Precise Determination of Plutonium by Potentiometric Titration GLENN R. WATERBURY and CHARLES F. METZ University of California, los Alamos Scientific laboratory, 10s Alamos, N. ,Macro amounts of plutonium are quantitatively oxidized with fuming perchloric acid, and plutonium(V1) is reduced to plutonium(1V) with a slight excess of standard iron(l1) solution, which is then titrated automatically with cerium(1V). Large sample sizes and weight burets are used for precise results. An average value for the plutonium content of a high purity metal sample of 99.98%, with a standard deviation of 0.02%, was obtained for 1 1 determinations, using 3- to 5-gram aliquots of a solution containing 63.1 1 1 mg. of the metal per gram. From the total of the concentrations of the trace impurities, the plutonium content of the sample was calculated to be 99.98%. Of 26 foreign metals investigated, chromium, gold, manganese, vanadium, and platinum seriously interfered. The spectrophotometric titration of plutonium is also discussed.
B
of the need for highly precise assays for plutonium, a simple procedure for routine determinations which had a standard deviation of less than 0.03 relative yoeven in the presence of iron, one of the common trace contaminants of plutonium, was investigated. Gravimetric, radiochemical, titrimetric, and spectrophotometric methods(10,lS) have been used for determining plutonium, but none possess the desired precision, and many are seriously affected by iron. A spectrophotometric determination of the end point in the titration of plutonium(II1) with cerium(1Tr) gives highly precise results for pure plutonium, but iron interferes seriously with the end point detection. The investigation of potentiometric titration methods led to the method described for determining plutonium in the presence of iron. Potentiometric titration methods for plutonium involve the oxidation of plutonium(II1) to plutonium(1V) with cerium(1V) or other suitable oxidant (10) or the reduction of plutonium(V1) t o plutonium(1V) with a reductant, such as electrolytically generated iron(I1) (4, IO). I n oxidimetric titrations, the samples are usually evaporated to fumes with sulfuric acid to eliminate undesirable anions and the plutonium is rpECAUSE
1144
ANALYTICAL CHEMISTRY
M.
duced with zinc amalgam or titanium(111) before the titrations. These pretreatments frequently involve sample transfers, which are sources of sample loss, or the determination of two end points. I n the titration of plutonium(V1) with a reducing agent, fuming perchloric acid may be used in the pretreatment to oxidize the plutonium, and the excess oxidant may be removed by diluting the solution. Thus the initial fuming to remove undesirable anions and the oxidation of plutonium may be performed in one operation, and there is no transfer of sample. This procedure has been used successfully for the oxidation of chromium (2, 5, 7, 9, 15-17, 10, 60), and of plutonium (10) in the absence of sulfate which stabilizes plutonium(1V). The use of iron(I1) as the titrant eliminates any interference caused by iron. Up t o 10-mg. samples of plutonium have been used in titrimetric methods with precision varying from 0.15 to 5% (10). T o obtain highly precise results, 300- to 500-mg. samples of plutonium are titrated by this method, and weight burets are used for measuring sample aliquots and for adding titrant. I n addition, to obtain a larger potential change and better end point detection than are possible in the direct titration of plutonium(V1) with iron(II), a back-titration technique is employed. The plutonium(V1) is reduced to plutonium(1V) with a slight excess of standard iron(I1) solution, which is then titrated with cerium(1V). The back-titration method with the larger potential break more readily permits the use of a n automatic titrator of the differential-potentiometric type having a small capacity buret. The automatic titrator greatly reduces operator time and eliminates errors of judgment inherent in manual titrations. Although the use of a modified automatic titrator is described, this method is equally applicable for the determination of plutonium by manual titration. SPECIAL APPARATUS A N D REAGENTS
DRY BOXES. Because the radiation froin plutonium consists of alpha particles, no shielding is required, but the extremely low maximum permissible daily body intake of 6.5 X mp. of plutonium (10) requires elaborate pre-
