V O L U M E 25, NO. 1 1 , N O V E M B E R 1 9 5 3 Methods of separation are naturally important in the industrial preparation of these metals. Radiochemistry has emphasized the need for chemical separations of these metals. It has also placed a special demand-that of speed-upon the chemist concerned with the operations of solution and separation. An unusual example of difficulty occurring in radiochemistry, in the author’s laboratory, is the greatly increased resistance to chemical attack of iridium which has been bombarded in the cyclotron. Consequently it becomes necessary to look for more rapid chemical methods. The fourth and last paper of the symposium is concerned with separation by means of ion exchange resins. The principal limitation in platinum group analysis a t present therefore is that the methods, excellent as they are as to results, are too slow to fit the needs of radiochemistry. More rapid meth-
1613 ods of quantitative separation are especially needed. Platinum group chemistry has also been largely empirical. Systematic quantitative studies of solubilities of precipitates, of adsorption phenomena, of complex formation and stability, of photometric behavior, and of electrochemical behavior are needed. The systematic application of such information should lead to an improved analytical scheme for the platinum group metals. LITERATURE CITED
(1) Beamish, F. E., et aZ.;“Applied Inorganic Analysis,” Hillebrand and Lundell, 2nd ed., Chap. 20, New York, John Wiley &
Sons, 1953. (2) Gilchrist, R., Chem. Revs., 32, 277 (1943). (3) Gilchrist, R., and Wichers, E., J . Am. Chem. SOC.,57, 2565 (1935).
6th Annual Summer Symposium-Less Familiar Elements
Concentrating and Dissolving Platinum Metals F. E. BEAMISH AND W. A. E. McBRYDE Department of Chemistry, University of Toronto, Toronto, Ont., Canada This is an interim report of the efficiency of the fire assay for ruthenium, osmium, rhodium, and iridium. Reference is made to the losses caused by cupellation and to the methods of effecting quantitative corrosion of platinum metals and their alloys. Data are provided to proFe that certain types of fluxes may “slag off” some of the platinum metals. Of particular significance is the fact that prefusion of ore mixtures may result in verj- unsatisfactory assay recoveries. This observation suggests the possibility that similar phenomena may be found in natural occurrences. Furthermore, these researches provide the first experimental data on the efficiency of lead collection.
T
HE treatments ahich go to make up the fire assay for pre-
cious metals go back a great many years. In discussing the history of assaying, Smith ( 1 7 ) mentions that many of the procedures such as cupellation and parting are recorded as early as the 12th or 14th century. The refining of silver by fire in the manner of cupellation is recorded by Jeremiah in the Old Testament. The operations are by no means well known to many chemists. At universities, assaying is taught in schools of mines or of mining engineering, and the average chemistry student hears very little or nothing about it. Severtheless, the operations involved when an assay is required for the platinum metals call for a high degree of chemical skill. Thus, Bugbee ( 7 ) states, in connection with the platinum metals, “their correct determination is considered the most difficult anal) sis in the realm of inorganic chemistry.’’ The success of the crucible assay depends on several factors. One is having the ore reduced to a very fine powder and intimately mixed with the flux. The choice of composition for the flux is governed by the composition of the gangue to be slagged off. For instance, an ore rich in silica requires a compensating amount of soda ash and litharge to form a suitable slag. The composition of the flux and the temperature of the furnace must be controlled so that reduction of lead occurs while the slag is still quite viscous. In this way the extraction of the precious metal occurs throughout the body of the charge, Later, the temperature may be raised, and as the slag becomes more fluid the minute droplets of lead fall and collect together in the base of the crucible. Some knowledge of the approximate composition of the gangue is necessary t o predict the amounts of acidic or basic oxides to be added in order to form a suitable slag and to predict the amount of reduction of litharge which may take place by the action of sulfides. The characteristics of a good slag are numerous. I t
