ANALYTICAL CHEMISTRY, VOL. 51, NO. 11, SEPTEMBER 1979
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Determination of Osmium by Atomic Absorption Spectrophotometry R. C. Mallett,’ S. J. Royal, and T. W. Steele The National Institute for Metallurgy, Private Bag X30 15, Randburg, 2 725. South Africa
A sensitlve method has been developed for the measurement of osmium by atomic absorption spectrophotometry. With the use of sbnpty constructed apparatus, volatile osmium generated in a small furnace is pulsed Into a nitrous oxide-acetylene flame, permitting levels of osmium down to 0.2 pg to be measured. A large range of concentrations has been tested for possible interferences. Where these exist, osmium should first be separated by distillation. For the determination of 5 Fg of osmium, the method has a coefficient of variation of about 12%.
The determination of osmium is probably more difficult than the determination of the other noble metals, since this metal has poor sensitivity by conventional flame atomic absorption spectrophotometry (AAS) ( I ) and, unless the concentration level is sufficiently high, it is not possible to determine osmium by this method. Electrothermal AAS yields equally unsatisfactory results, since osmium is partially lost as the tetroxide or chloride during drying and ashing and, even if this loss could be prevented, temperatures in excess of 3000 “C would be required to vaporize the metal ( 2 ) . T o date, the most sensitive methods for the determination of osmium are probably spectrophotometric methods used after separation by distillation (3)or by solvent extraction ( 4 ) . However, even for these methods the lower limit of determination is about 5 Fg of osmium, the limiting factor being the volume of solution that must be taken for measurement. Skogerboe et al. ( 5 )showed that several elements could be converted to volatile halides, which, when pulsed into a flame, yield improved sensitivity by AAS. Those investigators generated the chloride by bubbling a stream of air through hydrochloric acid into a silica tube containing the dried sample, which was heated in a small tubular furnace. The outlet from the furnace was connected to the spray chamber of an atomic absorption spectrophotometer, and the absorption of the element was measured in an air-acetylene flame. In the present investigation, a similar, but somewhat modified, apparatus was used to generate osmium as a volatile species and measurements were made in a nitrous oxideacetylene flame. However, preliminary tests for osmium indicated that bubbling of the flush gas (nitrous oxide) through hydrochloric acid was not necessary. EXPERIMENTAL Apparatus. Measurements were made with a Varian Techtron AA5 atomic absorption spectrophotometer equipped with a nitrous oxide-acetylene burner, the peak heights being recorded on a Corning 840 chart recorder. Flush-gas flows were measured with a Techtron AA3 flowmeter that was calibrated in liters per minute. The furnace was constructed from clear quartz tubing of internal diameter 10 mm (see Figure 1). The flush-gas was brought in through a side arm, as shown. Nichrome wire was wound closely over the section shown and covered with asbestos string. Heating was regulated by a variable transformer, and the furnace was connected to a five-way tap as shown. In the “flush“ position, a bleed of nitrous oxide was passed into the furnace and carried 0003-2700/79/0351-1617$01.00/0
Table I. Instrumental Settings lamp current, mA spectral band pass, nm wavelength, nm Corning 840 recorder speed cm/min
15
0.2 290.9 1
out the volatile osmium, which then passed into the flame. In the “bypass” position, the nitrous oxide was passed direct to the flame, the furnace was sealed off, and the Teflon bung could be removed without a flash-back occurring. The bung had a length of quartz rod inserted in such a way that the sample tubes were correctly and reproducibly located. The sample tubes, consisting of quartz tubing having an internal diameter of 7 mm and a length of 50 mm, held up to 2 mL of solution. Solutions were usually transferred to the tubes with disposable plastic-tipped Lancer dispensers, and were dried by placing of the tubes in holes drilled in an aluminum plate heated on a hot-plate (Figure 2). Evaporation was speeded up considerably