Analyst, May, 1974, Vol. 99, $9. 285-295 285 The Determination of Iridium and Ruthenium in Rhodium Sponge by Solvent Extraction Followed by Atomic-absorption Spectrophotometry* BY M. A. ASHY AND J. B. HEADRIDGE (Department of Chemistry, The University, Sheffield, S3 7HF) A method is described for the determination of trace amounts of iridium and ruthenium in rhodium sponge. The sponge is dissolved by treatment with concentrated hydrochloric acid and sodium chlorate in a sealed glass tube a t 250 "C. Chloro-complexes of rhodium and other noble metals are produced. From such a solution, 2 M in hydrochloric acid, iridium and ruthenium are extracted into chloroform that is 1 per cent. rn/V in methyl- triphenylphosphonium chloride ; rhodium is not extracted. The organic phase is evaporated to dryness and the residue dissolved in acetonitrile containing lithium perchlorate, which is the interference suppressant for the subsequent determination of iridium and ruthenium by atomic-absorption spectrophoto- metry. Osmium can be determined in rhodium sponge in a similar manner if precautions are taken so as to prevent loss of osmium when the tube is opened.The results are in good agreement with those obtained by emission spectrography and the limits of detection for iridium and ruthenium in the sponge are 7 and 4.5 pgg-l, respectively. A COMPLEX series of chemical operations is required for the preparation of pure rhodium sponge from ore samples. In the final stages of the refining process rhodium sponge can be produced that contains trace amounts of other noble metals.The efficiency of the process can be ascertained by determining the iridium and ruthenium contents of the sponge. When this work was started in 1972, the detection limits for the spectrographic determination of platinum, ruthenium, osmium and iridium in rhodium sponge were 5, 30, 30 and 40 pg 8-1, respectively. It was felt that the detection limits for iridium, osmium and ruthenium could be improved by solvent extraction of these trace elements from an aqueous solution of the rhodium metal into an organic solvent, and subsequent spraying of the organic solvent into the flame of an atomic-absorption spectrophotometer. Headridge and co-workers have found that the detection limits for the determination of aluminium,l antimony,2 bismuth3 and tin4 in steels following solvent-extraction procedures are improved by factors of thirty to sixty compared with methods involving direct atomic- absorption spectrophotometry in aqueous solutions.Slavin5 states that the concentrations of iridium, osmium and ruthenium that produce 1 per cent. absorption are 8, 1 and 0.3 pg m1-1, respectively, using the atomic lines at 208.9 nm, 290-9 nm and 349.9 nm, respectively. Iridium and ruthenium were determined with the air - acetylene flame but osmium was determined with the nitrous oxide - acetylene flame. A search of the chemical literature has revealed that 10 to 150 pg of iridium can be separated from 1 to 2 mg of rhodium by means of a single extraction.6 The extraction was carried out by adding 3 ml of a 2 per cent.solution of tetraphenylphosphonium bromide in water to 15 to 20 ml of iridium(1V) in approximately 0.1 M hydrochloric acid and shaking the mixture for 3 minutes with 10 to 25ml of chloroform. An extractable ion-association complex is formed with the doubly charged hexachloroiridate(1V) ion, but not with the triply charged hexachlororhodate(II1) ion. F0k7 has also shown that iridium(1V) can be extracted quantitatively from 0.1 M hydro- chloric acid with an equal volume of chloroform containing 1 per cent. m/V of tetraphenyl- arsonium chloride. The hexachlororhodate(II1) anion is not extracted. He also states that palladium(I1) and (IV), osmium(IV), ruthenium(1V) and platinum(1V) also form extractable compounds with the tetraphenylarsonium cation. * Presented a t a meeting of the Society for Analytical Chemistry and Analytical Division of the Chemical Society, Sheffield, July 12th and 13th, 1973.0 SAC and the authors.286 ASHY AND HEADRIDGE: DETERMINATION OF Ir AND Ru IN Rh SPONGE [Analyst, Vol. 99 If rhodium sponge containing trace amounts of osmium, ruthenium, iridium, palladium and platinum could be brought into solution in hydrochloric acid such that the oxidation states of the chloro-species were rhodium(III), ruthenium( IV), osmium(IV), iridium( IV), palladium(I1) or (IV) and platinum(IV), then it seemed likely that trace amounts of iridium, and possibly also ruthenium, osmium, palladium and platinum, could be readily separated from rhodium by solvent extraction. Dissolution of the noble metals can be achieved by reaction at 250 "C in a sealed tube with chlorine produced from sodium chlorate and concentrated hydrochloric acid.