should remain rather viscous early in the fusion while the lead drops are forming, but become more fluid a t somewhat higher temperatures; it should separate cleanly from the lead button after cooling; and its composition must not be so basic as to corrode out the clay crucible. Ideally, about 25 grams of lead should be produced and form the so-called button on cooling. For ores containing but little sulfide, the quantity of lead in the button is regulated by the amount of flour or other reducing agent added with the charge. Sulfide ores themselves will reduce litharge, and it is customary to express the reducing power of an ore as the number of grams of lead produced by the reducing action of 1 gram of ore. Since it is desired to limit the size of the buttons and also t o leave some unreduced litharge in the slag, the custom is t o add potassium nitrate as an oxidizing agent in the assay of sulfide ores; this is the socalled niter assay. Another practice, somewhat rarer and certainly more time-consuming than the foregoing, is to roast the ore with free access of air in order to convert sulfides to oxides. hlention should also be made of the iron nail assay. I n this method, the reduction of litharge is accomplished by an excess of iron, usually in the form of nails. These are removed before the melt is poured. This type of assay is applied to sulfide ores and usually gives rise t o a ferrous sulfide matte. With‘this treatment a large amount of soda ash is added to the flux and much of the iron matte dissolves i n the slag in the form of sodium ferrous sulfide. A large proportion of base metals is found in the button in the case of the iron nail assay. The contents of the crucible or pot are poured into a conical mold, and on cooling the slag breaks aB-ay or can be removed from the lead button. The scorification assay consists of heating together ore and lead, plus a small amount of borax, in an oxidizing atmosphere. Thiq is carried out in a muffle furnace on a shallow saucerlike
1614
ANALYTICAL CHEMISTRY
dish. The lead is partially oxidized to litharge which exerts a solvent action on the silica and other dross of the ore, while the remainder of the lead collects the precious metals in a button. Any sulfides are first osidized, and the resulting oxides form solid solutions with litharge. The scorification method finds principal application in the assay of silver and rich gold ores, but it is also of interest in platinum-metal work where scrap, meepings, and the like are to be treated for the concentration of precious metal.. CUPELLATION
The two preceding operations are customarily folloivecl by cupellation. Cupels are small thick cups, generally made of hone ash or similarly porous materials. The button from the fusion treatment is placed on the previously heated cupel and heated in a muffle furnace with free access of air. The lead oxidizes to litharge which, at the temperature of its formation, is a liquid: some of this vaporizes, but most of it soaks into the porous cupel. While the molten litharge wets the cupel and runs into the interstices of the bone ash, the remaining lead alloy has a higher surface tension and stays as a globule on the surface. The litharge here acts, as in the scorificntion assay, as a solvent for oxides of base metals. Gold, platinum, and probably silver are not oxidized while the other platinum metals may form oxides. The precious metals or their oxides eventually remain as a head on the cupel after the lead has all been consumed. Usually a known weight of silver is added to the ore or the button so that the bead is of a convenient size and, much more important, gives a proper parting when treated with acid. The procedures just described have been used for a very long time in the analysis of ores for gold and silver. Errors have been encountered, especially a loss of silver during cupellation. This is considered to be due mainly to the formation of oxide which is dissolved and absorbed vith the litharge. Quantitative data for this loss are available but hard to interpret owing to the failure of many experimenters to control their conditions with sufficient care. I t is known that the losses are greater the higher the temperature during cupellation and also the grpntcr the amount of lead in the button. POSSIBLE LOSSES OF PLATINUM M E T 4 L S
