(11) Marshall, E. D., Rickard, R. R., ANAL. CHEM.22, 795 (1950). (12) Mellon, M. G., “Colorimetry for Chemists,’] Columbus, Ohio, G. F. Smith Chemical Co., 1945. (13) Ogburn, S. C., Jr., J . Am. Chem. SOC. 48, 2493 (1926). (14) Ogburn, S. C., Jr., J . Chem. Educ. 5, 1371 (1928). (15) Sandell, E. B., “Colorimetric Deter-
mination of Traces of Metals,” Interscience, New York, 1944. (16) Singleton, W., Znd. Chemist 3, 121
(20) Yaffe, R. P., Voigt, A. F., J . Am. Chem. SOC.74,2500 (1952). (21) l b i d . , p. 2503. (22) l b i d . ., p. _ 3163.
(1927). (17) Steiger, B., Mikrochemie 16, 193 (1934-5). (18) Wilson, R. F., Ph.D. thesis, University of Texas Library, 1953. (19) Wohler, L., Mete, L., 2. anorg. u. allgem. Chem. 138, 368 (1924).
RECEIVEDfor review February 27, 1957. Accepted May 20, 1957. Contribution No. 534. Work performed in the Ames Laboratory of the U. S. Atomic Energy Commission.
Cou I ometric Determination of PIutoniu m WILLIAM N. CARSON, Jr.1, JEANETTE W. VANDERWATER, and HERMAN S. GlLE Hanford Atomic Products Operation, General Electric
b A general chemical method for the determination of small amounts of plutonium in solution consists of a coulometric titration based on the preliminary oxidation of the plutonium(VI), followed by titration with electrolytically generated ferrous ion to plutonium(1V). It is subject to few interferences. It can b e used for samples containing 3 y to 10 mg. of plutonium. The precision expressed as the standard deviation of a single result is within about 5% at the 3-7 level and 1% a t the 1-mg. level. Both manual and automatic procedures were found suitable for the entire range of sample size. Simple modifications overcome the few interferences. Thus amounts of plutonium heretofore determined only b y radioassay methods can be determined chemically and larger amounts are more readily determined and with higher precision than with previous methods. The work extends the application of coulometric titration methods to a new substance.
G
volumetric, radiochemical, and spectrophotometric methods have been applied to the determination of plutonium. Because of the extreme hazard to personnel in handling materials that contain more than trace amounts of plutonium, only micro- and semimicroprocedures are normally used. Excellent examples of the use of all four types of methods are given in the Plutonium Project Record (IO). Only the methods based on alpha particle counting have had wide application. Previously reported volumetric methods of analysis involve the pluRAVIMETRIC,
Present address, General Engineering Laboratory, General Electric Co., Schenectady, N. Y.
Co., Richland, Wash.
tonium(II1)-(IV) couple. Koch (8) has described a n oxidimetric titration in which the plutonium is converted to plutonium(II1) in a Jones reductor and then titrated with standard ceric sulfate to plutonium(IV), using ferroin as indicator. Obvious adaptations of this method, such as the use of chromous or titanous ion for the preliminary reduction and the use of electrometric end points, can be made. Experimentally, iron has been found to be a n excellent chemical stand-in for plutonium, and, in general, volumetric methods for the determination of iron can be used for plutonium. However, such methods must be applied to samples free of iron, or corrections made for the iron content by a separate assay. The factor for plutonium in iron is large and thus solutions containing even small amounts of iron have large correction factors. An attempt was made to adapt the titration involving the plutonium(II1)(IT)couple to a coulometric titration employing the electrolytic generation of ceric ion as described by Furman, Cooke, and Reilley (7). The best results were obtained by fuming the sample with sulfuric acid to convert it to a sulfate medium, diluting with water, and reducing the plutonium with an excess of titanous sulfate. The solution was then titrated using potentiometric end points. Both the titanous and the plutonium end points are sharp, even for small amounts of plutonium. However, a large reagent blank was always found, due largely to the traces of iron in the reagents, particularly in the cerrous sulfate and the sulfuric acid used in the electrolyte. Attempts to reduce the iron blank by purification of reagents or changes in the procedure were not successful, and the method was unsuitable for the titration of microgram amounts of plutonium.
