398 Analyst, June, 1967, Vol. 92, $9. 398-402 The Recovery of Trace Elements after the Oxidation of Organic Material with 50 per cent. Hydrogen Peroxide BY J. L. DOWN* AND T. T. GORSUCHf ( United Kingdom Atomic Energy Authority, The Radiochemical Centre, Amersham, Buckinghamshire) The recovery of several elements, a t the p.p.m. level, from various organic materials after oxidation with sulphuric acid and 50 per cent. hydrogen peroxide has been studied. Most of the elements investigated could be recovered quantitatively, but germanium, arsenic, selenium and ruthenium suffered losses under some or all of the conditions examined. The causes of these losses are discussed. THE last few years have seen the publication of several papers dealing with the oxidation of organic materials with 50 per cent.hydrogen peroxide.lS2 The advantages claimed for the procedure include speed of oxidation, ability to deal with difficult materials such as plastics, low blank values and the fact that the only decomposition products are water and oxygen. So far, little information has appeared regarding the behaviour of specific trace elements during this oxidation procedure. The work reported here was carried out in an attempt to fill this gap. APPARATUS- The work was carried out with the apparatus shown in Fig. 1, and which has been described el~ewhere.~,~ With the tap in position 1 the solution in the flask can reflux indefinitely. With the tap in position 2 the solution in the flask will distil into the reservoir, and when in position 3, liquid collected in the reservoir can be run out through the side-arm.EXPERIMENTAL I 2 3 Fig. 1. Apparatus for controlled decomposition of organic material * Present address : Barking Regional College of Technology, Longbridge Road, Dagenham, Essex. t Present address : Ranks Hovis McDougall (Research) Limited, Cressex Laboratories, Lincoln Road, High W’ycombe, Bucks.DOWN AND GORSUCH 399 Counting equipment-All the determinations of activity were made by y-counting with an Ekco scintillation counter, type N664A. This has a thallium-activated sodium iodide crystal, about 2 inch high x 4 inch diameter, as detector, and the sample, in solution, is contained in an annular polythene cup surrounding the crystal. REAGENTS- Sulphuric acid, copzcentrated, sp.gr. 1.84. Hydrogen peroxide, 50 per cent.-Supplied by British Drug Houses Limited.Radioactive fracers-Solutions were prepared of several nuclides, with radioactive con- centrations ranging between 1 and 30pC per ml, and chemical concentrations of about 2 pg per ml. The nuclear and chemical data for the tracers used are given in Table 1. Nuclide Antimony-124 . . Arsenic-74 , . . . Bismuth-207 . . Cadmium-109 . . Chromium-51 . . Chromium-51 . . Germanium-68 . . (plus gallium-68) Indium-114m . . (@us indiuni-114) Manganese-54 . . Ruthenium-106 . . (plus rhodium- 106) Selenium-75 . . Tellurium-132 . . (plus iodine-132) Tin-113 . . . . Vanadium-48 . . (plus indium-l13m) Zinc-65 . . . . Zirconium-89 . . TABLE I NUCLEAR AND CHEMICAL DATA FOR TRACERS Chemical form Half-life Antimony chlorides 60 days Sodium arsenate 18 days Bismuth chloride 28 years Cadmium chloride 470 days Sodium chromate 27.8 days Chromic chloride 27.8 days Germanium chloride 280 days Indium chloride 50 days Manganese chloride 314 days Ruthenium chloride 1 year Sodium selenite 121 days Sodium tellurite 78 hours Tin(I1) chloride 118 days Vanadyl chloride 16 days Zinc chloride 245 days Zircon$ chloride 78 hours Principal photon emission w \ MeV percent, MeV percent.0.60 0.59 0.63 0-57 0.088 0.32 0-32 0.51 0.19 98 60 14-5 98 4 9 9 174 19 1.69 48 0-51 59 plus X-rays 1-06 76 plus X-rays 0-022 X-rays 0-005 X-rays 0.005 X-ravs 1.08 4 0.009 X-rays 0-024 X-rays 0.84 100 0.005 X-rays 0.51 21 0.62 11 0.27 56 0.14 54 0.28 23 0.12 16 0.40 13 plus X-rays 0.67 100 0.23 96 0.78 84 0.65 26 0.52 22 and others 0.39 67 0.024 X-rays 0.51 112 0.99 100 1.31 98 plus X-rays 1.12 49 plus X-rays 0.9 100 0.51 50 plus X-rays NOTE-The X-rays produced by elements of atomic number below 30 will be of too low an For elements of higher atomic number energy to be recorded by the counting equipment used.the X-rays will make an increasing contribution t o the counts recorded. PROCEDURE- Weigh 2 g of organic material into the 250-ml round-bottomed flask shown in Fig. 1, add, by pipette, 1 ml of radioactive tracer solution and assemble the apparatus as shown. Add 20 ml of concentrated sulphuric acid by way of the condenser and reservoir. With the tap in position 1, heat the flask until the organic material chars, and continue heating for between 30 minutes and 1 hour. Add 10ml of 50 per cent.hydrogen peroxide to the mixture in small amounts through the condenser. When the vigorous reaction has sub- sided