cautions. Stainless steel glove boxes with safety glass nindows are used for handling solid samples, and similar enclosures with the gloves removed or open front dry boxes are used for handling solutions. Surgeon’s gloves, coveralls or smocks, and canvas shoe covers are worn. ELECTROPOLISHING~PPARATUS. This consists of a glass crystallization dish containing a small amount of 20% potassium carbonate solution, a tantalum cathode, a tantalum wire anode, and a 22.5-volt dry cell battery. I n cleaning plutonium metal, the sample is placed in the carbonate solution and touched with the anode until the surface is bright. The sample is then placed on a fritted-glass disk and washed with water, acetone, and ether while the wash liquids are removed by suction through the disk. FUMIKG APPARATUS.T o prevent perchloric acid fumes from condensing in the plutonium dry boxes, a special fuming hood was fabricated from borosilicate glass with a stainless steel frame (Figure 1). A multiple-unit electric hot plate mith a 5 X 24 inch top surface fits inside the hood, leaving sufficient vertical clearance to place 250-ml. tall-form beakers on the hot plate. Heat from both the hot plate and heat lamps is used in the fuming operation. The exhaust duct of the dry box is washed continuously with water from three fine spray nozzles during fuming operations. & l O D I F I E D SARGEST-~1.4LMSTADT TITRATOR. ii commercial titrator was adapted for remote-control use in plutonium dry boxes or hoods (18). Cerium(1V) solution, O.O5M, is used in the 1-ml. buret. M‘eighing burets, Friedman and La Mer, 60-ml. CERIUM(IV)SOLUTION, 0.05.V. Dissolve about 30 grams of ceric ammonium sulfate, Ce(S04)2.2(SH4)2S04.2H20, in 500 ml. of 2M sulfuric acid, dilute to 1 liter with water, mix thoroughly, and let stand overnight. Standardize against alkaline arsenite solution or qodiuni oxalate, and calculate the grams of iron(I1) solution equivalent to 1 ml. of ceric sulfate solution. IROS(II) SOLUTIOF,0.5 to 0.7.V. Dissolve 200 grams of .Nohr’s salt. FeS04.(NH4)&3O4.6H20, in 500 ml. of I M sulfuric acid and dilute to 1 litcr with water. Bubble nitrogen through the solution to remove any oxygen and then store in a glass-stoppered bottle. Standardize daily or immediately before use against primary standard grade
HEAT LAMPS,
Figure 1.
Fuming hood inside plutonium dry box
potassium dichromate. high purity plutonium metal, or carefully standardized ceric sulfate solution. From the standpoint of arailability and purity, potassium dichromate is recommended as the standardization solution. An analysis of a typical sample, No. 136A, of potassium dichromate from the Kational Bureau of Standards shows a purity on the basis of effective oxidizing power of 99.9970. However, the loss in R-eight of 0.0270 upon drying at 240' C. introduces an uncertainty in this figure. T o use, weigh and dissolve a t least 2 grams of the dried salt, and transfer aliquots containing about 200 mg. of potassium dichromate by \\-eight buret to 250-ml. tall-form beakers. Titrate as described in the Recommended Method. High purity plutonium metal is the preferred standard because the titration conditions are the same for standardizing the iron(I1) solution and for analyzing the samplrs. A purity of 99.987, was calculated for a plutonium metal sample by subtracting the total impurity concentration obtained for oxygen b y tlie platinum-bath fusion method and for about 25 other elements by spectrographic methods. However, there is a small uncertainty in using plutonium as a standard because of t h e surface cleaning operation (see Sampling) required and the possibility of other impurities, which are not determined, b6,ing in the metal. If used as a standard, the Recommended Alrthod should be follon-ed and grams of plutonium per gram of iron(I1) solution should be calculated from the titration data. The standard ceric sulfate solution niay also be used if it has been standardized to within O . O l ~ O against a primary standard. (The ceric sulfate solution used in the analysis of the samples need not be standardized so carefully becausc only a small amount is used as compared w t h the amount of iron(I1) solution.) For ceric sulfatr,
