Analjst, July, 1068, Vol. 93, $9. 429432 429 The Rapid Dissolution of Plutonium Dioxide by a Sodium Peroxide = Sodium Hydroxide Fusion, Followed by Determination of the Plutonium Content by Controlled-potential Coulometry BY G. W. C. MILNER AND D. CROSSLEY (Analytical Sciences Division, U. K.A .E.A . Research Group, Atomic Energy Research Establishment, Harwell) A method is described for the dissolution of plutonium dioxide, followed by determination of the plutonium content by controlled-potential coulo- metry. The plutonium dioxide is brought into solution by fusion with a mixture of sodium peroxide and sodium hydroxide at 600" C for 15 minutes in an alumina crucible. The cold melt is leached with water, which is then acidified with sulphuric acid. The solution is heated for 15 minutes to de- compose hydrogen peroxide and, after cooling, diluted to a suitable volume.The plutonium content of an aliquot containing about 4 mg of plutonium is determined by controlled-potential coulometry. A potential of + 0.30 volt versus a S.C.E. is used for reduction to plutonium(III), whereas +Om70 volt veuus a S.C.E. is used for the quantitative oxidation to the plutonium(1V) state. Mean recoveries on 100-mg amounts of plutonium dioxide that had been ignited at 850" C were 99.95 per cent., with a coefficient of variation of 0.11 per cent. For the complete dissolution of samples previously ignited at higher temperatures (about 1600" C), an increase in the ratio of the weight of fusion mixture to sample is necessary. Mean recoveries on 50-mg amounts of plutonium dioxide that had been ignited at 1600" C were 99-85 per cent., with a coefficient of variation of 0.54 per cent.IN earlier work from this laboratory a sodium peroxide sinter technique1 was described for the rapid dissolution of plutonium dioxide, including high-fired material. The plutonium content of the resulting solution was then determined by differential spectrophotometry. Subsequent experience with this method has shown that occasionally inconvenience can result from the existence of small amounts of undissolved sample in the final solution. This problem was traced to the difficulty of obtaining satisfactory mixing of the sodium peroxide and the sample under glove-box conditions. It was considered that this difficulty might disappear on modifying the sinter so that fluid conditions occurred; these conditions would be produced by increasing the temperature to above 500" C.In addition, reduction of the size of the sample required for analysis would help to improve and speed up the dissolution procedure. This objective could be achieved if the differential spectrophotometric method for plutonium was replaced by an electrochemical method. For example, it is possible to determine as little as 1 mg of plutonium to within k0.2 per cent. by controlled-potential coulometry, as compared with the 30mg of plutonium needed for a single determination by differential spectrophotometry. Another advantage is that by handling smaller sample weights, the final determination can be carried out in a fume cupboard, instead of a glove-box, with a consequent improvement in the speed of analysis.It was expected that a peroxide fusion would give a rapid method for the dissolution of plutonium dioxide, either alone or in mixtures with certain other materials. It should, for 0 SAC; Crown Copyright Reserved.430 MILNER AND CKOSSLEY: THE RAPID DISSOLUTION OF PLUTONIUM [Ana&St, VOl. 93 example, be much quicker than the ammonium hydrogen sulphate fusion,2 which takes 4 hours to effect the dissolution of refractory plutonium dioxide. Moreover, it should also be satisfactory for the dissolution of other materials mixed with plutonium dioxide, particularly those which are not dissolved by fusion with ammonium hydrogen sulphate. These materials include ruthenium metal, ruthenium dioxide, silica and chromium oxides.In view of these possible advantages, the peroxide fusion technique has been examined in some detail. EXPERIMENTAL SELECTION OF A SUITABLE CRUCIBLE- Although a platinum crucible is satisfactory for sodium peroxide sinters carried out at about 450" C, it is not suitable for fusions at temperatures in excess of 500" C. In considering suitable crucible materials, it was thought that a metal crucible would be preferable to one made from a ceramic because of such factors as ease of handling and the absence of porosity, difficulties usually associated with ceramics. Specimens of various metals were, therefore, tested by fusing small pieces with 0.5 g of sodium peroxide plus 0-5 g of sodium hydroxide for 15 minutes at 600" C in alumina crucibles.By this means it was hoped to identify those metals undergoing negligible attack in the fusion process, and a summary of the results is shown in Table I. These results indicated that, of the metals examined, zirconium was the most suitable for further study. Several zirconium crucibles were made, therefore, by a cold-drawing process. Initially, they had rough internal surfaces, which led to losses caused by melts creeping up the crucible walls, but this effect was reduced by polishing the internal surfaces of each crucible. Evidence of the removal of some zirconium from the crucible