THE PLATINUM METALS 881 



IrCl 4 , bat they pass into RhCl a and IrCl 3 9 bi8 very easily when heated 



whence 424 L= 227,264, and the Second portion of the equation 226,144, which is 

 sufficiently near, within the limits of experimental error, see Chapter I., Note 11.) 

 The same equation is applicable to the dissociation of Na2H and K 3 H (Chapter XII., 

 Note 42) but it has only been verified in this respect for Pd 2 H, since Mbutier, by 

 calculating the amount of heat L evolved', for = 20, according to the variation of 

 the tension (dp/dt) obtained 4'1 thousand calories, which is very near the figure 

 obtained experimentally by Favre (see Chapter XII., Note 44). The absorbed hydrogen 

 is easily disengaged by ignition or decreased pressure. The resultant compound 

 does not decompose at the ordinary temperature, but when exposed to air the metal 

 sometimes glows spontaneously, owing to the hydrogen burning at the expense of 

 the atmospheric oxygen. The hydrogen absorbed by palladium acts towards many 

 solutions as a reducing agent ; in a word, everything here points to the formation of a 

 definite compound and at the same time of a physically-compressed gas, and forms one 

 of the beat examples of the bond existing between chemical and physical processes, to* 

 which wo have many tunes drawn attention.' It must be again remembered that the 

 other metals of the eighth group, even copper, are, like palladium and platinum, able to> 

 combine with hydrogen. The permeability of iron and platinum tubes to hydrogen is 

 naturally due to the formation of similar compounds, but palladium is the mosfe 

 permeable. 



s W Rhodium is generally separated, together with iridium, from the residues left 

 after the treatment of native platimun, because the palladium is entirely separated from 

 them, and the ruthenium is present in them in very small traces, whilst the osmium at 

 any rate is easily separated, as we shall soon see. The mixture of rhodium and iridium 

 which is left undissolved in dilute aqua regia is dissolved in chlorine water, or by the> 

 action of chlorine on a mixture of the metals with sodium chloride. In either case both 

 metals pass into solution. They may be separated by many methods. In either case 

 (if the action be aided by heat) the rhodium is obtained in the form of the chloride 

 RhClj, and the iridium as iridious chloride, IrClj. They both form double salts with 

 sodium chloride which are soluble in water, but the iridium salt is also partially soluble 

 in alcohol, whilst the rhodium salt is not. A mixture of the chlorides, when treated with 

 dilute aqua regia, gives iridic chloride, IrCl^, whilst the rhodium chloride, RhClj, re- 

 mains unaltered ; ammonium chloride then precipitates the iridium as ammonium ividio- 

 chloride, Ir(NH4) 2 Cl 6 , and on evaporating -the rose-coloured filtrate the rhodium gives 

 ft crystalline salt, RhtNH^Cle. Rhodium and its various oxides are dissolved when 

 fused with potassium hydrogen sulphate, and give a soluble double sulphate (whilst 

 iridium remains unacted on) ; this fact is very characteristic for this metal, which offers 

 in its properties many points of resemblance with the iron metals. When fused with 

 potassium hydroxide and chlorate it is oxidised like iridium, but it is not afterwards 

 soluble in water, iu which respect it differs from ruthenium. This is taken advantage of for 

 separating rhodium, ruthenium, and iridium. In any case, rhodium under ordinary 

 conditions always gives salts of the type EX 3 , and not of any other type ; and not only 

 halogen salts, but also oxygen salts, are known in Ibis type, which is rare among 

 the platinum metals. Rhodium chloride, RhCl s , is known in an insoluble anhydrous 

 and also in a soluble form (like CrX 5 or salts of chromic oxides), in which it easily gives 

 double salts, compounds with water of crystallisation, and forms rose-coloured solutions. 

 In this form rhodium easily gives double salts of the two types RhMjClg and RhM 2 Cl 5 

 for example, K 3 RhCl 6 ,8H 2 O and K 2 RhCl 5 ,H 2 O. Solutions of the salts (at least, the 

 ammonium salt) of the first kind give salts of the second kind when they are boiled. If 

 a strong solution of potash be added to a red solution of rhodium chloride and boiled, a 

 black precipitate of the hydroxide Rh(OH) 3 is formed ; but if the solution of potash is 

 added little by little, it gives a yellow precipitate containing more water. This yellow 

 hydrate of rhodium oxide gives a yellow solution when it is dissolved in acids, which 

 only becomes rose-coloured after being boiled. It is obvious a change here takes place, 

 like the transmutations of the salts of chromio oxide. It is also a remarkable fact thai 