* This method of dissolution was used as chloro-complexes of the noble metals are produced.A consideration of the inorganic chemistry of the noble metals indicated that treatment with sodium chlorate and concentrated hydrochloric acid at 250 "C should produce hexachloro- rhodate( 111) , hexachloroiridate( IV) , hexachloroplatinate( IV) , probably mainly hexachloro- ruthenate(1V) and hexachloropalladate(IV), and probably osmium(VII1) tetroxide together with the dioxotetrachloroosmate(V1) anion, [OSO,C~,]~-.~~~O After diluting the solution to make it 2 M in hydrochloric acid (before the solvent extraction), the osmium will probably be present as a mixture of osmium(V1) and osmium(IV), particularly if the solution is allowed to stand at some stage.Both osmium tetroxide and the dioxotetrachloroosmate(V1) ion will oxidise the chloride ion in hydrochloric acid to chlorine, the former much more rapidly than the latter. In fact, when standard solutions of rhodium, iridium, ruthenium and osmium in 6 M hydrochloric acid were prepared by the above treatment with concentrated hydrochloric acid and sodium chlorate, and these solutions diluted to make the hydrochloric acid concentration 2 M, it was found that the ultraviolet and visible absorption spectra of the solutions indicated that they contained predominantly the hexachloro-complexes of rhodium(III), iridium(IV), ruthe- nium(1V) and osmium(1V). In this paper, a method is described for the dissolution of rhodium metal with concen- trated hydrochloric acid and sodium chlorate in a sealed glass tube at 250 "C, and the removal of iridium(1V) and most of the ruthenium(1V) from a solution of the rhodium sponge in 2 M hydrochloric acid by extraction into chloroform containing methyltriphenylphosphonium chloride.The chloroform layer is evaporated to dryness, the residue taken up in acetonitrile, lithium perchlorate added as an interference suppressant and the solution made up to 5 ml with acetonitrile. The concentrations of iridium and ruthenium in this solution, and hence in rhodium sponge, are determined by use of atomic-absorption spectrophotometry. The method should also be suitable for the determination of trace amounts of osmium, if present, in rhodium sponge provided that care is taken to prevent any loss of trace amounts of volatile osmium tetroxide when the tube containing the treated rhodium sponge is opened.EXPERIMENTAL APPARATUS- Pyrex glass tubes, 9 mm in internal diameter, 1.5 mm in wall thickness and sealed at one end, were prepared. These were drawn out near to the open end to form a constriction in the tube with a minimum internal diameter of 2 to 3mm. The tubes could be sealed by heating these constrictions with an oxy-propane torch. When 100 mg of noble-metal sponge were to be dissolved, the sealed tube was 25 cm long, but for samples of 500 mg, longer tubes (50 cm) were used. Before heating was started, the tube was enclosed in a steel casing with a steel screw-top.The steel casing contained 20 g of calcium carbonate powder in order to neutralise acid, which would escape from the sealed tube in the unlikely event of the tube exploding. A small escape hole for gases was also drilled through the side of the casing. Atomic-absorption spectrophotometric measurements were made with Unicam SP90 Series 1 and Unicam SP1900 instruments fitted with air - acetylene or nitrous oxide - acetylene burners. REAGENTS- Hydrochloric acid, sp. gy. 1-18-Analytical-reagent grade. Sodium chlorate-General-purpose reagent. Lithium @erchlorate-General-purpose reagent, dried at 100 "C. Chloroform-Analytical-reagent grade.May, 19741 BY SOLVENT EXTRACTION AND ATOMIC-ABSORPTION SPECTROPHOTOMETRY 287 Acetonitvile-General-purpose reagent, redistilled at 82 "C before use.~ethyltriphenylphosphonium chloride-This reagent was prepared from methyltriphenyl- phosphonium bromide (Koch-Light Laboratories Ltd.) by passing an aqueous solution of the salt through a column of De-Acidite FF resin in the chloride form. The effluent was evaporated at 100 "C nearly to dryness and the solid was then dried in an oven at 90 "C for 6 hours. Rhodium, iridium, ruthenium and osmium sponges-These were of Specpure quality and were supplied by Johnson Matthey Chemicals Ltd. Potassium hexachloropnlladate ( I V ) and potassium hexaclzloroPlatiizate(rV)-These salts were prepared from palladium( 11) chloride and platinum (IV) chloride, respectively. Standard iridium solution A (1000 pg ml-l)-Weigh accurately 100 mg of iridium sponge into a small glass tube, transfer it quantitatively into a Pyrex tube (25 cm long when sealed) and add 5 ml of concentrated hydrochloric acid (sp. gr.1-18>. Immerse the tube in liquid nitrogen or cardice until the contents just start to solidify and then add 0.2 g of sodium chlorate crystals. Remove the tube from the coolant, seal it and allow it to attain room temperature. Place it in a steel casing fitted with a screw-cap and heat in an oven at 250 "C for 6 hours so as to dissolve all of the sponge. Allow the tube to