342 



NA TURE 



{^August 7, 1879 



destined to serve as the International Prototype Standards. 

 Their investigations are set forth at great length, and 

 analytical chemistry is thus enriched by an elaborate 

 memoir, in every way worthy of its distinguished authors. 

 The alloys of platinum iridium, of -which these standards 

 are made, was furnished by Mr. George Matthey, of the 

 well-known firm of Johnson, Matthey, and Co., and, in the 

 April number of the Annales de Ckimie, there is a paper by 

 MM. Deville and Mascart, describing the experimental 

 determinations of the various physical constants of the 

 metal of which the Regie Gdoddsique is made. We can- 

 not do better than quote their words as indicating the care 

 and skill bestowed by Mr. Matthey in the preparation of 

 this standard : " En fabriquant un pareil alliage avec une 

 telle puretd, M. Matthey a r^solu un probl6me de mdtal- 

 lurgie des plus difficiles et des plus compliquds. On ne 

 peut s'imaginer, k moins qu'on ne connaisse dans tous 

 leurs ddtails les procddds si pdnibles employes &. la purifi- 

 cation de r iridium et meme du platine, combien il a fallu 

 d'intelligence, de patience et de ddvouement k la Science 

 pour rdussir dans une pareille oeuvre." ^ 



As the matter is of much importance, a paper recently 

 communicated to the Royal Society by Mr. G. Matthey 

 possesses special interest, as in it he describes the methods 

 employed for preparing the metals in a state of purity. 

 The following is an abstract :— 



"The six metals (of which platinum is the chief) 

 usually found more or less in association, present charac- 

 teristics of interest beyond their metallurgical utility, 

 which are, perhaps, worth alluding to. It is, for instance, 

 a curious fact that the group should consist of three light 

 and three heavy metals, each division being of approxi- 

 mately the same specific gravity — the heavier having (in 

 round figures) just double the density of the lighter series. 



Thus we find osmium, iridium, platinum forming the 

 first division, of the respective specific gravities of 22'43, 

 22'39, 2 1 '46; whilst ruthenium, rhodium, and palladium 

 are represented by the figures ii*4o, ii"36, 11, theaverage 

 densities of the heavy and light divisions thus being 

 respectively 22"43 and 11-25. 



But a more interesting and important classification is 

 what I may designate as a first and second class series, 

 from the more important view of their relative properties 

 of stability. Thus platinum, palladium, and rhodium 

 form the first or higher class, not being volatihsable in a 

 state of oxide ; iridium, osmium, and ruthenium forming 

 the second or lower class, their oxides being more or less 

 readily volatilised. 



The oxide of iridium is affected at 700° to 800" C, and 

 entirely decomposed at 1,000°, whilst osmic and hypo- 

 ruthenic acids are volatihsed at the low degree of 100'', the 

 latter exploding at 108°. The chlorides of these metals 

 can be sublimed at different temperatures (as also the 

 protochloride of platinum). 



Platinum 



The preparation of this metal in a state of purity is an 

 operation of extreme delicacy. I commence by taking 

 ordinary commercial platinum ; I melt this with six times 

 its weight of lead of ascertained purity, and, after granu- 

 lation, dissolve slowly in nitric acid diluted in the propor- 

 tion of I volume to 8 of distilled water. The more readily 

 to insure dissolution, it is well to place the granulated 

 alloy in porcelain baskets such as are used in the manu- 

 facture of chlorine gas for holding the oxide of man- 

 ganese. When the first charge of acid is sufficiently 

 saturated, a fresh quantity should be added until no more 

 action is apparent ; at this stage the greater part of 'the 

 lead will have been dissolved out, together with a portion 

 of any copper, iron, palladium, or rhodium that may have 

 been present. These metals are subsequently extracted 

 from the mother-liquors, the nitrate of lead by crystallisa- 

 tion, and the remaining metals by well-known methods. 