continue reffuxing for a few minutes, then turn the tap to position 2. Continue heating for 30 minutes, turn the tap to position 3 and collect the liquid that has distilled into the reservoir. Return the tap to position 1 and add a further 10ml of 50 per cent. hydrogen peroxide by way of the condenser. Again turn the tap to position 2 and continue heating until white fumes of sulphuric acid appear in the flask. Run the distillate out of the reservoir and combine it with the distillate previously obtained. Make the combined distillates up to 50 ml. Dilute the acid solution remaining in the flask and make this up to 100 ml. Dilute 1 ml of the tracer solution to 100 ml with 10 per cent.sulphuric acid to give400 [Analyst, Vol. 92 a reference solution corresponding to 100 per cent. of the radioactivity used. Compare the activities present in the distillate and the residue with the activity of the reference solution by counting 10-ml aliquots of each. Calculate the percentage of the original activity to be found in each fraction. DOWN AND GORSUCH : RECOVERY OF TRACE ELEMENTS AFTER THE RESULTS The results of experiments in which all of the tracers and several organic materials were used are listed in Tables I1 and 111. Table I1 shows the nuclides with which no difficulties were experienced, and Table I11 shows those where some losses were found. TABLE I1 ELEMENTS SHOWING GOOD RECOVERIES AFTER OXIDATION Nuclide Organic material Recovery, per cent.Cadmium-109. . .. .. .. .. PVC 94, 95, 97, 97 Cadmium-109. . . . . . .. .. Polythene 95 97 Bismuth-207 . . .. .. .. .. Cocoa 101 103 Vanadium-48.. .. . . . . . . Cocoa 100 101 Vanadium-48.. . . .. .. . . Cocoa + NaCl 98 101 Chromium-51 (chromic chloride) . . .. PVC 99 101 Chromium-51 (sodium chromate) . . . . Cocoa + NaCl 100 103 Tin-113 . . . . .. .. .. Cocoa + NaCl 100 103 Zinc-65 . . .. .. .. . . Cocoa + NaCl 100 102 Antimony-124 . . . . .. . . Cocoa + NaCl 97 98 Zirconium-89 . . . . . . . . . . Cocoa + NaCl 101 100 Manganese-54 . . .. . . . . PVC 99 102 Indium-114 . . . . .. . . . . PVC 97 101 Tellurium-132* (sodium tellurite) . . . . PVC 100 100 *The tellurium-132 solutions were allowed to stand for 20 hours before counting to permit equilibrium with the radioactive daughter, iodine-132, to be re-established.TABLE I11 ELEMENTS NOT COMPLETELY RECOVERED AFTER Nuclide Organic material Germanium-68 . . .. .. .. None Cocoa Cocoa + NaCl Urea Urea + NaCl Polythene PVC Ruthenium-106 . . .. .. . . None Cocoa Polythene PVC Arsenic-74 . . . . .. .. .. None Cocoa + NaCl Cocoa + NaCl Polythene PVC Selenium-75 . . .. .. .. .. None PVC Pol ythene OXIDATION Recovery, per cent. 97 92 69 48 10 13 100 101 12 9 92 94 3 3 55 27 72 51 8 3 37 33 98 99 66 59 62 53 97 99 3 3 98 99 66 72 11 16 DISCUSSION The apparatus and procedure used in this investigation were selected for their ability to provide the maximum amount of information, rather than for speed. With this technique it is possible to investigate the behaviour of the tracers in considerable detail; this was not done in the present survey but a standard procedure has been established.One advantage of the relatively complex apparatus shown in Fig. 1 is that the risk of mechanical loss during the very vigorous reaction, which occurs upon addition of 50 per cent. hydrogen peroxide to hot sulphuric acid solution, is minimised.June, 19671 OXIDATION OF ORGANIC MATERIAL WITH 50 PER CENT. HYDROGEN PEROXIDE 401 The radioactive concentrations of the tracer solutions were selected to give count-rates of between 30,000 and 80,000 c.p.m. in the 10-ml aliquot of the standard taken for measure- ment. The concentration chosen was governed by the efficiency with which each nuclide was detected by the scintillation counter used: this varied from about 0.5 per cent.for chromium-51 to nearly 15 per cent. for bismuth-207. The results in Tables I1 and I11 show that most of the tracers used were recovered in yields of well over 90 per cent.; in fact, of the tracers listed in Table 11, only the recovery of cadmium seems to be significantly below 100 per cent. Although the chemical nature of the tracers used varied somewhat, the initial period of heating with sulphuric acid should have been adequate to prevent differences of behaviour arising from this cause. Although the results cannot necessarily be used to predict the be- haviour of the elements studied if they occur in organic combination, it should be a good indication of the behaviour to be expected if they occur in inorganic form, The results in Table I11 show that with four of the elements studied, germanium, arsenic, selenium and ruthenium, losses occurred under some or all of the conditions studied, but that no single explanation can cover all four.When the tracers were carried