Similar quantitative data have not been available for the platinum metals and the assumption appears to have been generally made that, with minor modifications, the procedures long in use for the gold assay up to the parting of the head \\ill yield satisfactory results for platinum and its congeners. This assumption cannot be entirely warranted because of several features of the chemistry of these metals. In the first place, the platinum metals are much more disposed to form oxides than gold or silver. The characteristic inertness of these elements to surface corrosion and oxidation tends t o obscure the fact that they are thermodynamically much more active than silver or gold. Latimer (13 ) has given some values for the standard free energies of formation of some of the oxides (Table I). While these data are in some cases only estimates and do not permit any calculation of equilibrium constants a t the higher temperatures characteristic of furnace work, they do indicate that there is a greater tendency to oxide formation than is sometimes thought. For this reason, the possibility must he considered of these metals being retained in solution as silicates or borates in the slag during the crucible assay. Likewise, an even more appreciable loss may occur in cupellation with the dissolving of platinum metal oxides in the molten litharge. It is also known that two of the platinum metals form volatile oxides of known composition, and the existence of gaseous oxides has also been postulated to account for the small loss in weight that occurs when platinum, palladium, or iridium are heated in air. Loss of these metals during certain furnace treatments may
therefore be anticipated. This may he especially true in the latter stages of cupellation because the melting points of the final beads are much higher when they contain platinum metals than when they contain gold and silver only. Cupellation losses are knonn to occur with gold assays, especially if the temperature becomes too high or if the proportion of silver in the final head is too ]OM. This seems to he mainly a mechanical losq, perhaps arising from a lowering of the surface tension of the alloy under these conditions. When the platinum metals are considered, a further source of mechanical loss may be expected because of the insolubility of rhodium and iridium and the restricted solubility of platinum in silver. The presence of iridium seems particularly apt to cause precious metal to be left hehind on the cupel in a finely divided state. EXPERIMENTAL WORK ON FIRE 4SSAY
A thorough-going investigation of the efficiency of these fireassay techniques when applied to platiniferous ores and concentrates has been in progress in this laboratory over a period of years. Results of this study have now been published for ruthenium (f8),osmium ( I ) , rhodium ( S ) , and iridium (6). Examination of the distribution of palladium and platinum during these treatments is in progress. There is a considerable body of evidence or opinion that the most serious losses of platinum metals occur during cupellation, probably for the reasons outlined above, and these losses in cupellation have been verified. Accordingly a composite method of analysis for the platinum metals is being worked out based on dissolving or parting of the lead button. This has introduced a few analytical complications, but it seems probable that button parting will be required in the future for precision work.
Procedure. The procedure in these invatigations has been approximately the same for each of the metals investigated. Synthetic or naturally occurring ores of different types were salted with known amounts of the platinum metal. The salting technique was either by direct addition of weighed amounts of sponge metal or by addition of known amounts of standard solution followed by drying. The ore samples used always included a siliceous (nonreducing) and a sulfide (reducing) type. The ore samples were mixed with various types of fluxes, acidic, neutral, or basic, and fused in the crucible assay. Different procedures for the assay of the sulfide oreq were tried: preroasting of the sample and the niter assay in all cases and the iron nail assay in the study of ruthenium and of osmium. The buttons resulting were analyzed for the precious metal directly. In this connection perchloric acid parting proved of great use, especially in ruthenium and osmium analyses. The slag from the first fusion was all collected, ground fine, and re-fused withlitharge and flour to give a second button. In extreme cases, as the analysis required, further reassaying of the slag was continued until the platinum metal was reasonably well accounted for. In the cases of ruthenium and osmium the gases over the fusion mixture were aspirated from the crucible through suitable absorbing solutions. The latter were then analyzed for the appropriate metal. I n three of the four researches thus far reported (all hut rhodium) the loss of platinum metal during cupellation was examined. This examination required collecting and analyzing vapors given off in the case of ruthenium and osmium and also an assay of the ground cupel after the bead had been removed. I n all cases the beads were analyzed for the metal. Ruthenium analyses were performed by thionalid precipitation Table I.
