Rhodium Trichloride, RhCl₃

Insoluble Rhodium Trichloride, RhCl₃, may be obtained in a variety of ways:

1. By the action of chlorine at dull red heat. Berzelius observed that metallic rhodium, in a finely divided state, when heated to dull redness in chlorine absorbs some of the gas, yielding a product of empirical formula Rh₂Cl₅, which he regarded as a mixture of rhodium di- and tri-chlorides, RhCl₂ and RhCl₃. Claus, however, concluded that neither Rh₂Cl₅ nor RhCl₂ could be obtained as a separate entity by this method, and that Berzelius' product was a mixture of rhodium trichloride and unattacked rhodium. Nevertheless, Claus himself was never able to cause rhodium under these conditions to take up the theoretical quantity of chlorine required for complete conversion of the metal into trichloride. Leidie many years later experienced the same difficulty both with finely divided metallic rhodium and with rhodium black. The latter gave decidedly the best results at red heat, the mean of three results being as follows:
   
   

   	Found	Calculated
   Rhodium	51.6	49.41
   Chlorine	48.34	50.59

   
   
   It appears probable, therefore, that rhodium trichloride undergoes partial dissociation at dull red heat, i.e. at about 550° C. The reaction between chlorine and finely divided rhodium begins at about 250° C.
2. By prolonged heating of rhodium sesquisulphide in chlorine. It is exceedingly difficult, however, to expel the whole of the sulphur in this manner. Leidie4 found that even after exposure to chlorine for eight hours at a dull red heat the product contained 0.81 per cent, of sulphur.
3. A convenient method of obtaining insoluble rhodium trichloride consists in heating its alloy with tin, namely, RhSn₃, to 440° C. in a rapid current of chlorine gas. Air and moisture are first displaced by carbon dioxide; chlorine is then admitted and the temperature raised. The alloy is vigorously attacked, the tin escaping in the form of its volatile chloride, rhodium trichloride remaining behind. The product is cooled in chlorine, which is subsequently displaced by carbon dioxide. It is very important that both air and moisture should be excluded during the reaction, as otherwise complex oxy derivatives of tin are produced which are not volatile at the temperatures used, and incomplete separation of the metals occurs.
   
   As obtained in this manner rhodium trichloride is a brick-red powder, insoluble alike in water and in acids.
4. In a subsequent memoir Leidie recommends the following method:
   
   The anhydrous sodium, potassium, and ammonium chlor-rhodites (vide infra) on being heated in a current of pure, dry chlorine are decomposed into insoluble rhodium trichloride and the alkali chloride. The resulting mass is cooled in chlorine, which gas is finally replaced by dry carbon dioxide. If ammonium chlor-rhodite was employed initially the product now consists of pure rhodium trichloride, the ammonium chloride having completely volatilised. If the sodium or potassium salt was employed, the product is extracted with water, the alkali salt dissolving, leaving the insoluble rhodium trichloride behind. As in this latter case a trace of alkali is liable to be left behind, it is preferable to use the ammonium salt.
5. Finally, when soluble rhodium trichloride (vide infra) is heated to 440° C. in chlorine, it is converted into the insoluble salt.

Obtained by these methods rhodium trichloride is a red powder, insoluble in water and in acids. It is decomposed by concentrated aqueous potash, the resulting solution behaving like that of rhodium sesquioxides in the alkalies. It dissolves in concentrated solutions of potassium cyanide and of alkali oxalates, yielding complex cyanides and oxalates respectively.

When heated in chlorine with the chloride of an alkali metal, a double chloride or alkali chlor-rhodite is formed, which is soluble in water. Hydrogen reduces it at dull red heat to the pure metal, the reducing action taking place at temperatures as low as 190° C. with pure hydrogen. It undergoes slight dissociation at high temperatures, and if heated in a current of chlorine to bright redness a slight formation of sublimate takes place, the composition of which varies slightly, but approximates to that required for the formula RhCl₃. When raised to a bright red heat rhodium trichloride is gradually reduced to the metal.

xH₂O.Soluble Rhodium Trichloride, RhCl₃.xH₂O, may be prepared by dissolving the hydrated sesquioxide in concentrated hydrochloric acid and evaporating. The product is not quite pure on account of the presence of alkali in the sesquioxide. Consequently it is advisable to extract with alcohol, which dissolves the rhodium salt, filter, evaporate, and recrystallise from water.

Leidie recommends the following method of preparation: Finely divided rhodium and excess of sodium chloride are heated to the fusion point of the latter in chlorine, and the resulting mixture of double chloride and excess sodium chloride dissolved in twice its weight of water. Concentrated hydrochloric acid is now added, and the whole allowed to stand twenty-four hours, whereby sodium chloride crystallises out. The solution is decanted, cooled to 0° C., and hydrogen chloride gas passed in. After saturation with the gas the containing vessel is sealed and kept for several days at 0° C., whereby the remaining excess of sodium chloride crystallises out. After decanting, the solution is gently evaporated until syrupy, and finally exposed over potassium hydroxide until all uncombined water has been removed.

The product is hydrated rhodium trichloride, which, according to Claus contains eight molecules of water. Leidie, on the other hand, concluded that the amount of water varies and does not correspond to any definite hydrate. It is an amorphous, brick-red, deliquescent substance which, on heating to 90-95° C., still retains four to five molecules of water and two of hydrogen chloride. At 100° C. it loses water and hydrogen chloride simultaneously, and at 175-180° C. it is completely dehydrated. At 360° C. it becomes insoluble in water, but it is most readily converted into the insoluble form by heating to 440° C. in a current of chlorine.

Rhodium trichloride unites with chlorides of the alkali metals to yield two types of double salts, namely, the rhodohexachlorides or hexachlor-rhodites of general formula RhCl₃.3MCl or M₃RhCl₆, and the rhodopentachlorides or pentachlor-rhodites of general formula RhCl₃.2MCl or M₂RhCl₅, which correspond to the chlor-indites and chlor-ruthenites respectively.