cool to room temperature and cool it again in liquid nitrogen or cardice until the contents just start to solidify. Open the tube by scratching the glass near to the sealed end and giving it a sharp tap. (As the tube breaks, chlorine, under pressure in the tube, may escape.) Transfer the contents into a 100-ml calibrated flask containing 45 ml of concentrated hydrochloric acid, rinse the tube several times with distilled water and make the volume up to the mark so that the concen- tration of hydrochloric acid in the final solution is approximately 6 M.Standard iridium solution B (100 pg ml-l)-Dilute 10 ml of solution A to 100 ml with 2 M hydrochloric acid immediately before use. Standard ruthenium solutions A (1000 pg ml-l) and B (100 pg ml-1)These solutions are prepared in a similar way to the iridium solutions. Standard rhodium solution (5000 pg ml-l)-This is prepared in a similar way to the above solutions, except that 500 mg of rhodium sponge, 10 ml of concentrated hydrochloric acid and 0.6 g of sodium chlorate are used in a 50 cm long sealed tube.Standard osmium solution (1000 pg mZ-l)-This is prepared in a similar way to the above solutions, but before opening the tube, the contents are solidified completely by immersing the tube in liquid nitrogen. The opened tube is then placed upright in a 100-ml beaker and held clear of the sides of the beaker by means of tongs fitted with glass end-pieces. From a beaker immersed in cardice, 45 ml of cooled, concentrated hydrochloric acid are then added to the tube so as to fill it completely, the excess of acid being added to the 100-ml beaker. A long test-tube is inverted over the glass tube containing the osmium such that the open end of the tube is immersed in the acid in the beaker. This outer tube is used in order to direct into the beaker any solution that may spray from the inner tube as the solid chlorine melts and then vaporises.The beaker and tubes are allowed to stand until they attain room temperature; as the liquid in the tube expands, it overflows into the beaker. Rinse the outer test-tube with distilled water and collect the rinsings in the beaker. Then transfer approximately 5 ml of liquid from the tube into the beaker with a narrow diameter pipette and rinse the outside of the tube with distilled water, collecting the rinsings in the beaker. Add the contents of the tube to the beaker and rinse the tube with distilled water. Finally, transfer the contents of the beaker into a 100-ml calibrated flask and dilute to the mark. By use of this method no loss of volatile osmium tetroxide can occur.EXTENTS OF EXTRACTION OF THE NOBLE METALS- Rhodium-Two extractions with 10-ml volumes of chloroform that was 1 per cent. m/V in methyltriphenylphosphonium chloride were carried out on 40 ml of 2 M hydrochloric acid containing 1000 pg of rhodium, prepared from standard rhodium solution. The aqueous solution was diluted to 50 ml. The amount of rhodium in the aqueous phase was determined by atomic-absorption spectrophotometry. Palladium and platinum-A similar procedure to that for rhodium was applied to solutions of palladium(1V) and platinum(1V) in 2 M hydrochloric acid.288 ASHY AND HEADRIDGE : DETERMINATION OF Ir AND Ru IN Rh SPONGE [Analyst, Vol. 99 Iridiwm-Two extractions were carried out with 60-ml volumes of chloroform that was 1 per cent. m/V in methyltriphenylphosphonium chloride on 240 ml of 2 M hydrochloric acid containing 3000 pg of iridium, prepared from standard iridium solution A.The aqueous phase was evaporated to dryness and the residue taken up in 5 ml of 2 M hydrochloric acid. The concentration of iridium was determined in this solution by atomic-absorption spectro- photometry. Ruthenium-A similar procedure was applied to a solution of ruthenium in 2 M hydro- chloric acid, prepared from standard ruthenium solution A. Osmium-A similar extraction procedure was applied to a solution of osmium in 2 M hydrochloric acid prepared from standard osmium solution. As solutions in 2 M hydrochloric acid prepared from the standard osmium solution absorb strongly at 372 nm, solution absorp- tion spectrophotometry was used in order to determine the concentration of osmium remaining in the aqueous phase.The extents of extraction of the above six elements into chloroform are shown in Table I. TABLE I EXTENTS OF EXTRACTION OF NOBLE METALS INTO CHLOROFORM Element Amount extracted, per cent. Rhodium . . .. . . 0 Iridium . . .. .. .. 100 Ruthenium . . .. .. 90 Osmium.. .. .. .. 100 Palladium . . .. .. 43 Platinum . . . . .. 