weigh out aliquots of the standardized solution containing about 4 meq. of cerium(1V) into 250-ml. tall-form brakers and proceed with the titration. Calculate the equivalents of potassium dichromate or ceric sulfate per gram of iron(I1) solution and multiply by the molecular weight of the plutonium used in the samples to determine the grams of plutonium per gram of iron(1I) solution. T h e molecular weight of the plutonium used in this work is 239.06. Because of variations in the isotopic composition of plutonium, the molecular weight of t h e plutonium in each sample and standard should be calculated from its isotopic analysis data. T h e method of standardizing should duplicate the procedure followed in actually analyzing a sample of plutonium as described under Recommended Procedure. Because the titrator is triggered or turned off at the inflection point in the titration curve, which may not be exactly the equivalence point, other methods of end point detection could introduce small errors. SOLVTIOKS OF FOREIGN IOKS.Either the metal or the chloride or nitrate salt of the metal was dissolved in water, hydrochloric, nitric, or perchloric acids, or mixtures of the acids, to prepare a solution containing about 10 mg. per nil. of the metal. SAMPLING AND SAMPLE DISSOLUTION
To take full advantage of the high precision and accuracy offered b y this method, samples of at least 2 grams, properly cleaned, should be used to reduce the effect caused by weighing errors. If the surface of a sample is tarnished, the sample should be electropolished as described under Apparatus. Large solid samples may also be cleaned mechanically by filing away the oxidized surfaces. Cleaned samples should either be refrigerated to prevent oxidation or weighed and dissolved im-
mediately. The sample weight should be at least l o 5 times as large as the standard deviation of the balance for differential weighings. A semimicrobalance (either a Mettler Type B 6, Mettler Instrument Corp., or a n Ainsworth T C X , Wm. Ainsn.orth & Sons, Inc.) was used. Plutonium metal and most plutonium alloys are readily dissolved in dilute hydrochloric acid. Hon-ever, if the excess acid is depleted much below 0.1J4, hydrated plutonium oxide forms and complete solution is difficult. To prevent spray loss caused by the vigorous dissolution at acid concentrations of 1 to 331, a long-necked container of large volume should bt, u w l . A weighed 250-ml. or 500-nil. volumetric flask, held at an angle of aimut 45 degrees, was satisfactory. \Then dissolution was complete, the solution was diluted with 3Jf hydrochloric acid to 100 to 200 ml and weighed on a large capacity balance. Solution aliquots of a t least 2 grams and containing over 200 mg. of plutonium were weighed using a semimicrobalance. If the sample is not completely solublr in hydrochloric acid. or mixtures of hydrochloric, nitric. and perchloric acids, the sealed tube technique of Gordon, Schlecht, and Wichers ( 6 ) , using hydrochloric and perchloric acids. is recommended. Long solution methods requiring fusions or many sample transfers are not recommended if results are to be highly accurate. Plutonium solutions in hydrochloric, nitric, or perchloric acids may also be assayed for plutonium, but phosphate and sulfate solutions interfere. Weight aliquots as drscribed above should be taken for analysis. RECOMMENDED PROCEDURE
Transfer the weighed aliquot containing over 200 mg. oi plutonium to a 250-ml. tall-form beaker and cover with a ribbed watch glass. Evaporate to near dryness using heat lamps and low heat from the hot plate. Cool, wash down the beaker walls with 63f nitric acid, and add 5 to 10 ml. of 70% perchloric acid. Evaporate the solution to strong fumes of perchloric acid using sufficient heat from the hot plate and heat lamps to maintain the temperature slightly belon- the boiling point. Cool, and add 5 to 10 ml. of 20 to 30% perchloric acid. Again evaporate the solution until the final volume is 2 to 3 ml. Do not evaporate to dryness or results will be low. If the beaker accidently becomes nearly dry and the residue is not completely soluble in perchloric acid, add a few milliliters of 16M acid and 1.11 hydrofluoric acid and heat until solution is complete. Then repeat the fuming operations. After evaporating the aliquot to 2 to 3 nil., cool tlie solution rapidly by placing the beaker on a cool metal surface, and then dilute with about 50 ml. of n-ater as soon as it msy be added without spattering. d modified automatic VOL. 31, NO. 7, JULY 1959
1145