Standard Free Energy of Formation of Noble &letalOxides Oxide AgzO AuzOs PdO PtO
Rho Rh2Os IrzOs RuOz RuOi OSOI
F " , Kcal. -2.6 35.0 -14 4
-iko
-50.0
-4 2 . 0
(estimated) -40.7 -33 (estimated) -67.9
1615
V O L U M E 25, NO. 1 1 , N O V E M B E R 1 9 5 3 ( 1 5 ) and by counting of radioactivity; the two procedures enabled cross checking in the case of large samples and t'he radioactive counting enabled quick estimation of small amounts of ruthenium in vapors and filtrates. Osmium analyses were all performed colorimetrically by means of thiourea ( 2 , 4). It was found that the distillation procedure introduced considerable arid variable amounts of silica into the sample which precluded the use of gravimetric analysis. Rhodium was determined after precipitation with thiobarbituric acid (8). This reagent was applied to chloride solutions and had the advantage that lead in moderate amounts could be tolerated in solution during the precipitation. When a double precipitation of rhodium was carried out, results of greet accuracy were obtained for rhodium in the presence of 10 grams of lead. Rhodium was first isolated from large amounts of lead by treatment of the solution with zinc. The small amounts of lead carried down by the zinc were not precipitated by thiobarbituric acid. Iridium analyses werz carried out on the residues from nitric acid or perchloric acid parting of the lead button. A short procedure consisted of treating the ignited residue repeatedly with sulfuric acid and hjrdrofluoric acid t o remove silica and subjecting the final residue to leaching Tvith dilute nitric and hydrochloric acids in succession. Other residues were treated by hot chlorination on a bed of sodium chloride to render the iridium soluble (2,O). Base metals and silica were then removed by precipitation with phosphate i n the presence of sodium nitrite, and finally the iridium w:is determined hydrolytically ( 5 ) .
Results. The results of these investigations are very interesting. It, was found, for instance, t,hat neither ruthenium nor iridium was completely collected into the lead in one fusion. One additional fusion x i t h litharge and flour sufficed t o extract the remainder of the rut,henium, but a t least two extra fusions were required in the case of iridium. Recovery of ruthenium in the first fusion was found t o be from i5 t o 95% on a 6-nig. sample, and no diffcrenw could be found among the slag losses for acidic, basic, or neutral fluxes. The distribution of iridium was more eshaustively studied. The first fusion estracted about 90% of a 5-mg. sample nhen various fluxes were used. \Vith acidic or neutral slags, the remainder of the iridium was recovered in two reassays, but more than 5% of a 5-mg. sample was lost when basic slags were used. Even more startling losses were recorded when the assay was attempted on copper sulfide or nickel oxide ores. These required a basic flux with escess litharge for slagging, and the loss of iridium in these assays was from 20 t o over 60%. Furthermore, copper and nickel ah-ays appeared in the lead button in these cases. The failure to recover ruthenium or iridium in one fusion is thought to be due, in part a t least, to limited solubility of these metals in lead. The loss of iridium to basic slags is a much more subtle phenomenon. I t appears necessary to postulate that iridium forms a compound with some of t,he constituents in the flux, itrid as t'he observation is confined to basic slags, this suggests the formation of an acidic oxide of iridium. The presence of iridium in the slag could not be proved by spectrographic examination. If a flux salted with iridium were fused without button formation, and then assayed and reassayed the total recovery of iridium was less than 80%. There is evidence that this phenomenon is not confined to iridium. Rhodium and osmium gave much more satisfactory assays. Data on rhodium are t'he least complete, but they reveal that on one fusion, plus a reassay of the slag, recovery of 10 mg. of metal was of the order of 97 to 99% complete. Collection in the first button was of the order of 94 to 98% irrespective of the ore t,ype and flux used. Nickel oxide ores gave slightly poorer recovery in the first button. The study of the distribution of osmium was very complete and disclosed that the efficiency of the assay depended somewhat on the type of flux used. The best collection seemed to be obtained with neutral fluxes and was of the order of 96 to 99%. Moreover, in these cases the collection was virtually- complete in the first button. X representative acid flux of medium viscosity yielded poor collection i n one button; at least two reassnys