' Aim. de C/iim. et de Phys., sme Sirie, t. jivi., April, 1879. 



The metallic residue now obtained will be found in the 

 state of an amorphous black powder (a form most suit- 

 able for further treatment), consisting of platinum, lead, 

 and small proportions of the other metals originally 

 present — the iridium existing as a brilliant crystalline 

 substance insoluble in nitric acid. After digesting this 

 compound in weak aqua regia, an immediate dissolution 

 takes place of the platinum and lead, leaving the iridium 

 still impure, but effecting a complete separation of the 

 platinum. 



To the chloride of platinum and lead after evaporation 

 is added sufficient sulphuric acid to effect the precipita- 

 tion of the whole of the lead as a sulphate, and the 

 chloride of platinum, after dissolution in distilled water, 

 is treated with an excess of chloride of ammonium and 

 sodium, the excess being necessary in order that the pre- 

 cipitated yellow double salt may remain in a saturated 

 solution of the precipitant. The whole is then heated to 

 about 80°, and allowed to stand for some days ; the am- 

 monio-chloride of platinum will settle down as a firm 

 deposit at the bottom of the vessel, whilst if any rhodium, 

 as is generally the case, is present, the surface liquor will 

 be coloured a rose tint, occasioned by a combination of 

 the salts of the two metals. 



The precipitate must be repeatedly washed with a 

 saturated solution of chloride of ammonium and subse- 

 quently with distilled water charged with pure hydro- 

 chloric acid. This is necessary for its purification. The 

 small quantity of the double salt which will be taken up 

 and held in solution is of course recovered afterwards. 

 Rhodium may still exist in the washed precipitate, which 

 must therefore not be reduced to the metallic state until 

 its separation is completed, and this is best effected by 

 mixing with the dried compound salts of chloro-platinate 

 and chloro-rhodiate of ammonia, bi-sulphate of potash 

 with a small proportion of bi-sulphate of ammonia, and 

 subjecting to a gradual heat brought by degrees up to a 

 dull red in a platinum capsule, over which is placed an 

 inverted glass funnel. The platinum is thus slowly re- 

 duced to a black spongy porous condition freed from 

 water, nitrogen, sulphate of ammonia, and hydrocloric 

 acid, the rhodium remaining in a soluble state as bi- 

 sulphate of rhodium and potash, which can be dissolved 

 out completely by digesting in boiling distilled water; a 

 small quantity of platinum will have been taken up in the 

 state of sulphate, but is regained by heating the residue 

 (obtained on evaporation) to redness, which reduces it to 

 the metallic condition, the rhodium salt remaining un- || 

 decomposed. 



By the method above described the platinum is freed 

 not only from rhodium, but from all other metals with 

 which it may have been contaminated, and is brought to 

 a state of absolute purity, of the density 2 1 '46, the highest 



degree obtainable. 



Iridium 



f 



In practice, the purest iridium which can be obtained 

 from its ordinary solution (deprived of osmium by long 

 boiling in aqua regia and precipitated by chloride of am- 

 monium) will almost invariably contain traces of platinum, 

 rhodium, ruthenium, and iron. 



I fuse such iridium in a fine state of division with ten 

 times its weight of lead, keeping it in a molten state for 

 some hours, dissolve out the lead with nitric acid, subject 

 the residue to a 'prolonged digestion in aqua regia, and 

 obtain a crystalline mass composed of iridium, rhodium, 

 ruthenium, and iron, in a condition suitable for my fur- 

 ther treatment. By fusion at a high temperature with an, 

 admixture of bi-sulphate of potash, the rhodium is almost 

 entirely removed, any remaining trace being taken up 

 together with the iron in a later operation. The iridium 

 so far prepared is melted with ten times its weight of dry 

 caustic potash, and three times its weight of nitre, in a 

 gold pan or crucible ; the process being prolonged for a 

 considerable time to effect the complete transformation of 