titrator was used in subsequent steps of the determination, but manual titration using a potentiometer with platinum and calomel electrodes may be substituted. Operation of the modified titrator has been described (18). Place the beaker on the magnetic stirrer of the modified automatic titrator and insert the clectrodes, stirring bar, and buret tip into the solution. Start the stirrer, set the sensitivity adjustment on the titrator meter box to maximum sensitivity, and add 10 ml. of 50% sulfuric acid. [Because the potential change in the reduction of plutonium(V1) with iron(I1) is less than that in the titration of the excess iron(I1) with cerium(IV), the meter sensitivity is increased to make detection of the first end point rasier.] Weigh the weight buret containing the standard iron(I1) qolution; add the reagent to the beaker until a largc potential change is shown by the meter. Because meter fluctuations warn of the approach of the end point, the excess iron(I1) may be kept t o 1 drop. Reweigh the buret. Turn t h e sensitivity knob back t o a predetermined setting that gives nearly full scale deflection in the titration of iron(I1) with cerium(IV), and adjust the meter zero setting if necessary. Read the dial of the Gilmont buret and turn on the automatic titrator. When the end point is reached and the titrator turns off, read the Gilmont buret and calculate the net amount of iron(I1) solution required to reduce the plutonium in the aliquot. Plutonium, %
=
[grams Fe(I1) solution] [grams Pu/gram Fe(I1) solution] 11001 [grams of sample in aliquot]
EXPERIMENTAL RESULTS A N D DISCUSSION
Oxidation of Plutonium. I n addition t o electrolytic oxidation of plutonium t o t h e sexivalent s t a t e ( 8 ) , several reagents have been described as suitable oxidants, including bromate, dichromate, hot dilute nitric acid, hot permanganate, hot perchloric acid, and ozone (10, 11). Many of these oxidants or oxidizing methods are either slow or incomplete, and some require additional treatments to destroy excess oxidant or leave undesirable ions in the solution. Hot perchloric acid was used in this work because it rapidly and quantitatively oxidizes plutonium to its highest oxidation state and leaves no undesirable ions. I n addition, anions such as chloride and nitrate are eliminated from the sample solution during the oxidation procedures. Conflicting reports of the reliability of perchloric acid as an oxidant for chromium have been summarized ( 2 ) , and the possibility of low results in the oxidation of plutonium with perchloric acid was considered. However, b y following some of the techniques recommended for the oxidation of chromium, hot perchloric ucid proved very reliable as an oxidant tor plutonium. I n the final fuming with perchloric acid, the solution was reduced t o 2 to 3 ml. which cooled rapidly when 1146
the beaker was placed on a cool metal surface. The cooled solution was diluted as quickly as possible and titrated. Boiling or addition of sodium bicarbonate to remove chlorine was not necessary under these conditions. Plutony1 chloride, if formed during the oxidation, is not volatile, and a refluxing still head, as used with chromium, is not required. Results were low only when the oxidized solutions were not titrated soon after dilution. Detection of End Point. Automatic or manual titration, using a weight or volumetric buret, gave equally precise results. Because of differences in starting potentials, caused by the different acid and metal ion concentrations of the solutions, for most accurate results it was necessary to estimate the end point from a plot of each titration curve rather than to titrate to a set potential value. This method of end point determination, while accurate, was tedious and timeconsuming. The automatic titrator shut off at the inflection point of the titration curve regardless of the starting potential or the potential at the shut-off point. For example, the reading on the titrator meter at shutoff was between 1.2 and 1.3 for the standardization titrations of cerium(1V) solution and between 1.4 and 1.6 for the titrations
ANALYTICAL CHEMISTRY
