dryness, but even after this treatment a certain amount of unchanged thorium acetate is usually present.