by a small stream of air blown into the tubes. For this purpose, a manifold was made from thin, flexible, plastic tubing attached to a thicker, rigid length of plastic tubing as shown. The system could dry 20 samples (0.5-mL volume) within 15 min. INSTRUMENTAL PARAMETERS Instrumental parameters were established using solutions of osmium made by the dissolution of ammonium chlorosmate in a dilute solution of hydrochloric acid. Instrument a n d B u r n e r Adjustment. The settings used for the spectrophotometer are summarized in Table I. For adjustment of the burner position and the flame conditions, a strong solution of osmium (1 g/L) was aspirated in the usual way and adjustments were made for maximum absorption, the flow of nitrous oxide and acetylene being noted, as well as the height of the red feather. The flame was then extinguished, and the nebulizer and holder were removed. The main support-gas supply tube was removed from the nebulizer and connected to the inlet of the AA3 flowmeter, and the outlet from the flowmeter was connected to the five-way tap inlet (Figure 1). With the tap in the by-pass position, the nitrous oxide was allowed to pass, and the flow through the AA3 flowmeter was adjusted to the required flow rate for flushing. The auxiliary supply on the AA5 gas-control unit was increased until the total flow rate corresponded to that previously noted. The nitrous oxide was then turned off and the burner was ignited in the usual way (Le., on airacetylene) and then switched over to nitrous oxide-acetylene. The acetylene was not readjusted. By use of only the auxiliary control, the flame was finally adjusted to give the same height of red feather previously noted. Time of Heating. With the furnace temperature set a t 850 “C (measured with a thermocouple), the absorbance peaks were measured for 5 pg of osmium after the samples had been heated in the furnace for various periods of time. The osmium, before drying, was added as a solution of ammonium chlorosmate in 10% hydrochloric acid. The results are shown in Figure 3. (No scale expansion was used.) A maximum was reached after 20 s, after which there was a gradual drop, levelling out to a plateau a t about 100 s. This gradual drop in response with time of heating may be attributed to the 0 1979 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 51, NO. 11, SEPTEMBER 1979 A C - 3
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Key A 5-way tap glass B Teflon connector c Teflon bung wtth qua& rod D Qua& tube E Nlchroms wlre F Asbestos strlng G Plastic connector
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Figure 1. Generation apparatus (in flush position)
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Figure 4. Change in absorbance with furnace temperature for 5 pg of osmium
Figure 2. Drying apparatus
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80 lbo 1 0 Heating time in furnace, 5 6'0
Figure 3. Change in absorbance with duration of heating in furnace for 5 pg of osmium
diffusion out of the sample tube of a small amount of gaseous osmium to an area between the sample tube and the furnace wall where it is not subject to the pulse of flush gas. A time of 40 s was chosen for subsequent tests, since this period was well beyond what appeared to be the most critical time, but was not unduly long. Furnace Temperature. With a heating time of 40 s, 5 pg of osmium was measured at various furnace temperatures, which had been ascertained with a thermocouple before each sample was placed in the furnace. The results shown in Figure 4 indicate that little or no improvement in sensitivity was obtained after 800 "C, and that the precision a t 900 "C was very poor. A temperature of 850 "C was chosen for subsequent tests. Flow Rate of Flush Gas. With a furnace temperature of 850 "C and a heating time of 40 s, 5-wg amounts of osmium were measured while being flushed with nitrous oxide at various flow rates. The results are shown in Figure 5, where maximum sensitivity was obtained with a flow rate of between 1 and 2 L/min. The drop in sensitivity at high flow rates is probably due to excessive dilution of the vapor and shorter residence time in the flame, and, below this setting, the vapor passes into the flame so gradually that the pulse effect is not completely achieved.