93 TENTATIVE METHOD FOR THE DETERMINATION OF IRIDIUM AND RUTHENIUM I N RHODIUM SPONGE- A tentative method for these determinations could now be devised and was as follows. To 0.5 g of rhodium sponge contained in a Pyrex tube, add 10 ml of concentrated hydrochloric acid. Cool the tube and its contents in liquid nitrogen and add 0.6 g of sodium chlorate, then seal the tube and heat it for a minimum of 6 hours at 250 "C.Open the tube and transfer the contents quantitatively into a 250-ml separating funnel, then add 10 ml of concentrated hydrochloric acid and dilute the solution to 120 ml. Add 30 ml of chloroform that is 1 per cent. m/V in methyltriphenylphosphonium chloride to the mixture in the funnel and shake it vigorously for 2 minutes. Allow the layers to separate and transfer the organic phase into a 100-ml beaker, then add 30ml of chloroform solution and extract again. Combine the chloroform phases and evaporate them to dryness on a steam-bath, as chloroform has unsatisfactory burning characteristics in flames. Take up the residue in 2 ml of acetonitrile and transfer the solution into a 5-ml calibrated flask, washing the beaker with acetonitrile and finally diluting the solution to the mark with the same solvent.Determine the flame absorbances for iridium and ruthenium in this solution by nebulising the solution into the appropriate flame of an atomic-absorption spectrophotometer. By use of suitable calibration graphs, calculate the concentration of these elements in the rhodium sponge. ATOMIC-ABSORPTION SPECTROPHOTOMETRIC DETERMINATION OF IRIDIUM, RUTHENIUM AND Iridium-By using the Unicam SP90 spectrophotometer, the 263.9 nm atomic line from an iridium hollow-cathode lamp and an air - acetylene flame, a calibration graph of absorbance Venus concentration was constructed for iridium (0 to 2000 pg ml-l) in 2 M hydrochloric acid by appropriate treatment of standard iridium solution A. The instrument conditions for this graph are given in Table 11.In order to obtain the calibration graph for iridium in acetonitrile, the following method was used. To six separating funnels add 0, 1, 2, 3, 4 and 5 ml of standard iridium solution B and make up the volumes of the solutions to approximately 40 ml by adding 2 M hydrochloric acid. In each instance extract the solution twice by vigorous shaking with 10-ml volumes of chloroform that is 1 per cent. m/V in methyltriphenylphosphonium chloride. Transfer the combined chloroform phases into 100-ml beakers and evaporate to dryness on the steam- bath. Take up the residue in each beaker with 2 ml of acetonitrile and transfer the solutions OSMIUM-May, 19741 BY SOLVENT EXTRACTION AND ATOMIC-ABSORPTION SPECTROPHOTOMETRY 289 into 5-ml calibrated flasks marked 0, 20, 40, 60, 80 and 100 pg ml-1 of iridium, Dilute to the marks with the acetonitrile solvent and obtain the absorbance for each solution nebulised into the flame under the conditions given in Table 11.Calibration graphs for iridium in solution in 2 M hydrochloric acid (0 to 250 pg ml-1) and in acetonitrile (0 to 80 pg ml-l) were also obtained by using the Unicam SP1900 spectro- photometer under the conditions given in Table 111. Ruthenium and osmium-By using the Unicam SP90, calibration graphs were also obtained for ruthenium in solution in 2 M hydrochloric acid (0 to 400 pg ml-l) and in aceto- nitrile (0 to 100 pg ml-l), and for osmium in 2 M hydrochloric acid (0 to 2000 pg ml-l) and in acetonitrile (0 to 200 pg ml-1). The solvent-extraction procedures were identical with those used for iridium. The conditions for the atomic-absorption spectrophotometric deter- minations are shown in Table 11.TABLE I1 INSTRUMENT CONDITIONS FOR THE ATOMIC-ABSORPTION SPECTROPHOTOMETRIC DETERMINATION OF IRIDIUM, RUTHENIUM AND OSMIUM WHEN USING THE UNICAM SP90 INSTRUMENT Instrument parameter Air a t 30 p.s.i. . . .. . . C2H2 a t 7 p.s.i. (aqueous solution) N,O a t 30 p.s.i. . . .. . . C2H2 at P.s-i. (CH3CN) . . C2H2 a t 15 p.s.i. (CH3CN) . . CaH, a t 15 p.s.i. (aqueous solution) .. .. .. .. . . .. . . . . .. Wavelength of line used/nm Slit width/mm . . . . Lamp current/mA . . . . . . .. Distance of centre of light Air - C2H2 .. ..path above burnerlmm. {N20 - C2H2 .. .. Iridium 5 1.2 1.0 - 263.9 0.03 15 9 Ruthenium 5 1.4 1.2 5 2.8 2-6 349.8 0.03 15 5 5 Osmium - 5 3 2.8 290-9 15 10 0.03 - Calibration graphs for ruthenium (0 to 100 pg ml-l) and osmium (0 to 400 pg ml-1) in 2 M hydrochloric acid, and for ruthenium (0 to 40 pg ml-1) and osmium (0 to 80 pg ml-1) in acetonitrile, were also obtained by use of the Unicam SP1900 instrument under the conditions given in Table 111. TABLE I11 INSTRUMENT CONDITIONS FOR THE ATOMIC-ABSORPTION SPECTROPHOTOMETRIC DETERMINATION OF IRIDIUM, RUTHENIUM AND OSMIUM WHEN USING THE UNICAM SP1900 INSTRUMENT Instrument parameter Air a t 30 p.s.i. . . . . .. C2H, a t 10 p.s.i. (CH,CN) N20 a t 30 p.s.i. . . .. . . C2H2 a t 10 p.s.i. (CH,CN) C2H2 a t 10 p.s.i. (aqueous solution) C,H2 a t 10 p.s.i.