separating rhodium from iridium

8/6/2019 Separating Rhodium From Iridium

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QUAN'rITATIVE ANALYSIS OF HHODIUM INMATEHIAL CONTAINING IHIDIUM

Paul Blumberg

Ledoux & Company359 Alfred Avenue, Teaneck, NJ 07666

Rhodium-containing material with iriditun is decomposedby acid ormetal fusion techniques to yield a chloride solution of rhodiumand iridium. Classical approaches are evaluated for theseparation of rhodium and iridium. Finally, the rhodium is reducedwith formic acid and the iridium is removed as a hydrated oxide.An evaluation of gravimetric and instrumental techniques ispresented for optimizing accuracy of the dete~Tninations.

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92QUANTITATIVENALYSISFRHODIUMN MATERIALONTAININGRIDIUM

The accurate analysis of rhodium is canplicated by two features ofrhodiumchemistry:1) There is no specific agent for rhodiumprecipitation.2) Materials with high rhodium concentration are hard tosolubilize.

The first feature is particularly important if the materialcontains iridium. The second feature requires special attentionespecially if the material is metallic and coarse. To showho"these two cond.i+Lonsaffect the analysis of these materials, weshall follow the analyst through the procedure by a parallel pathuntil we have a solution of the material in a chloride medium.DISSOLVINGl'HEMATERIAL

Platinum alloys with rhodium and iridium can be dissolved in aquaregia if the Rh and Ir concentrations together do not exceed 10%.Alloys which contain more than 10%, of Rh and Ir, are usuallyfused with zinc. The zinc fusion is used to break up largerparticles of metal into fine powder. It requires 10 times theweight of zinc, and the mixture is heated under a layer ofammoniumchloride to prevent the zinc fran oxidizing. Zinc isdissolved fran the fused alloy by 6NHCl and the platinum metalsare filtered off. The filter paper and metallic residue isignited in a crucible, reduced under hydrogen and thenchlorinated.Material which is finely powderedor no larger than 40 meshcan bechlorinated directly. The dry chlorination of the platinum metalsis extensively covered in literature. Most of the literature (I,2) indicates that the chlorination can be accanplished by heatingthe sample in a stream of chlorine under NaCl to a temperature of550C. Wehave found this to be impractical because the meltingpoint of NaCl is over 800C and the salt layer inhibits permeationof chlorine throughout the sample.A more efficacious procedure is to convert the material toinsoluble anhydrous RHCl1by heating it in a stream of chlorinefor one hour at 650C. This is a preliminary purification of theRh as well as a concentration. Then the REC13is covered withthree times the sample weight of NaCl. The mixture is heatedrapidly to 825C. The sodium salt of rhodium so formed readilydissolves in water. Most of the iridium and platinum remaininsoluble as the metal. Sometimes a second chlorination isrequired to extract all of "the rhodium fran larger quantities ofplatinum and iridium.Another source of mixed rhodium and iridium chloride solutions isaqua regia soluble platinum alloys. In this case, these elementsare separated from platinum by hydrolytic precipitation carried


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93out under strict pH control. (3) The chlorides of the dissolvedmaterial are adjusted to a pI-]of 5.5. At this point NaBrO isadded and the solution heated to oxidize the elements. 3Thesolution is cooled and adjusted to a pI-]of 8 wi-th NaI-lC03.Morebromate is added and the sample again heated. This procedurequantitatively precipitates rhodium, iridium and palladium Whileplatinum remains in solution.'l'he hydrated oxides are dissolved in I-ICIand the palladium isremoved with din-ethyl glyoxime. The filtrate is evaporated andfhe DMGs destroyed with nitric, perchloric and sulfuric acids.'l'he material is dissolved in 10% I-]Cland filtered. The filterpaper is ignited and fused with Na202. The fusion is leachedin water, acidified with I-]CLnd comblneclwith the decomposedIMGfiltrate. Again we have reached the point where we have achloride solution of rhodium and iridimn for separation.