of the slag were required to make the collection nearly complete, and gas losses were appreciable in every fusion. A different acid flux, rich in borax and quite fluid, gave recovery in one fusion of $16to 99% and no gas losses. Basic fluxes showed some retention of osmium by the slag and noticeable gas losses. The iron nail assay was tried with pyritic ores salted with ruthenium or osmium and in both cases proved to be entirely unsatisfactory because much of the precious metal together with some lead clung tenaciously t o the nail. Except as noted for certain fluxes with osmium (basic fluxes and one acid flux) there were no appreciable losses of ruthenium or osmium as volatile products during the crucible fusion. This was true even of the niter assays of certain pyritic ore samples. Evidently the reducing conditions of the fusion do not permit the production of volatile oxides. CUPELLATION LOSSES
The examination of cupellation losses proved to be very interesting. Ruthenium showed high losses, even on partial cupellation to a button weighing 6.5 grams but the loss was to the cupel, not to the air as volatile oxides. Presumably, then, lower oxides of ruthenium are formed and may be absorbed by litharge. IVhen buttons containing 5 mg. of osmium and 100 mg. of silver were completely cupelled almost all the osmium was lost, mostly as vapor but a small amount to the cupel. Partial cupellation of buttons, from which silver was omitted, t o a weight of 5 to i grams showed a significant loss as vapor and to the cupel, the loss amounting to 5 or 10%. Cupellation losses of iridium were examined by preparing synthetic buttons with test lead and iridium sponge and subjecting these to an ordinary cupellation. The beads were parted with nitric acid and the insoluble residue determined as iridium with allowance for a cupellation blank. Even when the silver to iridium ratio was as high as 200 to 1, small losses of iridium were observed. Black particles of iridium oxide \$-ere scattered about the surface of the cupel near the silver bead. This mechanical loss of iridium is presumably due t o the very low solubility of iridium in silver. A comparable study of the cupellation of buttons containing rhodium remains to be done. CONCESTRATION BY ION EXCHAhGE
A procedure of an entirely different sort for concentrating the platinum metals, or a t least of separating them from moderate amounts of associated base metals, is being examined in this laboratory. This involves the use of ion exchange resins of the anion exchange type. Sdvantage is taken of the fact that in dilute solutions of chlorides the platinum metals occur mainly a s anionic complexes like chloroplatinate (PtCI,--) while the base metals are present largely as cations or cationic complexes. The work has not progressed to a point where these separations can be included n-ithin a scheme of analysis. METHODS OF DISSOLVIRG
The dissolving of platinum metals presents a number of special problems. The behavior of the metal toward dissolving reagents is markedly influenced by its state of subdivision and also by its purity. The most reactive forms are the blacks; sponges and powdered metal are less reactive than blacks but usually much more reactive than compact metal. N o sharp line of demarcation can be drann between these states of fineness so that a good deal of uncertainty and many apparent contradictions have found their wav into the literature dealing with dissolving these elements. One of the commonly practiced techniques for decreasing the compactness of these metals is t o form an alloy by fusion with a more active metal and later to dissolve the active metal with an appropriate acid. Lead is frequently used for this purpose when preparing scrap platinum metals, sweepings, and the like for refin-
ANALYTICAL CHEMISTRY
1616 ing or for analysis. Zinc or tin are also used in the same way. Silver is always added in moderate amounts to ores or buttons preparatory to cupellation in order to render certain of the platinum metals soluble when the bead is parted by acids. Among the compact metals, palladium is attacked by hot concentrated nitric acid and by boiling sulfuric acid. The latter acid slowly attacks platinum and rhodium. Both platinum and palladium are dissolved by aqua regia. The remaining metals are resistant t o all single acids and to aqua regia. When the metals are very finely divided they are more susceptible to attack by acids. Thus, finely divided rhodium can be dissolved by hot concentrated sulfuric acid. Ruthenium, in a fine state of division Is attacked by aqua regia; the osmium is attacked by fuming nitric acid. Potassium hydrogen sulfate or potassium pyrosulfate when fused will dissolve finely divided rhodium with the production of some type of sulfate. -4lkaline oxidizing fusion mixtures are effective for rendering finely divided ruthenium or osmium soluble. Sodium peroxide, or potassium hydroxide mixed with potassium nitrate or potassium perchlorate, will convert the metal to a water-soluble ruthenate or osmate during fusion. Platinum and palladiuni are badly corroded by these fused reagents, and all the platinum metals, if they are finely divided enough, are converted by surh oxidizing fusions to forms which are water-soluble or arid-soluble. Molten alkali cyanides also will dissolve appreciable amounts of platinum, palladium, or rhodium. Aqueous solutions of sodium hypochlorite (chlorine in alkali) rapidly dissolve finely divided ruthenium or osmium. This method of dissolving ruthenium is used routinely in this lalmratory and it is very convenient. CIILORIR & T I 0 1