(aqueous solution) .. . . .. .. .. .. .. .. .. .. .. .. .. ,. .. .. .. .. .. . . .. .. . . .. .. .. Wavelength of line used/nm Slit width/mm . . Sensitivity . . Integration timels Lamp current/mA . . Distance of centre of light Aqueous solution .. path above burner/mm { CH,CN . . .. .. Iridium 5 1.2 0.7 - 263.9 0.03 490 4 15 9 9 Ruthenium 5 1.2 5 2-8 349.8 0.03 580 4 15 8 10 - - Osmium - 6 3 2.8 290.9 0.06 518 4 15 10 10 The flow-rates shown in Tables I1 and I11 were those found to produce the maximum absorbances for solutions of the noble metals. The nitrous oxide - acetylene flames were lit and extinguished via air - acetylene. The concentrations of iridium, ruthenium and osmium in 2 M hydrochloric acid and in acetonitrile that produce 1 per cent.absorption using both the Unicam SP90 and Unicam SP1900 atomic-absorption spectrophotometers are shown in Table IV.290 ASHY AND HEADRIDGE: DETERMINATION OF I r AND Ru IN Rh SPONGE [Analyst, Vol. 99 TABLE IV CONCENTRATIONS OF IRIDIUM, RUTHENIUM AND OSMIUM PRODUCING 1 PER CENT. ABSORPTION Sensitivity in aqueous Sensitivity in aceto- Element Flame type solution/pg ml-l nitrilelpg ml-l & r- SP90 SP1900 SP90 SP1900 Iridium .. . . Air- C,H, 17 16 2.2 1.6 Ruthenium . . . . Air-C,H, 14 0.6 20 - NZO - CZH, 29 2.5 10-7 0.8 Osmium .. . . N,O-C,H, 65 5 12.5 1.5 For 0.5-g samples of rhodium sponge, the figures in the last column of Table IV corres- pond to 16, 8 and 15 pg g-l of iridium, ruthenium and osmium, respectively. The limits of detection would be lower than these values and a method based on the tentative method for the determination of iridium, ruthenium and osmium in rhodium sponge will be more sensitive than the spectrographic method (see above).However, before such a method could be applied to rhodium sponges, it was necessary to investigate the possible interfering effects of other extractable noble metals on the absorbances of solutions of iridium, ruthenium and osmium. INTERFERING EFFECTS OF RUTHENIUM, OSMIUM, PLATINUM AND PALLADIUM ON IRIDIUM- Twenty solutions, each containing 200 pg of iridium in addition to 200 to 1000 pg of ruthenium, osmium, palladium or platinum, were extracted in a similar way to that described for the iridium in acetonitrile calibration graph. The flame absorbances (air - acetylene flame) of the solutions of iridium containing different amounts of other co-extracted noble metals in acetonitrile were measured on the Unicam SP90 instrument and compared with the flame absorbance of acetonitrile containing iridium alone. All of these added elements affected the iridium absorbance, as shown in Fig. 1.0.1 1 0.1 0 L - ; 0.09 01 2 0.08 - 0 - 8 007 c 7J I L 2 0.06 a 0 05 Os, Pd, Pt or R u with LiCIO, added 0.04 Amount of interfering element added/pg Fig. 1. Effects of extracted osmium, palladium, platinum and ruthenium on the absorbance for extracted iridium. The amounts of elements are those originally present in the aqueous solutions before extraction INTERFERING EFFECTS OF IRIDIUM, OSMIUM, PALLADIUM AND PLATINUM ON RUTHENIUM- Twenty solutions, each containing 300 pg of ruthenium in addition to 250 to 1250 pg of .. .. .. .. . .. . ~ * . . .. I . - . . iridium, osmium, palladium or platinum, were extracted in a similar way to that describedMay, 19741 BY SOLVENT EXTRACTION AND ATOMIC-ABSORPTION SPECTROPHOTOMETRY 291 for the iridium in acetonitrile calibration graph. The flame absorbances (nitrous oxide - acetylene flame) of the solutions of ruthenium in acetonitrile containing different amounts of other co-extracted noble metals were measured on the Unicam SP90 instrument and compared with the flame absorbance of acetonitrile containing ruthenium alone. All of these added elements affected the ruthenium absorbance, as shown in Fig. 2. =I oeo6 Ir, Os, Pd or Pt with LiCIO, added + - - -1- - -I-- - -1- - + - - [I v- 0.05 cn a 0 c3 0.04 z cc 0) 2 0.03 5 8 2 0.02 0.01 0 200 400 600 800 1000 1200 1400 L I Pt Pd Ir O.O1 0 1 200 400 600 800 1000 1200 1400 Amount of interfering element added/pg Fig.2. Effects of extracted iridium, osmium, palladium and platinum on the absorbance for extracted ruthenium. The amounts of elements are those originally present in the aqueous solutions before extraction INTERFERING EFFECTS OF RUTHENIUM, IRIDIUM, PALLADIUM AND PLATINUM ON OSMIUM- Twenty solutions, each containing 250pg of osmium in addition to 250 to 1250pg of ruthenium, iridium, palladium or platinum, were extracted in a similar way to that described for the iridium in acetonitrile calibration graph. The flame absorbances (nitrous oxide - acetylene flame) of the solutions of osmium in acetonitrile containing different amounts of other co-extracted noble metals were measured on the Unicam SP90 instrument and com- pared with the flame absorbance of acetonitrile containing osmium alone.Iridium did not interfere but the other added elements affected the osmium absorbance, as shown in Fig. 3. ELIMINATION OF INTERFERENCES IN THE DETERMINATION OF IRIDIUM, RUTHENIUM AND OSMIUM- Pannetier and Toff olill successfully used lithium sulphate as an interference suppressant when trace amounts of platinum, iridium, rhodium and palladium