of plutonium solutions. T o determine if this difference in meter readings at the end points introduced error, additional cerium(1V) solution was added from the automatic buret t o bring the meter reading to 1.6 after the titrator had indicated the end point. Using this second set of data, the plutonium recoveries were recalculated and were slightly over 100%. Obviously the end points selected by the titrator were more nearly correct than that obtained by titrating to a definite meter reading. Spectrophotometric titrations were also performed. I n these titrations, a weighed aliquot of standard plutonium sulfate solution was reduced by zinc amalgam and about 98% of the plutonium(II1) was oxidized by cerium(1V) standard solution which was added rapidly from a weight buret. The final titration was made using a 2-ml. volumetric buret with 0.01M cerium(1V) standard solution while argon gas was passed into the titration vessel to prevent air oxidation of the plutonium(II1). Absorbances were measured a t 380 mp using a Beckman D U spectrophotometer after each 0.05- t o 0.10-ml. addition of titrant near the end point. Although the absorbancy of cerium(1V) is a maximum near 320 mp, the high absorbancy of plutonium(1V) at this
wave length caused interference which could be greatly reduced by taking measurements at 360 mp. From a plot of the absorbance readings as a function of volume of oxidant added, the end point was estimated. By using large samples, 200 to 400 mg. of plutonium, the results for plutonium were precise to 0.03 relative %. A 250-ml. tall-form beaker was used as a titration vessel, and the 10-cm. cell compartment of the Beckman DE spectrophotometer was modified to hold the beaker by removing the usual cellholders and replacing the cover with a flat piece of sheet metal through which a circular hole was cut to hold the beaker. An inverted box-type cover with openings for the argon tube and the buret tip was placed over the beaker. Felt or rubber protectors around the openings made the cover lighttight, and a slate black finish reduced reflections. A magnetic stirrer (Mag-Mix, Precision Scientific Co.) with cover removed was fitted below the cell compartment to provide stirring during the titrations. The light path through the beaker was about 5 cm. and large absorbance readings were obtained. However, for this preliminary investigation, the simple container for the radioactive plutonium solutions seemed adequate, and a more complex cell with a shorter light path was not tried. More dilute solutions used to reduce absorbancc readings did not improve the results. Although the spectrophotometric detection of the end point was satisfactory for the titration of plutonium(II1) with cerium(IV), the large absorbance of iron(II1) below 400 mp caused interference in determinations in which plutonium(V1) was reduced with a slight excess of iron(I1) before the final titration with cerium(1V). Complexing agents for iron(II1) such as fluoride or phosphate were not satisfactory in reducing the interference because they formed precipitates with plutonium. By controlling the acidity and the phosphate concentration, a precision of 0.2 relative yo was obtained for the determination of plutonium by this method. One other method was tried to eliminate iron interference in the spectrophotometric titration of plutonium. The plutonium and iron were reduced by zinc amalgam, o-phenanthroline was added to raise the oxidation potential for the iron(I1) to above 1 volt, and the plutonium was titrated with cerium(1V) using diphenylamine sulfonic acid as an indicator. The color change of the indicator was followed spectrophotometrically. This method was not sufficiently precise because of the interference in the spectrophotometric determination of the end point caused by iron(I1) o-phenanthrolate, the nonlinearity of
~~
the absorbance of t h e indicator as a function of amount of oxidant added, and the incomplete complexing of iron(I1) b y o-phenanthroline in the acid solution which resulted in some oxidation of the iron. Neither permanganate nor dichromate was satisfactory as an oxidant for the spectrophotometric titrations. Fading of the permanganate color after the end point made the absorbance as a function of the volume of oxidant added nonlinear. Iron(II1) interfered strongly with absorbance measurement for dichromate. Because of the satisfactory potentiometric results, the spectrophotometric titrations were not pursued further. Effect of Foreign Ions. T o determine t h e elements which interfere with the determination of plutonium, measured amounts of solutions containing foreign elements were added to aliquots uf a standard plutonium solution before t h e fuming operation. T h e results obtained for plutonium are shown in Tabli, I. Little or no intrrference resulted from neiglit ratios of 3 to 1 for potassium, 0.2 to 1 for uranium, 0.12 to 1 for silver, 0.1 to 1 for