204 



this phenomenon being herewith extended to chroniluin, iron;cobalt, 

 platmuia, rhodium, and iridium. With respect to. the dispersion of 

 some of these salts, we hope yet to furnish some new data in the 

 near future. 



§ 2. The required racemic Potassium- Iridium- Oxalate-, K^\Ir{Ci 0^)\ -\- 

 -j- 41 H^O was obtained in the following way. A 3 "/o solution of 

 pure, hydrated iridiiimclUoride of commerce (Heraeus) is treated 

 by a solution of potassiumhydroxide in excess. A dirty brownish 

 precipitate is formed, which dissolves in the excess of KOH to form 

 potassium- iridiate. The alkaline solution is heated, and then some 

 perhijdrol (30 '/o ^a^J added: the colour ciianges to dark blue, and 

 the principal part of the iridium precipitates as Ir{OH)^, Another 

 part of it remains in the solution as a colloidal suspension of great 

 stability, not being precipitated or coagulated from it, even after 

 addition of electrolytes. These solutions are therefore better evaporated, 

 and the residue li-ansformed into ammonium-cldoro- iridiate to be 

 used afterwards in other experiments. 



The blue precipitate is washed by decanlalion with water slightly 

 acidified by means of o.valic acid; the filtrates and washings are 

 also later converted into the mentioned compound. When the removed 

 liqiiid gets colourless, the decantation may be considered complete, 

 and the precipitate is brought into a round bottom flask, the super- 

 fluous Ii(|uid removed after some hours, standing, and a hot, concen- 

 trated solution of o.valic acid added. The contents of the vessel are 

 boiled under a reflux-condenser during 30 or 40 hours; a part of 

 the oxalic acid is thereby decomposed, and the tetravalent iridium 

 reduced to trivalent according to the equation : 



IhiOH), + C\0,H, = 2Tr{0H), + 2H,0 + 26Y>„ 

 while iridium.-trioxalic acid is then formed from the derivative of 

 the trivalent iridium, conforming to the equation : 



2rr{()H), + QC,0,H, = 2J/r(C,0,),| H, -f 6H,(). 



The gold-yellow solution finally obtained is filtered, and almost 

 perfectly neulralised by means of KffCO^; it is then concentrated 

 on the waterbath, and the successive fractions of the crystals formed 

 are separately collected. Almost pure potassium-oxalate is first deposited, 

 and afterwards, besides this, the orange crystals of the salt required, 

 which is very soluble. These crystals have to be separated mecha- 

 nically, and they are afterwards recrystallised for purification^). 



The racemic compound crystallises in pale orange-coloured crystals. 



1) Dr. J. Kahn has aided most effectively in the preparation of a part of this 

 racemic compound, and in the troublesome working up of the zrirfmw-residues. 