Oue of the most satisfactory methods for rendering each of the platinum metals water-soluble is the direct chlorination of the metal, preferably on a bed of sodium chloride. This technique has been known for many years and has been applied recently t o several problems in this laboratory (10). Procedure. An examination was made of the chlorination of the pure metals alone and of the metals covered by dry sodium chloride. The apparatus consisted of a U-shaped silica tube heated by an electric furnace. A porcelain boat containing the sample was placed near the bend of the outer arm of the C. Chlorine was dried before passing through the tube by means of sulfuric acid and phosphorous pentoxide. The exit gases were bubbled through two receiving bottles usually containing 6 N hydrochloric acid. Under these conditions the metals were completely converted to chlorides or chloro complexes. Of the platinum metals, only ruthenium and osmium &ere partially carried through the apparatus and caught in the gas-receiving bottles. Some of the metals produced a sublimate a t the cooler end of the tube. Ruthenium and gold, and also lea?, produced a considerable amount of this deposit, while osmium, iridium, and platinum produced slight amounts of clublimate, Rhodium and palladium produced none.
Results. The chlorination of these metals revealed some curiphenomena for which no adequate explanation seems available. I n the chlorination of platinum, for instance, part of the metal was transferred several inches through the tube and condensed as a chloride on a cold surface. The temperature in the hot part of the tube was higher than the dissociation temperature of any known chloride of platinum, and yet platinum shows no appreciable vapor pressure a t this temperature. Ruthenium produced two quite different products as sublimate. These differed widely in appearance and properties, but both were unmistakably ruthenium trichloride (11). The residues in the porcelain boat proved to be soluble in 0.1 AT hydrochloric acid in all those cases where sodium chloride formed part of the charge. When no sodium chloride had been used the residues did not dissolve in this acid. By this chlorination procedure i t was shown that various OUR
weights of iridium were quantitatively converted to soluble forms from which the metal could be recovered by hydrolytic precipitation. It was also found possible to convert various native alloys of the platinum metals into acid-soluble products. In the analysis or refining of native platinum part of the metal resists prolonged attack by aqua regia; this part is generally said to consist of the alloy iridosmine, iridium, and a certain amount of nonplatiniferous material. Iridosmine is found in South Africa, Tasmania, and the Urals. These naturally occurring alloys are sometinies brought into solution only with great difficulty by alkaline fusions and acid extractions. By treatment lvith chlorine on a bed of sodium chloride, a Tasmanian iridosmine which had resisted complete attack by other methods m s brought into solution. Granular specimens of iridosmine and of native platinum were also converted to acid-soluble products in the same way. The conversion of these refractory alloys to soluble products in the absence of sodium chloride was accomplished, but n-as too slow t o be practticable. SE 4 LED-TUBE CH LORlN 4TIOll
B significant advance in dissolving iridium and platinum-metal alloya which do not respond t o ordinary :wid treatment has been made by Wichers and his associates (19) at the National Bureau of Standards. I n this procedure, the sample to be dissolved is heated in a sealed glas3 tube Tvith a solution of concentrated hydrochloric acid containing Oxidizing agents. The latter include chlorine, fuming nitric arid, sodium chlorate, or perchloric acid. Special care must he taheri in the filling aiid sealing of these tubes. If the amount of sample and acid requires a tube larger than 20 cm. hy 4 mm., the waled tube should be heated in a steel bomb. The temperatures of hesting are sometimes as high as 300” C. and pressures as gieat as 300 atmospheres