ceriumIIII), cobalt, copper, lead, niolybdenum, niobium, rhodium, ruthenium, thallium, tin, and titanium, 0.08 to 1 for zirconium, 0.075 to 1 for bismuth, 0 05 to 1 for strontium and magnesium, and 0.036 to 1 for cadmium. neodymium, .tnd nickel. Small amounts of chromium, gold, nianganme, platinum, and vanadium causcd serious interference. Cerium(1V) interfercnce mas eliminated by adding a few drops of hydrogen peroxide to rcducc the ci7rium before the fuming operation. I n the absence of sulfuric or phosphoric acid, perchloric acid does not oxidize cerium(II1) (17). Repeated evaporations with hydroc,hloric and perchloric acids to volatilize chromium as chromyl chloride reduced, but did not eliminate, the interfercnce caused by chroniiurn Sulfate ion prevented the quantitative oxidatiori of plutonium with perchloric acid, and phos,hate caused plutonium to prC6pitatc. Although no interference caused by iroii was expected, amounts up to 4070 of the plutonium weight n c w added to aliquots of the 1 ilutoiiium standard before the fuming 1 yeration without deleterious effects on the plutonium determination. The 300 mg. of potassium, as shorn-n in Table T, 1%ere aticled to determine the effect of a large salt concentration on the ( i d a t i o n of plutonium by perchloric wid. Although the final volume of colution could not bc reduced to 2 to 3 nil. in the fuming, quantitative results for plutonium n ere obtained. Separation of PIutonium. Although few of the foreign metals tested interfered in this titration method, the quantitative separation of plutonium
before its determination mould be necessary for samples containing chromium, gold, manganese, platinum, or vanadium. Several general types of separations have been applied to plutonium (IO), b u t no specific procedure described is as precise and accurate as the titration method. For this viork, a highly quantitative separation of the plutonium from the interfering metals with practically 100% yield was required. I n addition, no sulfate, phosphate, or organic materials that are difficult to destroy should be introduced. These requirements, as well as maintaining the high precision and accuracy of the over-all determination, imposed severe restrictions on the selection of a separation method. One method for each of three general types of separationanion exchange, precipitation, and liquid-liquid eutraction--n as investigated. Anion exchange separations have been described in which plutonium(1V) is selectively absorbed from 10M nitric acid or 12-11hydrochloric acid and then eluted 1% ith a reductant such as hydroxylamine or hydriodic acid (1, 5, 10,14). Using modified methods, recoveries of 99.8 to 100.2% were obtained for 200 mg. of plutonium in the presence of 10 to 20 mg. each of barium, cadmium, cerium, chromium, cobalt, iron. manganese, molybdcnum, nickel, tin, vanadium, and zircmium -4 double precipitation of the plutonium as the iodate from 1M nitric-0.253l iodic acid solutions containing the same 12 foreign elements gave crr:ttic results. An arerage for the plutoiiium of 99.9%, n i t h a standard deviation of 0.4%) nas obtained for five deterniinations of 200-mg. amounts of plutonium. Of the various liquid-liquid extraction separations describd for plutonium (10, 1%), only the extraction of plutoniuni(V1) into lievonc (4-methy1-2pcntanonc) froin aluminum nitrate solution n a s tried. For six determinations of 262 to 404 mg. of plutonium in the presence of the 12 foreign elements, recoverks of 97.2 t o 99.9% nere obtained. Although this titration method gives precis[> and accurate, results for the assay of high purity plutonium metal, the accuracy of the determination in materials from \+liich the plutonium must b- separated prior to its titration is limited by thc efficiency of the separation mpthod. Thir investigation indicates that none of the general qcparatioii methods tried is as precise and accurate as the titration method. By combining the radiochemical results for the trace amounts of plutonium lcft in the raffiiiates, superiiates, or n ash solutions, with the results obtained by the titration method for the separated plutonium, somenhat more accurate total plutonium values were
~
Table I.
Effect of Foreign Ions on Titration of Plutonium
Pu Taken,
Mg.
224.90 200.50 199.43 200.12 200.36 200.63 200.39 200,32 200.23 100.03 100.03 100.03 100.03 100.03 100.03 100.03 100,03 100.03 100.03 100.03 190.15 412.12 280.03 278.03 297.50
Foreign Metal Added, Ng.