RESULTS AND DISCUSSION Nature of the Volatile Osmium Compound. As has been stated, the tests that have been described were carried out on solutions of the ammonium chlorosmate salt, and, although
I/min
Figure 5. Change in absorbance with change in flushing rate of nitrous oxide for 5 pg of osmium
it was shown that flushing with hydrochloric acid vapor entrained with the nitrous oxide, as compared with flushing with nitrous oxide alone, did not affect the measurement, it was not clear whether chloride present in the residue after drying played a part in the generation and measurement of osmium. Osmium tetrachloride is volatile above 560 "C (6), and, as such, could be swept into the flame. However, under the conditions existing in the furnace, it is also possible that the tetroxide is formed. T o test the effect of chloride, a solution, free of chloride, was made by dissolution of osmic acid ( 0 ~ 0crystals ~ ) in water (solution A) and after suitable dilution, five aliquot portions representing 7.3 pg each were added to a set of silica test tubes containing approximately 0.5 mL of water saturated with SO2 (set A). The osmium was probably now present as the hydrated oxide Os02.2H20(6). The tubes were then stoppered. A second portion of the solution of osmic acid was diluted with a solution of HC1-SO2,stoppered, and set aside for a day so as to ensure reduction of the osmium to the stable 0sCb2-form (7) (solution B). A third solution of osmium made from the ammonium chlorosmate salt (NH,)20sC16 was prepared (solution C). Five portions of each of solutions B and C (sets B and C), containing 7.3 and 6.7 pg of Os, respectively, were dried in the normal way, together with set A, and the residues were measured in the furnace. Taking into account experimental error and the small difference in mass between set C and sets A and B, there was little difference in the peak heights (Table 11). This would indicate either that the sensitivities for osmium as the tetroxide and as a chloride species are the same, or that osmium entering the flame is in the same form irrespective of whether chloride is present or
ANALYTICAL CHEMISTRY, VOL. 51, NO. 11, SEPTEMBER 1979
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Figure 6. Interference effects of various metals and metal salts on the absorption of 5 pg of osmium
Table 11. Response for Osmium with and without Chloride mean peak
osmium mass, Crg 7.3 7.3 6.7
medium SO, only SO,-HCl
ammonium chlorosmate salt
height, precision mm mm 92 94 91
2 5
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+3
not (this form being Os04, since in set A no chloride is present). This argument hinges on whether there is a difference in sensitivity between the two forms. In an experiment in which Os(VII1) (as osmic acid) and Os(1V) (as chlorosmate) were measured by normal flame AAS and by emission spectroscopy using the inductively coupled plasma (ICP); it was the Os(VII1) species that yielded the significantly greater sensitivity: fourfold in the case of AAS and tenfold for ICP. If these observations can be compared with those obtained by the use of the generation method, the sensitivity of the measurement of sets B and C by the generation method should have been less than that obtained for set A, if, in the case of sets B and C, it was the chloride form entering the flame and for set A it was the osmium tetroxide form that entered the flame. In the normal AAS and ICP methods, the sample is
introduced in aerosol form, whereas in the generation method it is introduced in the gaseous form. It may therefore be argued that sensitivity ratios in the various systems between osmium(VII1) and osmium(1V) are not comparable. The experiments do, however, lend strength to the conclusion that it is probably Os04 that is formed in the furnace. Interference Studies. Aliquot portions of the solution of ammonium chlorosmate that contained 5 pg of osmium were transferred to the silica tubes, and various elements were added individually in amounts of up to 5 mg (Le., an excess of a thousand-fold). The results shown in Figure 6 are for those elements that yielded a recovery of less than 90%, and for which interference can be considered serious. In addition, the following interfering elements, also tested up to 5 mg, yielded recoveries of better than 90%: Pt, Pd, Rh, Ir, Ca, and Se. Of the noble metals, gold and ruthenium displayed the most interference, whereas for the base metals serious interferences were noted for nickel, iron, tin, lanthanum, and particularly for aluminum. These interferences were most severe when more than 1 mg of the element was present. To test whether the low recoveries were due to the loss of osmium during the drying stage, or to the suppression of the formation of osmium tetroxide during heating in the furnace, macro amounts of osmium (2 mg) in a solution of hydrochloric acid were heated separately in the presence of gold (100 mg), ruthenium (100 mg), and nickel (100 mg) in a closed system. The vapors
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ANALYTICAL CHEMISTRY, VOL. 51, NO. 11, SEPTEMBER 1979
Table 111. Effecion Interference of Increase in Heating Time interfering element
a
- peak height, mm 4.6 s
80 s
120 s
nil 50 Cu (5 mg) 39 ( 4 mg) 39 La ( 5 mg) 39 (4 mg) 38 Ni ( 5 mg) 49 ( 1 mg) 49 Sn ( 5 mg) 47 (1 mg) 51 Estimated from Figure 3.