mny be expected inside the tube. Khere larger tubes and a steel bomh are used, it is recommended that dry ice be added to the bomb in order to build up a compensating pressure outqide the tube. It is as well to add sufficient calcium carbonate to the bomb t o neutralize the avid should there be an uriIirenietlit:rtt.tl hreahagr of the glass tube. Wichers and coworkers have shown that this procedure will dissolve iridium, iridosmine, and a variety of platinum metal alloys. The method possesses one advantage over chlorination in that comparativPly large amounts of srdt are not introduced into the sample; this might be an important consideration in preparing sample$ for spectrographic examination or separation by ion euchange. Holyever, chlorination is rt somewhat leps demanding technique. n I s s o L w w P R E P ~ R E D~ L L O Y S
A somewhat different approach to the dissolving of platinum metals, with special attention t o iridium, is proposed by Pollard (14). I n this procedure, the sample is alloyed with tin; the latter is said to dissolve all the metal8 readily. The tin alloy is then dissolved by heating in a solution of lithium sulfate in concentrated sulfuric acid. Two hours’ heating is sufficient to dissolve 10 mg. of iridium, while rhodium is obtained in solution in much less time. Platinum and gold are left partly precipitated by this treatment, but these can be brought back into solution with the other metals as double sulfates, The unusual statement is made that “the platinum metal sulfates are more reactive than the chloro acids which result from an aqua regia attack, and reactions are possible , . . which do not occur in chloride solutions.” Alloying the platinum-metal sample with zirfc, and then dissolving out the active metal is one xell-known method of dispersing the precious metal so as to render it more easily susceptible to chemical attack. Gilchrist ( 1 2 ) recommends that the fusion be done a t red heat in a silica crucible with 10 parts of zinc and a
V O L U M E 2 5 , N O . 11, N O V E M B E R 1 9 5 3 cover of zinc chloride. After a t least 1 hour the crucible is cooled, the ingot removed, and treated without crushing, because of its brittleness, with dilute hydrochloric acid. Fusion of a sample of mixed platinum metals with lead in a carbon crucible is recommended by Schoeller and Powell (16) in order to render palladium, platinum, and rhodium soluble in acids. The fusion produces a button containing the platinum metals which is then subjected t o a parting with nitric acid. This type of procedure is used by Gilchrist for the determination oi iridium alloyed with certain other platinum metals (9). PARTING OF BE4DS
The cupellation of buttons containing the platinum metal.: plus deliberate additions of silver produces beads which a1e ready for the traditional parting procedures. It is recommended that where a complete analysis is required for the platinum metalq cupellation should be avoided and the button should be parted directly. Hon-ever, in the parting assay as applied to platinuni metals two possible parting acids may be used; the choice of the original parting acid will dictate certain of the subsequent stages in the separation of the metals. I t has been found t h a t when nitric acid is used for parting of a bead containing platinum and silver in certain egtablished ratios, the platinum di~solvesalong with the silver and most of the palladium. The platinum seems to pass into solution in a colloidal form. Sulfuric acid, on the other hand, causes the dissolving of the silver and part of the palladium, but leaves gold and platinum unaffected. These distributions actually depend to quite an extent on tht. relative proportions of the metah present, and deliberate addition of silver and gold is necessary to achieve the separations. The residue from a sulfuric acid parting may be recupeled with silver and the resulting bead parted with nitric acid to remove the
161’2 platinum. The weight of platinum, then, is the difference in the weight of the insoluble residue from the two partings. LITERATURE CITED
(1) Allan, W. J.. and Beamish, F. E., , ~ N A L . CHEM.,24, 1569 (1952). (2) Ibid., p. 1608. (3) Allen, W. F.. and Beamish, F. E., Ibid.. 22.451 (1950). (4) Ayres, G. H., and Wells, W. N., Ibid., 22, 317 (1950). (5) Barefoot, R. R., and Beamish, F. E., Ibid., 23, 514 (1951). (6) Ibid., 24, 840 (1952). (7) Bugbee, E. E., “A Textbook of Fire Assaying,” 3rd ed., Srw k’ork, John Wiley & Sons, 1940. (8) Currah, J. E., McBryde, W. 9. E., Cruikshank, A. J., and I3caniish, F.E., IND. ENG.CHEM., A 4 L . ED.,18, 120 (1946). (9) Gilchrist, R., J. Am. Chevz. Soc., 45, 2820 (1923). (10) Hill, 31. A., and Beamish, F. E., AN.~L. CHEM.,22, 590 (1950). (11) Hill, AI. A., and Beamish, F. E., J . Am. Chem. Soc., 72, 4855 (1950).