Pu Found,
20 Rh, 20 Pt, 15 Ti, 20 co, 20 T1, 20 Bi, 15 sn, 20 Pb, 20 Nb, 10 c u , 10 IIn>10 Ru, 10 x g , 15 Cr, 10 Cr, 10 20 Sr, 10; lfg) 10 v, 10 Ce, 30; Kd, 10 S i . 10: Cd. 10 zr,’25
112 02 100 01 89 6IQ 99 95 100 0; 100.02 100 08 100 05 100 00 100 07 100 05 100 0 3 100 06 100 20 100 83 100 19 100.05h 112 3’) 107 90 100 OT 100 00 104 00 99 96 100 08 89 Xi
.ill,
u.
7%
Drifting potential a t end point. Poor end point because of precipitate formation during titration. (L
b
obtained. It probablj would also be possible to obtain good results by uii~ig a simple scparation of only one particular metal in specific samples containing one interfering element. The separation of each interfering element TI ould require an iiidividual method, and none of these special cases was investigated. Because no general separation method efficient in the presence of all of the interfering metals seemed possible, this study n as not continued RELIABILITY
The reLnbility of the method 11 adetermined by analyzing a plutonium metal samplv which had been analyzed by spectrographic and vacuum fusion methods for trace impuritirs. B y subtracting the total of the trace impuritiw froin loo%, a value for the plutonium content of 09.9870 was calculated. T l i ~ sample T\ a i c.lectropolishcd, n-aslied in nater, acetone, anti ether, and driccl in air. Two samples each of 2 to 3 grams were weighed until constant weights were obtained. The sampl(3s wert’ dissolvcd in hydrochloric acid anti 1%eighed aliquots n cre analyzed. KO significant difference bet\\ een results for the tn o v, eighed purtions n ere observed, and an average value for the plutonium content was 99.98%, n i t h a standard deviation of 0.02%. -4portion of the sample which had not been clectropolished was dissolved in hydrochloric acid and analyzed by this method. The sample was not heavily coated with oxide, but the surface was discolored. An average value for t h e VOL. 31, NO. 7, JULY 1959
1147-
plutonium content was 99.903%, with a standard deviation of 0.01370, from five titrations. The effect of trace amounts of oxygen is clearly shown by this lower result. This procedure has been used for the determination of plutonium in high purity plutonium metal samples, alloys with noninterfering metals such as iron, and nitric or hydrochloric acid solutions. About 15 determinations may easily be performed per day. Using an automatic heating-cycle controller for the two evaporations with perchloric acid, the operator time per determination should be greatly reduced. After the fuming operations, the remainder of the determination requires less than 15 minutes when the automatic titrator is used.
LITERATURE CITED
(1) Aiken, A. M., 2nd Nuclear Engineer-
ing and Science Conference, Paper
57-NESC-100, Philadelphia, Pa., (1957). (2) Banks, C. V., O'Laughlin, J. W., Ax.4~.CHEK 28,' 1339 (1956).' (3) Browne, C. I., et al., J . Inorp. & AV:uclear Chem. 1, 254 (1957). (4) Carson, W. K.,Jr., Vanderwater, J. W., Gile, H. S., ANAL.CHEM.29, 1417 (1957). (5) Ewing, R. E., Banks, C. V., Ibid., 20, 233 (1948). (61 Gordon. C. L.. Schlecht. W. G.. ' Wichers, ' E., J . Research .fTatl. Bur: Standards 33, 457 (1944). (7) James, L. H., Chemist Analyst 19, No. 5, 14 (1930). (8) KO. Rov. AML. CHEM. 28, 274 ' (1956). (9) Lichtin, J. J., IND. ENG.CHEM.,AN.4L. ED. 2, 126 (1930). (10) Metz, C. F., AXAL.CHEW29, 1748 (1957). "
(11) Miller, H. W.,Brouns, R. J., Ibid., 24, 536 (1952). (12) Moore, F. L., Hudgens, J. E., Jr., Ibid., 29, 1767 (1957). (13) Phillips, G., Analyst 83, 75 (1958). (14) Phillips, G., Jenkens, E. ?;., J . Inorg. & Nuclear Chem. 4, 220 (1957). (15) Roger, Louis, Fonderie 1951,2359. (16) Schuldiner, Sigmund, Clardy, F. B., IND.EXG. CHEM.,ANAL. ED. 18, 728 (1946). (17) Smith, G. F., Ibid., 6, 229 (1934). (18) Waterbury, G. R.,ANAL. CHmf. 31 , 1138 (1959). (19) Willard, H. H., Gibson, R. C., IND.ENG.CHEX., AXAL. ED. 3, 88 (1931). (20) Willard, H. H., Young, P., J . Am. Chem. Soc. 50, 1379 (1928).