were determined in matrices of hexachlororhodic, hexachloroiridic and hexachloroplatinic acids. Lithium perchlorate is readily soluble in non-aqueous solvents and when it was added to solutions of the noble metals in acetonitrile the interferences were eliminated.Thus, 1 ml of a 5 per cent. m/V solution of lithium perchlorate in acetonitrile was added to the solutions of the noble metals in acetonitrile before the volumes were adjusted to 5 ml. The concentration of lithium ions in the solutions to be nebulised was, therefore, approximately 6OOpgml-l. As shown in Figs. 1, 2 and 3, the addition of lithium perchlorate not only removed the interferences due to the other noble metals, but also enhanced the flame absorbances for ruthenium and osmium and allowed these two elements to be determined with considerably greater sensitivity. No enhancement in flame absorbance occurred for iridium when lithium perchlorate was present in the solutions.292 ASHY AND HEADRIDGE: DETERMINATION OF Ir AND Ru IN Rh SPONGE [Analyst, Vol.99 0'036 I - + - - 1 - - -1- - + - _ I - - - -- Ir, Pd, P t or Ru with LICIO, added c; 0.028 0.020 - 0.004 - Pd lr P t Ru I I I I I I I 0 250 500 750 1000 1250 1500 1' i0 Amount of interfering element added/pg Fig. 3. Effects of extracted iridium, palladium, platinum and ruthenium on the absorbance for extracted osmium. The amounts of elements are those originally present in the aqueous solutions before extraction FINAL METHOD FOR THE DETERMINATION OF IRIDIUM AND RUTHENIUM IN RHODIUM SPONGE To 0.5 g of rhodium sponge, containing up to 0.1 per cent. each of iridium and ruthenium, in a Pyrex tube (50 cm long when sealed), add 10 ml of concentrated hydrochloric acid. Immerse the tube in liquid nitrogen or cardice until the contents just start to solidify and then add 0.6 g of sodium chlorate crystals.Remove the tube from the liquid nitrogen, seal it and allow it to attain room temperature. Place it in a steel casing fitted with a screw-cap and heat it in an oven at 250 "C for at least 6 hours so as to dissolve all of the sponge. Allow the tube to cool to room temperature and cool it again in liquid nitrogen or cardice until the contents just start to solidify. Open the tube by scratching the glass near to the sealed end and giving it a sharp tap. (As the tube breaks, chlorine, under pressure in the tube, may escape.) Transfer the contents of the tube quantitatively into a 250-ml separating funnel, then add 10 ml of concentrated hydrochloric acid and dilute the solution to 120 ml.Add 30 ml of chloroform that is 1 per cent. m/V in methyltriphenylphosphonium chloride to the mixture in the funnel and shake it vigorously for 2 minutes. Allow the layers to separate and transfer the organic phase into a 100-ml beaker, then add 30 ml of chloroform solution and extract again. Combine the chloroform phases and evaporate them to dryness on a steam-bath. Take up the residue in 2 ml of acetonitrile and transfer the solution into a 5-ml calibrated flask. Rinse the beaker with 1 ml of acetonitrile and add the rinsings to the flask, then add 1 ml of a 5 per cent. m/V solution of lithium perchlorate in acetonitrile, ignoring a small precipitate of methyltriphenylphosphonium perchlorate that forms, and dilute the solution to the mark with the same solvent.Determine the flame absorbances for iridium and ruthenium in this solution by using air - acetylene and nitrous oxide - acetylene flames, respectively, and the instrument con- ditions outlined in Table 111. If the rhodium sponge contains not less than 70 pg g-1 of iridium and 200 pg g-l of ruthenium, the instrument conditions outlined in Table I1 can be used. Read off the concentrations of iridium and ruthenium from calibration graphs prepared from solutions, the flame absorbances of which were determined at the same time as those of the solutions prepared from the rhodium sponge samples. PREPARATION OF THE CALIBRATION GRAPHS- Iridium-The method is identical with that described under Iridium in the section entitled Atomic-absorption spectrophotometric determination of iridium, ruthenium andMay, 19741 BY SOLVENT EXTRACTION AND ATOMIC-ABSORPTION SPECTROPHOTOMETRY 293 osmium, except that a l-ml volume of a 5 per cent.m/V solution of lithium perchlorate in acetonitrile is added to each 5-ml calibrated flask before diluting to the mark. Ruthenium-The method is identical with that used for iridium except that standard ruthenium solution B is used. ANALYSIS OF SYNTHETIC MIXTURES OF RHODIUM SPONGE WITH TRACE AMOUNTS OF IRIDIUM, Seven synthetic mixtures were prepared by adding fixed volumes of standard solutions of iridium and ruthenium to 0.5-g amounts of Specpure rhodium sponge in seven Pyrex tubes. Dissolution was achieved in a similar way to that described in the final method and the iridium and ruthenium contents of these solutions were determined as outlined in the