(12) Hillebrand, Wr.F., Lundell, G. E. F., Bright, H. A., and Hoffman, J. I., “Applied Inorganic Analysis,” Chap. 20, 2nd ed., Sew York, John Wiley 8: Sons, 1953. (13) Latimer, W. M., “Oxidation Potentials,” 2nd ed., New York, Prentice Hall, 1952. (14) Pollard, W. B., Bull. Inst. Mining M e t . , No. 497, 9 (1948). (15) Rogers, W. J., Beamish, F. E., and Russell, D. S., IND.ESG. CHEM.,ANAL.ED.,12, 561 (1940). (16) Schoeller, W. R., and Powell, A. R., “Analysis of Minerals and Ores of the Rarer Elements,” London, Chas. Griffin & Co.,
.
1940. (17) Smith, E;. A , “The Sampling and Asvay of the Precious N e t als,” 2nd ed., London, Chas. Griffin & Co., 1946. (18) Thiers, R. E., Graydon, I%-. F., and Beamish, F. E., ASAL. CHEM.,20, 831 (1948). (19) Wichers, Edward, Schlect, 1%’.G., and Gordon, C . L., J . Rescrtrch T o t / . Bur. Stnndlrrrds, 33, 363, 457 (1944). 1 1 ~ c i . r ~ i :for u revieiv July 13, 1953.
hc~ceptodJuly 17, 1953.
6th Annual Summer Syrnposium-Leess W’aniiliar Elements
Chemical Methods for Separating and Determining Metals Contained in Platiniferous Materials RliLEIGH GILCHRIST
iVational Bureau of Standards, Washington 25, D . C .
B
EFOlE World War I the Sational Bureau of Standards had
published a few papers which dealt with physical properties of the platinum metals. I n 1917 William Francis Hillebrand. chief of the bureau’s Chemistry Division, initiated a program of chemical investigation of the platinum metals, the object of a h i r h IWS the development of methods for preparing the metal. in a tiegrce of purity adequate for the most exacting requiremrnts, and tht. dcvclopment of methods for analyzing platiniferous niateria1.q. To appreciate the advances that were made in the next 25 years in perfec-ting nicthods for analyzing platiniferous materials, it is me11 first to examine the state of the analytical cheniistry of thc precious metals at the beginning of World War I. TRADITIONAL METHODS
Historical. Holtz (bo), after considering the methods (I, 2, 4, 6, 81) which had been proposed for the analysis of native grain platinum and platinum alloys, schematic diagrams of which arc reproduced in the book published by Duparc and Tikonowitsch ( 6 ) , attempted to incorporate their desirable features, together with ioinc of his own device.. into a procedure which he used to
aiialyze native platinum. Holtz’s modifications, however, added little to the solution of the riddle of analyzing crude platinum Two of the methods just cited deserve attention-namely, that of Deville and Stas for the analysis of platinum alloys, and that of I,eidiB, nhich was later developed a t the National Bureau of Standards into a procedure for analyzing dental gold alloy. (10). An important contribution to the analytical chemistry of the platinum metals made a t the University of Geneva was the application by Wunder and Thdringer (2’5) of dimethylglyoximc to the separation and determination of palladium. The precipitation of palladium by dimethylglyoxime deserves a place in the front rank of analytical reactions. Its very excellence serves to show liy contrast how great are the shortcomings of some of the traditional analytical methods for the platinum group. Separation of Platinum, Palladium, Rhodium, and Iridium from One Another by Ammonium Chloride. The most common operation in the analysis of platiniferous material has been the precipitation of platinum as ammonium chloroplatinate. This reaction has been generally used to separate platinum from rhodium and palladium, and, in some schemes of analysi-, from iridium after reducing this metal to the trivalent state.