The present study has shorr-n that the best possibility for a rapid and accurate coulometric determination of small amounts of plutonium is the titration of plutonium(V1) to plutonium(1V) with electrolytically generated ferrous ion. Iron will not interfere with the determination, and the technique can be applied to determination of 3 y samples of plutonium without undue difficulty. EXPERIMENTAL
Apparatus. The basic titration cell assembly is shown in Figure 1. The assembly consists of three parts: t h e titration cell, the pretreatment unit, and the scrubber. An exploded view of the first two parts is given in Figure 2; t h e titration cell in this figure is one used for the automatic high level titrations and differs from the manual cell shown in Figure 1 by having a special side arm added for the anticipation of end-point system required by the titrator. The titration assemblies are otherwise the same. The titration vessel is about 3 inches tall, and the pretreatment cell is about 4 inches tall and 0.75 inch in the small outside diameter. The details of size and shape may vary over a wide range; the titration vessel should be such that 4 to 6 ml. of electrolyte cover the electrodes and the pretreatment unit be about half filled with 2 to 3 ml. Single piece construction as indicated in the figures is required. The scrubber is used to remove acid fumes from the gases swept from the pretreatment unit. It is attached to a vacuum supply which pulls a stream of gas from the top of the pretreatment unit and is filled with marble chips and water to trap the acids; a coarse sintered gas dispersing tube is used to mix the gases with the liquid. The water in the scrubber is replaced periodically to prevent the accumulation of salts that would clog the scrubber tube and to prevent build-up of excessive radioactivity. VOL. 29, NO. 10, OCTOBER 1957
1417
stant current of 0.1 pa. obtained from a %volt battery with 30-megohm resistance in series. The value of the polarizing current is critical in this a p plication. The potential across the electrodes is measured by an electronic pH meter, such as the Beckman Model
H-2.
Figure 1.
Bosic titration cell assembly
The pretreatment unit is used to oxidize the sample prior to the titration. In the unit, the 2 m m . capillary tube that connects the unit to the titration cell extends to the bottom of the vessel; this is used to introduce the stirring gas and to siphon off the contents of the unit in the transfer of the sample to the titration vessel. The other end of the capillary tube terminates in the titration cell with a short drip tip. A side port on the pretreatment unit is provided for the introduction of the sample and reagents. This port is left open to provide a vent for the unit; in use, the suction from the scruhber pulls a rapidly moving stream of air across the upper region of the unit to sweep the acid fumes into the scrubher. As a sudden reaction in the pretreatment step could force the contents of the unit out of this vent, a safety tube is provided to act as a trap for the liquid, yet not interfere with the air flow. As an additional precaution, the side tube points to the side of the vessel rather than directly toward the front; this prevents the possibility of a sudden spurt of hot acid onto the operator. The pretreatment unit is heated as required by means of a small ring heater. The one shown in Figure 1 is the Fisher Hotspotter heating element (Catalog No 11-50215). The titration cell is placed back of the plane of the port on the pretreatment unit, to allow pipetting of reagents and sample. It contains the generation electrode system, the indicator electrode system, and the gas and vacuum inlet. The generation electrode system used is a divided cell system in which the anode, immersed in 1 to 1 perchloric acid, is separated from the rest of the cell by silica gel salt bridge. The anode compartment is a 5-mm. tube sealed a t one end by a short section of coarse frit in which the silica gel is deposited by filling the frit with a concentrated sodium silicate solution and 1418
ANALYTICAL CHEMISTRY
then dipping the tube in dilute acid for a few minutes. The anode (not shown) consists of a short length of 90% platinum-lO% iridium alloy wire, about 20 gage. The cathode (not shown) consists of about 2 inches of the same alloy wire wrapped in a coil around the lower end of the anode compartment. The lead to this wire is of the same alloy wire and is protected by a thin sheath of polyethylene tubing sealed to the wire to prevent entry of liquid between the sheath and the wire. The lead is introduced into the cell by way of the gar; inlet; a Tygon tube (not shown) makes the connection to the gas system, and holds the lead wire-in place. The same gas inlet and tube are used in applying vacuum to the titration cell in the transfer of the oxidized sample to the titration cell. The indicator electrode system is the derivative polarographic system of Reilley (9). The electrodes are two short wires of the same alloy as the generator electrodes sealed in a 7-mm. lead glass tube. These are polarized with a con-
Figure
2.