walls by the fusion process was detected. Moreover, this attack caused the formation of a grey deposit on the bottom of each crucible, which proved difficult to remove.At this stage, it was concluded that a satisfactory metal crucible would be difficult to obtain. In spite of known difficulties with ceramics, crucibles made only from this type of material were left for consideration. Experimental fusions were carried out, therefore, in thoria, magnesia and alumina crucibles. Thoria crucibIes were completely unattacked by the peroxide melt but, unfortunately, they had a poor resistance to thermal shock, which resulted in severe cracking. Magnesia crucibles were badly attacked during the fusion, and the melt crept up the wall surfaces. Fortunately, alumina crucibles withstood the fusion much better, and were only slightly attacked, 20 mg being a typical average loss in weight for a 15-minute fusion. Moreover, each crucible appeared to be usable for about four fusions, provided that careful drying of the crucible walls was carried out after each fusion. Also, problems connected with the creeping of the melt did not occur with the crucibles tested.On this evidence, an alumina crucible appeared to be the only satisfactory ceramic crucible readily available. The small amounts of aluminium passing into solution would not cause any interference in the coulometric determination of plutonium. Alumina crucibles of 15-ml capacity, as supplied by Thermal Syndicate Ltd., were, therefore, used exclusively in this investigation. DISSOLUTION OF PLUTONIUM DIOXIDE- Experimental fusions were carried out initially on a plutonium dioxide sample that had been ignited at 850" C. The mesh size of the material was less than 100 B.S.S.The sample (100 mg) was mixed with 0-5 g of sodium peroxide in an alumina crucible, 0-5 g of sodium hydroxide then added and the crucible heated at 600" to 620" C for 15 minutes. After cooling, the melt was leached by the method already described,l and the resulting solution acidified by adding it, dropwise, to 10 ml of water @us 4 ml of sulphuric acid (spgr. 1.84) contained in a 50-ml beaker. After warming for 15 minutes, the solution was cooled and diluted to 50 ml with water. The plutonium content was determined on a suitable aliquot by controlled- potential coulometry. The coulometric determination appeared to be fairly normal, with little trouble arising from any residual peroxide in solution. The only noticeable effect was that a slightly longer than normal reduction time (about 30 minutes) was required to reach a constant background current of less than 10 PA.A total of eight fusion experiments was carried out, and the behaviour in each instance was identical. Clear brown solutions were obtained without any trace of undissolved material. The mean recovery for plutonium from these determinations was 99.95 per cent., with a coefficient of variation of 0.11 per cent.July, 19681 431 Experimental fusions were next attempted on a plutonium dioxide sample that had been fired at 1600" C. A series of eight dissolutions with 100-mg portions of material was carried out, followed by coulometric determination of the plutonium. These experiments gave recoveries for plutonium that were up to 2 per cent.low. Further work showed that this bias was not caused by interference in the coulometry, but to slightly incomplete dissolution of the sample. Further dissolutions were then carried out with 50-mg portions of the plutonium dioxide sample to determine whether the higher ratio of fusion mixture to sample would improve the situation. Three fusions were carried out initially, and these gave good recoveries on subsequent coulometric titration. Ten more determinations were completed, and the mean recovery for the thirteen determinations was 99.85 per cent., with a coefficient of variation of 0.54 per cent. DIOXIDE BY A SODIUM PEROXIDE - SODIUM HYDROXIDE FUSION TABLE I OBSERVATIONS ON FUSING VARIOUS METALS WITH SODIUM PEROXIDE &US SODIUM HYDROXIDE Material Nickel .. .. .. Manganese - nickel alloy Stainless steel . . .. Zirconium (clean surface) Zirconium (oxidised surface) Silver . . .. .. Gold . . .. .. Observations .. . . Badly attacked .. . . Badly attacked .. . . -10% weight loss .. .. 2% weight loss .. .. 4% weight loss .. . . -30% weight loss .. . . Badly attacked METHOD APPARATUS- Thermal Syndicate Ltd. furnace. 1010-2, Solartron Laboratory Instruments Ltd., Chessington, Surrey). REAGENTS- Alumina crucibles-Recrystallised alumina crucibles, 15-ml capacity, as supplied by Mufle furnace-A "Hotspot," obtainable from A. Gallenkamp & Co. Ltd., or similar Controlled-fiotential coulomete@ p4-This was fitted with a digital voltmeter (Type LM Electrolysis cell for coulometry-As described previously.6 All reagents were of AnalaR grade.Sodium peroxide. Sodium hydroxide. Sul#huric acid, 18, 1 and 0-5 M. Distilled water. RADIOCHEMICAL SAFETY- Operation on dry samples containing plutonium dioxide, up to the point of complete dissolution, should be conducted in a glove-box. Aliquot portions for completion of the analysis can be handled in a fume cupboard with an efficient extraction and filtration system. PROCEDURE- Weigh 50 to 100 mg of sample ground