I
RECEIVED for review November 3, 1958. hccepted March 5, 1959. n7ork done under auspices of the U. S. Atomic Energy Commission.
Selective Liquid-Liquid Extraction of Iron with 2 -Thenoy It rifl uo roa cet o ne-Xy Ie ne Application to Homogeneous Reactor Solutions FLETCHER 1. MOORE Oak Ridge National laboratory, Oak Ridge, Tenn.
W. D. FAIRMAN, J. G. GANCHOFF, and JOHN G. SURAK Marquette University, Milwaukee 3, Wis.
b Extraction of ferric iron from 10M nitric acid solution with 0.5M 2-thenoyltrifluoroacetone-xylene is efficient and more highly selective than ether extraction or anion exchange from hydrochloric acid solution. Scrubbing the organic phase with 0.25M hydrofluoric acid-0.25M nitric acid increases the selectivity. Concentrated hydrochloric acid readily strips the iron from the organic phase. Excellent separation of iron is effected from the alkalies, alkaline earths, rare earths, ruthenium, zirconium, niobium, iodine, tin, antimony, cobalt, manganese, chromium, nickel, thorium, protactinium, and uranium. The technique has been successful in the determination of the specific activity of iron in homogeneous reactor solutions.
R
use of the versatile chelating agent, 2-thenoy1trifluoroacetoneJ suggested that the ferric iron chelate was highly stable in strong nitric acid solution (8). Very few metal ions form stable chelates with 2-thenoyltrifluoroacetone in the higher nitric acid concentration ranges, the known ECEST
1148
ANALYTICAL CHEMISTRY
elements being zirconium, hafnium, and protactinium. The necessity for a radiochemical method for the determination of the specific activity of iron in homogeneous reactor uranyl sulfate solutions prompted further studies on the behavior of iron in the nitric acid system with 2-thenoyltrifluoroacetone (Columbia Organic Chemicals Co., Columbia, S. C.). Iron-59 (t1,2, 45 days), a beta, gamma emitter, is formed according to the reaction: Fe-58 (n, 7) Fe-59. The presence of high level radioactivities precluded the direct use of gamma scintillation spectrometry without prior chemical separation. Studies (1, 2) of the extraction behavior of ferric iron with 2-thenoyltrifluoroacetone have suggested that the p H range of the aqueous phase of 2 to 4 is optimum for analytical separation work. Although ferric iron extracts readily under these conditions, the selectivity is very poor. Therefore, the major effort was directed to the use of higher concentrations of nitric acid. The use of 0.5M 2-thenoyltrifluoroacetone-xylene was based on experience with this reagent in metals analysis which was recently reviewed (9).
Ferric iron was also observed to extract efficiently from perchloric acid solution. The procedure developed is rapid and highly selective and may be readily adapted t o remote control operation. Yields average 70 to 80%. PROCEDURE
Pipet a suitable aliquot of the sample solution into a 50-ml. Lusteroid tube (Lusteroid Container Co., Inc.). Add 20 mg. of ferric iron carrier. Stir well and precipitate ferric hydroxide by adding excess concentrated ammonium hydroxide. Centrifuge for 1 minute in a clinical centrifuge and discard the supernatant solution. Wash the precipitate by stirring with 15 ml. of 1M ammonium hydroxide. Centrifuge for 1 minute and discard the supernatant solution. Dissolve the ferric hydroxide in 3 ml. of concentrated nitric acid. Add 10 ml. of 1OM nitric acid and 1.3 ml. of hydrogen peroxide (30 to 35%). Mix well, add 15 ml. of 0.5-If 2-thenoyltrifluoroacetone-xylene, and extract for 5 minutes. Centrifuge for 1 minute and carefully remove the aqueous phase with a transfer pipet or micropipet attached by rubber tubing to a vacuum trap, By using mild suction and squeez-
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