45a 33 35 35 33 41 40 32 43
450 32 33 30 28 36 40
28 40
generated were sucked off and collected in SO,-HCl solution. Analysis of these solutions for osmium showed that the low recovery in the presence of gold was almost entirely due to the reduction of gold during the evaporation stage, which caused the oxidation of osmium and its loss as the tetroxide. Ruthenium caused some loss in this way, but was not the only factor contributing to the low recoveries of osmium. In the instance of nickel, the amount of osmium that was detected in the solution was insignificant: losses during evaporation did not account for the low recovery of osmium. For aluminum, the severe losses were attributed mainly to the presence of water of crystallization associated with aluminum chloride. This caused the samples to spatter in the furnace. As a result, the precision of measurement was very poor. Iron caused the flame to be strongly colored and very turbulent, resulting in unreliable measurements. The interference from large amounts (up to 50 mg) of sodium sulfate, sodium nitrate, and sodium chloride was also assessed (Figure 6). These salts could be present in solutions of sample materials after fusion. The apparent loss in recovery of osmium with an increase in salt content was attributed mainly to the physical occlusion of osmium due to the preponderance of salt, rather than to chemical interference. Because the drop in osmium recovery with an increase in the concentration of the interferent may have depended on the duration of heating, some of the elements for which this drop was most pronounced (e.g., Cu, La, Ni, and Sn) were tested using heating times of up to 1.20 s. In all cases, there was a further drop in recovery. The peak heights are shown in Table 111. Some of this loss may be attributed to diffusion (as discussed earlier, Figure 3), but in addition it can be concluded that these elements interfere with the formation of osmium tetroxide in the furnace. We do not think copper, lanthanum, or tin would cause a loss of osmium during evaporation. I t will be noted that tests were done in the presence of very large amounts (up to a thousand-fold excess) of interfering elements. This testing was aimed a t the determination of osmium a t low levels in ores, or matte-leach residues, and similar materials, without the use of separation techniques. However, because of the interferences that would occur in these cases, it is essential to separate the osmium first, preferably by a distillation procedure, because the distillate would be collected in a solution of hydrochloric acid and sulfur dioxide that is amenable to the further treatment required for the evolution technique. This solution could be evaporated down and the whole sample used for measurement, so that extremely good sensitivity would be achieved. For a volume of sample solution of 2 mL, the sensitivity by this method is approximately 400 times better than by the
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Peaks of the
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conventional flame AAS method where no releasing agent is added, and 200 times better where uranium is added as the releasing agent. The sensitivity in relation to the distillation (3) or solvent-extraction ( 4 ) procedures normally used for this determination is greater by a factor of about 10. This method is applicable where the level of osmium is very low and the osmium cannot be determined easily by the extraction method. Sensitivity and Precision. The peak heights obtained without the use of scale expansion for standards in the range 2 to 5 pg of osmium are shown in Figure 7. The limit of detection, based on twice the standard deviation a t or near the detection limit, is 0.05 pg. Precisions of 42%, 14%, and 12% were obtained for 0.1,1, and 5 pg of osmium, respectively. These figures include the dispensing of between 10 and 100 pL of solution, evaporation, and measurement.
CONCLUSION A very sensitive method has been developed for the determination of osmium by AAS. The sensitivity is between 200 and 400 times greater than that obtained by conventional methods of flame atomic absorption spectrophotometry. Because interferences can occur, it is recommended that a separation step involving normal distillation procedures should be used where fairly large amounts of base metals or some noble metals are present. Alternatively, although not tested, the method of standard additions could be used to compensate for these interferences. ACKNOWLEDGMENT The authors thank the National Institute for Metallurgy for permission to publish this paper. LITERATURE CITED (1) Maliett, R. C.; Breckenridge. R. L.; Steele, T. W. "The determination, by atomic-absorptionspectrophotometry, of osmium and iridium in solution". Johannesburg, National Institute for Metallurgy, Report 1318, 22nd Sep., 1971. (2) Guerin, B. D. J. S.Ah. Chem. Inst. 1972, 25, 230-243 (Paper presented at the Symposium on Analytical Chemistry of the Platinum-group Metals, National Institute for Metallurgy, Johannesburg, 2nd-4th Feb., 1972.). (3) Jones, E. A,; Kruger, M. M.; Wilson, A,; Steele, T. W. "The determination of osmium", Johannesburg, National Institute for Metallurgy, Reporl 1232, 1st Apr., 1971. (4) Dixon, K.; Kruger, M. M.; Radford, A. J.; Steele, T. W. "The determination of osmium in platiniferous media", Johannesburg, National Institute for Metallurgy, Report 1654, 27th Mar., 1975. (5) Skogerboe, R. K.; Dick, 0. L.;Pavlica, D. A,; Lichte. F. E. Anal. Cbem. 1975, 4 7 , 568-570. (6) "Treatise on Analytical Chemistry"; Kolthoff, 1. M.; Elvlng, P. J., Eds.; Interscience: New York, 1963; Part 11, VoI. 8, pp 404-405. (7) Allen, W. J.; Beamish, F. E. Anal. Chem. 1952, 2 4 , 1608-1612.
RECEIVED for review July 25, 1978. Accepted May 4, 1979.