SEPARATIONFRHODIUMROMRIDIUMThe classical approach at this point is to separate the rhcx:liumfrom the Lr.id.ium by reducing the rhodium with a reducing agent.Titanous chloride, mercurous chloride and freshly precipitated Cusponge have been used successfully for the reduction. (4, 5) Therecommendedmediumfor this reduction is sulf-uric acid.All these reducing agents introduce base metals which have to beremoved. The rhodium can be easily cleanecl of the mercury byignition. HOt/ever, titanium and copper require dry chlorinationand acid treatments for purification. Also, the iridium solutionis contaminated with large quantities of base metals whichcomplicate tlle iridium analysis.The methodology so far described indicates the complexity, and thelarge number of manipulations involved in rhodium analysis. Eachtransfer carries with it the possibility of loss and introducessome ezror , The final rhodium sponge has to be examined forimpurities, providing another source of ezror ,Also the recovery solutions from the various separations must bechecked instrumentally for rhodiumwit~ additional e=ors.The statistical errors can be approximated both for thegravimetric analysis and +he instrmnental detiermi.nat.i.on, Franthis evaluation, a decision can be madeas to howto proceed. Therhodium sponge should weigh more than 100 milligrams. On astandard analytical balance this limits the error less than .3%.The impurities in the weighed rhodium sponge can be determinedwith a accuracy of 25%spectrographically. If the impurity leveldoes not exceed 1%, this introduces another .25%error. Impuritylevels above 1% are not acceptable, especially by emissionspectroscopy, because the estimates are less reliable. The errorassociated in determining the rhodium instrmnentally is 3%. Itcan be much better than this, but when all the various recoverysolutions are combined, the instrlUnent of choice is the DCplasmaor AA. If less than 5%of the rhodium is in +he recovery I theerror associated with this measurement is .15%. Within these


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parameters, then, the gravimetric determination should be betterthan 1%relative.The instrumental analysis can be very accurate. The amount ofwork required for this accuracy is just as demanding. First, allinstrumental techniques are canparative methods, and they requireexacting calibration. 1'0 achieve better than 1% standarddeviation on the measurementalone requires a superior instrument.It cannot be done on sequential ICP or OCPunits. It can beapproached by AA, if all the solution parameters are very closelymatched. Usually this is not practical, because rarely are allB1ecomponentsof a rhodiumsample that well characterized.A simultaneous ICP can be calibrated to provide very accurateresults. The uncertainties associated wi.th sample introductionand plasma instability can be corrected by use of internalstandards. More than one internal standard should be used toavoid erroneous results caused by spectral line interferences. Twolines for the element should be used for the same reason. Thiswill provide a 1%or better error in measurement and identifyinterferences in the analysis.Unfortunately, there are other problems associated withinstrumental work, namely dilutions. The mul.t.Ipe suJx1ivision ofa solution to reach the desired concentration range also haserrors associated wi+h it. One way to improve the accuracy ofdilution is to makeweighed aliquots.With these considerations in mind, we proceed to our separation ofrhodium from iridium. If we have isolated these two elements asdescribed by the dry chlorination and NaCl fusion, most of thebase metals are already removed. The dissolution of the sodiumsalts in water will solubilize the rhodium and allow for theremoval of refractories and most of the iridium by filtration.The next step is to acidify the solution to 5%HCland gas the hotsolution with H2S. The gassing is repeated, and the rhodiumsulfide is filfered off. The sulfide and filter paper aredestroyed by heating with a combination of 20ml HN03, 5mlHCl04 and 25ml H2S04 to fumes of H2S04_. The watchglassand 5eaker are washeaWlth metal free water ana refumecl.The cooled solution is diluted to 25%H2S04concentration andthe rhodium is reduced with 20ml of formic acid by vigorousboiling until the solution volume is reduced to 50ml. Thesolution is cooled, diluted to lOOmland the rhodium is filteredoff and ignited in a platinum crucible. The sponge is treatedwith !-IFand HN03to remove Si02. Finally, it is reduced in aRose crucible and weighed. The sponge is checked for purity,preferably by emission spec.The emission spectrograph is invaluable in rhodiumanalysis, firstbecause it is so comprehensive that it can identify unexpectedimpurities and second because the rhodium is hard to dissolvewhich is necessary for instrumental analysis. Anydissolution canintroduce impurities, especially silica. The dilution effects ofthe dissolution will obscure sane impurities.


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CONCLUSION

In summary,manysamples which contain rhodium and iridium can beanalyzed quite successfully using instrumentation available today.Still econanic considerations require splitting limits so tightthat instrwnental determinations alone cannot meet theserequirements. A combination of the gravimetric and instrumentaldeterminations can meet the requirements, if care is taken tomaximize +he purity of the sponge and to limit the rhodiumconcentration in recovery streams.

separating rhodium from iridium

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