final method. Four of these solutions also contained known trace amounts of osmium, which were later determined by atomic-absorption spectrophotometry in a manner similar to that for iridium, but this osmium was added after the dissolution procedure and before the solvent extraction (see Discussion).ANALYSIS OF RHODIUM SPONGE SAMPLES- by means of the final method, as was a l-g sample of rhodium trichloride. RUTHENIUM AND OSMIUM ADDED- Eleven samples of rhodium sponge were analysed for iridium and ruthenium contents RESULTS The sensitivities of the atomic-absorption spectrophotometric determinations of iridium, ruthenium and osmium in acetonitrile containing lithium perchlorate are given in Table V.TABLE V CONCENTRATIONS OF IRIDIUM, RUTHENIUM AND OSMIUM PRODUCING 1 PER CENT. ABSORPTION I N ACETONITRILE CONTAINING LITHIUM PERCHLORATE Sensitivity in acetonitrile containing LiClO,/pg ml-1 Element Flame type SG90 SP1400 Iridium . . .. Air-C2H, 2.2 1.6 Ruthenium . . . . Air-C,H, 15 N2O - C,H2 3 0.4 - Osmium . . .. NZO-CZH, 7 1 Results for the analysis of synthetic mixtures of rhodium containing trace amounts of iridium, ruthenium and osmium are shown in Table VI. These results are corrected for the trace amount of iridium present in the Specpure rhodium sponge; no ruthenium or osmium could be detected in this sponge. The results for mixtures 1, 2, 3 and 4 were obtained with the Unicam SP90 instrument. The Unicam SP1900 instrument was used in the analysis of mixtures 5, 6 and 7.TABLE VI RESULTS FOR THE ANALYSIS OF SYNTHETIC MIXTURES OF RHODIUM CONTAINING TRACE AMOUNTS OF IRIDIUM, RUTHENIUM AND OSMIUM Mixture 1 2 3 4 5 6 7 Iridium, per cent. & Added Found 0.060 0.058 0.040 0.037 0.020 0.017 0.100 0.099 0.020 0.022 0.080 0.082 0.160 0.155 Ruthenium, per cent. +--7 & Added Found Added Found 0.040 0.041 0.120 0.115 0.100 0.100 0.080 0.075 0.020 0.019 0.160 0.150 0.060 0.061 0.040 0.042 0.080 0.075 - - 0.060 0.065 0.020 0.020 - - Osmium, per cent. - - Results for the determination of iridium and ruthenium in rhodium sponges by the described method are given in Table VII, together with results for the spectrographic deter- mination of these elements in the sponges. The atomic-absorption results are the averages of two determinations on each sample.294 ASHY AND HEADRIDGE: DETERMINATION OF Ir AND Ru IN Rh SPONGE [Analyst, Vol.99 TABLE VII COMPARISON OF RESULTS FOR THE DETERMINATION OF IRIDIUM AND RUTHENIUM I N RHODIUM SPONGE BY ATOMIC-ABSORPTION SPECTROPHOTOMETRY AND EMISSION SPECTROGRAPHY Iridium content, per cent. f A > Spectrographic Atomic-absorption Sample analysis spectroyhotometr y A 0.1 0.098 B 0.001 N.D. C 0.01 0.012 D 0.015 0.014 E 0.007 0.008 F 0-03 0-027 G 0.006 0.010 H 0.03 0.029 0.03 0.027 0.007 0.008 J K Specpure sponge - 0.005 RhCl,.xH,O - 0.042 N.D. = Not detected. Ruthenium content, per cent. A f \ Spectrographic Atomic-absorption analysis spectrophotometry 0.07 0-0770 N.D. N.D. 0.020 0.0190 0.0060 0.005 0.005 0.0070 0.007 0.0075 0-007 0~0080 0.015 0.0150 0.001 N.D.0.005 0.0055 - N.D. - 0.02 18 DISCUSSION The results obtained for the determination of trace amounts of iridium in actual and simulated rhodium sponge samples are considered to be satisfactory. Only with iridium for sample G is there some discrepancy between the spectrographic and atomic-absorption spectrophotometric results. The limits of detection for the determination of iridium and ruthenium in rhodium sponge were 7 and 4-5 pg gl, respectively, where the limit of detection is defined as that concentration of the element which gives a signal equal to twice the standard deviation of a series of ten determinations near to the blank level. The spectrographic results were obtained in the laboratories of Johnson Matthey Chemicals Ltd.by using solid samples in a d.c. arc, intensities being recorded photographically on an Ebert 3-m instrument. Rhodium sponge seldom contains osmium in detectable amounts but if such a deter- mination has to be made, it is advisable to freeze completely the contents of the Pyrex tube by immersing it in liquid nitrogen before it is broken and to treat the contents in the manner specified in the method for preparing a standard solution of osmium. If a known trace amount of osmium is added to 0.5 g of rhodium sponge and the sponge dissolved according to the procedure of the final method, in which the contents of the tube are cooled until they just start to solidify, there is only a 97 per cent. recovery of osmium, presumably because a small amount of volatile osmium tetroxide escapes with the excess of chlorine from the tube when it is broken.Rhodium sponge is particularly difficult to dissolve, but the use of sealed glass tubes in the dissolution procedure was very helpful. If simple precautions are taken, these tubes are not dangerous and are easy to handle. No explosions occurred when tubes containing 1 g of sodium