to less than 100 B.S.S. mesh size (50 mg for samples of plutonium dioxide that have been ignited at temperatures above 1000" C) and transfer it into an alumina crucible containing 0.5 g of sodium peroxide. Mix by rotating the crucible by hand at an angle of 45", then add 0.5 g of sodium hydroxide in pellet form. Heat the crucible in a muffle furnace at 600" to 620" C for 15 minutes, keeping the crucible covered with an alumina (or silica) lid.Then remove the crucible from the furnace and allow it to cool. Add 1 ml of water to the crucible and allow the dissolution reaction to proceed for 5 to 10 minutes. Then add a further 0 6 m l of water, and gently swirl the contents of the crucible. If any of the melt is left undissolved, warm the crucible cautiously on a hot-plate, but avoid prolonged heating. (All of the melt should dissolve to give a brown to black- coloured suspension.) Transfer the extract, dropwise, with a Pasteur pipette into a 50-ml432 MILNER AND CROSSLEY beaker containing 10 ml of water fiulus 4 ml of sulphuric acid. Mix the solution during the addition by gently swirling the contents of the beaker.Wash the crucible by transferring about 2 ml of the acidic solution back into the crucible, and rinse the crucible walls with this solution. Wash the crucible with three further 2-ml portions of 0.5 M sulphuric acid and then with 2 mi of water. Combine the washings with the solution in the beaker, then warm on a hot-plate €or 15 minutes, or until all de-gassing has ceased. Cool the solution and dilute to 50 ml with M sulphuric acid. Use suitable aliquots, containing about 4 mg of plutonium, for determination by cont rolled-po t en t ial coulomet ry. Couulometric determination-Transfer the aliquot of sample solution to the coulometer cell, and add sufficient M sulphuric acid to cover the working electrode. Remove oxygen from the solution by passing a stream of nitrogen through it.Then reduce the plutonium to the tervalent state by electrolysing at a potential of +0.30 volt veysus a S.C.E. until the current attains a low constant value (10 pA or less). After adjusting the coulometer to zero, carry out the quantitative oxidation of plutonium to the quadrivalent state by electrolysing at +0.70 volt veysus a S.C.E. until the cell current reaches its previous low value (10pA or less). Correct the digital-voltmeter reading, Q, for a blank determination, carried out in exactly the same way with an aliquot of solution from a blank sodium peroxide-sodium hydroxide fusion. Calculate the weight of plutonium from the expression- Q (corrected) x F x 239.1 x I' 96,487 x A plutonium, mg = where F is the calibration factor in millicoulombs per millivolt for the coulometer range used, A is the volume of the aliquot taken for analysis and I' is the total volume of the sample solution.RESULTS The procedure was tested on two samples of plutonium dioxide, one having been ignited at 850" C and the other at 1600" C. The results, which are shown below, are expressed as percentage recoveries, assuming PuO,.,, stoicheiometry. Any error in this assumption is very small and is negligible relative to the precision obtained. Temperature of sample ignition 850" C; sample weight 100 mg; recovery, per cent. 99.83, 100916, 100.04, 100.05, 100*01, 99.89, 99434 and 99.93 (mean 99.95); coefficient of variation 0.11 per cent. Temperature of sample ignition 1600" C; sample weight 50 mg; recovery, per cent.99-95, 100-23, 99.70, 99.15, 100.95, 100.04, 99.50, 100.29, 99.05, 99.60, 100.25, 99-27 and 100*08 (mean 99435); coefficient of variation 0-54 per cent. CONCLUSIONS The sodium peroxide - sodium hydroxide fusion technique is a rapid and simple method for the complete dissolution of refractory plutonium dioxide, and it is by far the most rapid method for dissolving high-fired material. The sulphuric acid solution obtained on dissolving the melt is suitable for the direct determination of the plutonium content by controlled- potential coulometry. Although a glove-box is necessary for the fusion and dissolution of the melt, the resultant solution can be transferred into a fume cupboard and the determination completed there because of the small amount of plutonium needed for coulometry. This method represents an improvement and a simplification over the method involving a sodium peroxide sinter followed by differential spectrophotometry. The analysis by this latter method must be carried out entirely in a suite of glove-boxes because of the high concentration of plutonium involved. We thank Mr. D. Wicks for carrying out some of the coulometric determinations. REFERENCES 1. 2. 3. 4. 5. Milner, G. W. C., Crossley, D., Jones, I. G., and Phillips, G., Analyst, 1965, 90, 732. Milner, G. W. C., Wood, A. J., Weldrick, G.. and Phillips, G., Ibid., 1967, 92, 239. Rockett, J . J., U.K. Atomic Energy Authority Report AERE-R 3784, H.M. Stationery Office, Milner, G. W. C., and Edwards, J. W., U.K. Atomic Energy Authority Report AERE-R3772, Phillips, G., and Milner, G. W. C., in Shallis, P. W., Editor. "Proceedings of the SAC Conference, Received February 7th, 1968 London, 1961. H.M. Stationery Office, London, 1961. Nottingham, 1965," W. Heffer and Sons Ltd., Cambridge, 1965, p. 240.