through the oxidation cycle in the absence of organic matter, only ruthenium suffered serious loss. This is readily explicable by the formation of the volatile ruthenium tetroxide on treatment with 50 per cent. hydrogen peroxide. This element showed serious losses in all the experiments, and osmium, although it was not itself studied, might reasonably be expected to behave similarly. Of the other three elements, selenium was the only one showing large losses when both PVC and polythene were the organic materials being oxidised.A mechanism involving reduction to a selenium hydride or simple alkyl during the early charring stage has been proposed previously3 to explain such a loss. The two remaining elements, arsenic and germanium, show large losses in the presence of PVC but little or none in the presence of polythene. Further, with germanium, large losses were shown in the presence of urea and sodium chloride, while there was none in the presence of urea alone. The addition of sodium chloride to cocoa also caused a large drop in the percentage of germanium recovered. The obvious and reasonable explanation is to attribute the loss to the formation of volatile chlorides of arsenic and germanium when material containing ionic or covalent chlorine is heated with sulphuric acid.To reconcile the loss of germanium in the presence of cocoa with this explanation, tests were carried out in which 2 g of cocoa were charred with dilute sulphuric acid, and the distillate collected and tested for chloride ion. This was readily demonstrated. It is worth recording one further experiment, even although it is not concerned with the recovery of trace elements. It has been reported2 that the oxidation of liquid paraffin with sulphuric acid and hydrogen peroxide is dangerous, being accompanied by flashes of flame and small explosions. To investigate this, 2 g of liquid paraffin were oxidised by the procedure described above, but the initial charring period was extended to the full hour. Under these circumstances the oxidation proceeded quite smoothly, although more 50 per cent.hydrogen peroxide was required than is usual. TABLE IV BOILING-POINTS OF VOLATILE CHLORIDES Chloride Boiling-point, "C SbC1, . . .. .. .. 223 AsCl, . . . . .. .. 130 CrO,Cl, .. .. .. 117 voc1, . . . . .. .. 127 GeC1, . . .. . . . . 83 SnC1, . . .. . . .. 114 TeCl, . . .. .. . . 327 TeC1, . . . . .. . . 390 SbCl, . . .. . . .. 79 Most of the results obtained are very much as would be expected. The volatility of ruthenium tetroxide is well known, the loss of selenium is similar to previously reported losses, and the loss of arsenic and germanium is reasonable in view of the volatility of their402 DOWN AND GORSUCH [Analyst, Vol. 92 chlorides (see Table IV). However, the figures in Table IV show that antimony, chromium, vanadium and tin also have very volatile chlorides, yet these elements were recovered in good yield.This is probably due to either reduction of the elements, during the original charring stage, to the lower valency states or to hydrolysis of chlorides to oxy compounds, which are less volatile. For antimony, hydrolysis of chloride occurs readily. Similarly, vanadium halides and oxy halides are easily hydrolysed giving rise to involatile products, and it has been reported5 that tin chloride will not distil from chloride - sulphuric acid solutions, which is precisely the system that has been under investigation. Although this study has not been exhaustive, it is believed that, with the exception of mercury, most of the elements that are likely to cause difficulty have been included. As the apparatus used should effectively prevent mechanical loss, it is most probable that the losses found are caused by the formation of genuinely volatile species, always assuming that retention of radioactive material on the glassware is not a serious source of error. That this assumption is valid is supported by the fact that with all the volatile elements, excluding ruthenium, complete recoveries were obtained when no organic material was present. This indicates that absorption of the active material upon the glassware is unlikely from the strongly acid solution remaining after digestion. REFERENCES 1. 2. 3. Gorsuch, T. T., Ibid., 1959, 84, 135. 4. 5. Whalley, C., in West, P. W., Macdonald, A. M. G., and West, T. S., Editors, “Analytical Chemistry Taubinger, R. P., and Wilson, J. R., Analyst, 1965, 90, 429. Analytical Methods Committee, Ibid., 1960, 85, 643. Hillebrand, W. F., Lundell, G. E. F., Bright, H. A., and Hoffmann, J. I., Edztovs, “Applied Received September 26th, 1966 1962,” Elsevier Publishing Company, Amsterdam, London and New York, 1963, p. 397. Inorganic Analysis,” Second Edition, John Wiley & Sons Inc., New York, 1953, p. 285.