chlorate and 10 ml of concentrated hydrochloric acid were heated to a maximum temperature of 260 "C. The use of a PTFE-lined steel pressure vessel for the dissolution of rhodium, iridium and ruthenium with sodium chlorate and concentrated hydrochloric acid has been investigated. Unfortunately, neither rhodium nor iridium could be dissolved completely at a temperature of 220 "C on heating for 14 hours. When the temperature was increased, the PTFE started to soften and gases escaped from the pressure vessel.Ruthenium was found to dissolve at 220 "C on treatment in the pressure vessel for 24 hours but iridium still resisted complete dissolution under similar conditions. It is well known that noble-metal hollow cathodes have complicated emission spectra, and the lack of sensitivity for ruthenium and osmium in atomic-absorption spectrophotometric determinations when the Unicam SP90 instrument was used is due to the inexpensive mono- chromator of the instrument, which could not resolve the resonance lines from the emission spectra. When iridium was determined, the less sensitive resonance line at 263.9 nm was used as it was less noisy than the primary resonance line at 208.9 nm.Precipitates slowly formed during the extraction procedure when more than 600pg ofMay, 19741 BY SOLVENT EXTRACTION AND ATOMIC-ABSORPTION SPECTROPHOTOMETRY 295 iridium, 800 pg of ruthenium and 1200 pg of osmium were extracted into 10 ml of chloroform that was 1 per cent. m/V in methyltriphenylphosphonium chloride. Therefore, an immediate transfer of the chloroform layer was necessary when the amounts of noble metals exceeded these limits. The precipitates are very soluble in acetonitrile. The mutual interferences have been eliminated by the addition of lithium perchlorate. Not only were the interferences overcome, but also, for ruthenium and osmium, an enhance- ment in the absorption signals occurred. The causes of mutual interferences among the noble metals are not understood and very little is known about the mode of action of the reagents that suppress them.As can be seen from Tables IV and V, the concentrations of iridium, ruthenium and osmium in acetonitrile containing lithium perchlorate that produce 1 per cent. absorption with the Unicam SP1900 instrument are 1.6, 0.4 and 1 pg ml-l, respectively, compared with the best values of 16, 0.6 and 5 pg ml-l for aqueous solutions. If, for a direct method without solvent extraction, a typical concentration of rhodium in aqueous solution is taken as being 1 per cent. m/V, then the concentrations of iridium, ruthenium and osmium in rhodium sponge corresponding to 1 per cent. absorption are 1600,60 and 500 pg g-l, respec- tively. With the solvent-extraction procedure, the concentrations of iridium, ruthenium and osmium in rhodium sponge corresponding to 1 per cent. absorption are 16, 4 and 10 pg g l , which is a marked improvement. No scale expansion was used on the Unicam SP1900 instrument in these determinations, but if scale expansion is used it is anticipated that the limits of detection for iridium and ruthenium in rhodium sponge by the described method will be 4 and 1 pgg-l, respectively, or even better.12 Very recent results for the limits of detection for iridium and ruthenium in rhodium sponge using a d.c. arc on an Ebert 3-m spectrograph are 10 and 1 pg g l , respectively. The atomic-absorption spectrophotometric method gives similar sensitivity. We are indebted to the University of Riyadh, Saudi Arabia, for a maintenance grant for M. A. Ashy and to Johnson Matthey Chemicals Ltd. for the loan of Specpure noble metals and rhodium sponge samples. We also thank Johnson Matthey Chemicals Ltd. for allowing us to use their Unicam SP1900 atomic-absorption spectrophotometer and for providing us with information on the detection limits for noble metals in rhodium sponge when carrying out emission spectrography. REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. Headridge, J. B., and Sowerbutts, A., Analyst, 1973, 98, 57. Headridge, J . B., and Smith, D. R., Lab. Pract., 1971, 20, 312. Headridge, J. B., and Richardson, J., Analyst, 1970, 95, 930. Headridge, J. B., and Sowerbutts. A., Ibid., 1972, 97, 442. Slavin, W., “Atomic Absorption Spectroscopy,” Interscience Publishers, New York, 1968, pp. 114, Neeb, R., 2. analyt. Chem., 1957, 17, 154. Beamish, F. E., “The Analytical Chemistry of the Noble Metals,” Pergamon Press, Oxford, 1966, Cotton, F. A., and Wilkinson, G., “Advanced Inorganic Chemistry,” Third Edition, Interscience Griffith, W. P., “The Chemistry of the Rarer Platinum Metals,’’ Interscience Publishers, New Pannetier, G., and Toffoli, P., Bull. Soc. Chim. Fr., 1971, 3775. Thomerson, D. R., Scan, 1973, 1, 12. 138 and 154. Fok, J . S.-K., D ~ s s . Abstr., 1965, 25, 3815. p. 22. Publishers, New York, 1972, p. 990. York, 1967. Received November 7th, 1973 Accepted December 28th, 1973