Thorium Chloride, ThCl₄

Thorium Chloride, ThCl₄, was prepared by Berzelius by the chlorination of a mixture of thoria and carbon, heated in a porcelain tube; by Smith and Harris through the interaction of thoria and phosphorus pentachloride; and by Kruss and Nilson by heating the metal to redness in a stream of hydrogen chloride gas. Thorium chloride has been obtained by Rosenheim, Samter, and Davidsohn, together with two hydroxychlorides, by the interaction of freshly precipitated thorium hydroxide and alcoholic hydrogen chloride. Thus the two crystalline products Th(OH)₂Cl₂.5H₂O and Th(OH)₃Cl.7H₂O separated under different conditions; and from the mother-liquor ThCl₄.8H₂O and ThCl₄.9H₂O were obtained in crystals. Matignon and Bourion prepared anhydrous thorium chloride in large prismatic needles by passing a mixture of chlorine and the vapour of sulphur monochloride (S₂Cl₂) over heated thoria, and Bourion has found that the vapour of sulphur monochloride alone is sufficient; whilst Moissan and Martinsen obtained thorium chloride in colourless crystals by the action of chlorine on thorium carbide at high temperature; and, lastly, Matignon and Delepine have prepared it by passing a current of carbon monoxide and chlorine over thoria heated strongly in a porcelain tube, or by the action of carbon tetrachloride vapour on thoria heated below the melting-point of the chloride, the principal reactions being:

ThO₂ + CCl₄ = ThOCl₂ + COCl₂;

ThOCl₂ + CCl₄ = ThCl₄ + COCl₂.

According to Chauvenet, however, thorium chloride is most conveniently prepared, free from oxychloride, by heating the dioxide in a current of carbonyl chloride. The heat of formation of thorium tetrachloride is given by the equation

Th + 2Cl₂ = ThCl₄ + 300,200 ± 0.67 per cent, calories.

Thorium chloride forms colourless crystals of density 4.59; it sublimes at 720°-750° C., forming shining tablets, and melts at 820° C. It is deliquescent and dissolves readily in water, whence it crystallises as the hydrate ThCl₄.8H₂O. When this hydrate is dried at ordinary temperature and pressure it yields ThCl₄.7H₂O; and this salt, when heated to 50° C. in a current of dry hydrogen, passes into ThCl₄.4H₂O, which at 100° C. yields ThCl₄.2H₂O. Above 100° C. basic chlorides are formed. The heats of solution of the anhydrous salt and the different hydrates are: ThCl₄, 56,700 calories; ThCl₄.2H₂O, 41,080 calories; ThCl₄.4H₂O, 26,230 calories; ThCl₄.7H₂O, 14,700 calories; ThCl₄.8H₂O, 11,450 calories.

The hydrated chloride is readily prepared by dissolving the hydroxide in concentrated hydrochloric acid and saturating the cold solution with hydrogen chloride gas. The salt separates out as a beautiful white crystalline precipitate, and the yield is practically quantitative if an equal volume of ether be added while the gas is being passed into the solution.