The titration cell assembly is placed in a Lucite box, fitted with waste traps, vacuum line traps, helium flow controls, vacuum flow controls, stirrer controls, heater controls, etc. A sliding door gives additional shielding in front of the pretreatment cell during pretreatment to minimize further the danger of an acid spurt onto the operator. It was convenient to cover all the surfaces of the box with polyethylene film in order to facilitate cleaning up of any spills. Two constanhcurrent sources were used to provide the range of currents necessary for the titration of 2 y to 10 mg. of plutonium. For 300 y or more of plutonium, the electronic source previously described ( 1 ) was used. For lesser amounts of plutonium, a battery and series resistor type constant-current source was used: a large 45-volt B battery and a bank of resistors wired with a selector switch so that currents from 20 to 150 pa. could he obtained. Currents of less than 20 pa. do not give satisfactory end points. The exact value of the current was measured by determining the I R drop across a precision resistor in series with the leads to the generator electrodes. For the work reported, a 10-ohm =t0.05% resistor and a Leeds & Northrup (Catalog No. 8667) potentiometer were used with the electronic source. A 1000-ohm 2~0.05% resistor and a modified Leeds & Northrnp (Catalog No. 8667) potentiometer were used with the battery source. The modification of the potentiometer was to replace the galvanometer in order to increase the sensitivity, and to replace the original fine adjust resistor with a 10-turn, 25-ohm Helipot, so that a finer adjustment of the working voltage could he made. During part of the work reported,
Details of titration cell and pretreatment cell
"SELECT"
++ Figure 3. F. s-1, 5-2. s-3.
r.
c-1. c-2. c-3. K. KA. KB.
P.L.
Control circuit
Fuse 1 ampere DPDT toggle 3PST wafer SPST toggle Timer, 3-minute cycle, 3 cams Industrial Timer Corp., Newark, N. J. Cam switch, on from 2.5 to 2.7 minutes each cycle Cam switch, on from 0 to 0.03 minute each cycle Cam switch, on from 0 to 0.1 minute each cycle Relay, Struthers-Drum, 1 XBX, DPDT Relay contact-clock Relay contact-current Pilot light, Dialco 9 0 1 048
automatic titration equipment was used with good success. The equipment was similar to that reported previously ( 2 , 4). For high level samples (50 y of plutonium or more) anticipation of the end point (3) is required. For low level titrations (less than 50 y of plutonium), the entire titration is made by the addition of increments in the manner of the last stages of titration with the high level titrator. No particular difficulties were encountered during the investigation in the use of automatic equipment ; the precision and accuracy obtained are as good as for manual titrations. Only the manual procedures are described in detail in this report. The automatic titration follows the same pretreatment and operating procedures. For the manual titration of small amounts of plutonium, it was convenient to have a means for controlled addition of increments of current required by the procedure. For this purpose, a control circuit (Figure 3) provides for the selection of any of the following operations : continuous addition of current, addition of current in 0.1-minute increments a t the rate of one increment every 3 minutes, or addition of current in 0.03-minute increments a t the rate of one increment every 3 minutes. The heart of the device is a cycle timer that provides the two increment patterns. A pilot light warns the operator to take the reading of the indicator potential a t the correct time to get the best results. The manual switch is used for continuous addition of current a t the start of the titration. Reagents. Reagents were of C . P . grade, except for perchloric acid, which was of analytical reagent grade, and urea which was U. S. Pharma-
copeia grade. Special solutions used were made up as follows. Potassium permanganate solution, 0.1M, was made up within 4 hours of use from dry salt and distilled water. Yo special precautions were taken, other than having the solution fresh; no precautions in storage or preparation Ivere found to prevent the formation of a species of manganese dioxide that titrates in the procedures as plutonium and thus gives high results if the solution is over 4 hours old. Formaldehyde, 37%, should be kept free of formic acid, which inhibits the reaction with manganese dioxide. A fresh supply should be used if the manganese dioxide is not destroyed under the conditions of the pretreatment within 1 minute. Stock electrolyte, 0.6M ferric ammonium sulfate in 25% sulfuric acid1070 phosphoric acid solution. Working electrolyte, 6.5 grams of crystalline urea in 100 ml. of stock electrolyte. Lanthanum nitrate solution, 10 grams of lanthanum per liter: 3.1 grams of lanthanum nitrate hexahydrate in 100 ml. of water. Lanthanum fluoride solvent solution : 6 X perchloric acid, 0.2M boric acid, and 1.5M nitric acid. K a s h solution, 131 nitric acid and 1X hydrofluoric acid Pretreatment Procedures. For samples (which should be less than 500 p l , in volume and contain 0.003 to 10 mg. of plutonium) that can be analyzed directly, place the sample in the pretreatment vessel. Add 200 pl. of 0.1M potassium permanganate solution and 100 pl. of 5 to 6M nitric acid if the sample is less than 100 kl in size or less than 1M in nitrate, and
rinse sides of the vessel with 2 ml. of 1 t o 1 perchloric acid. Replace safety, and set shield in front of the apparatus. Adjust sweep gas (oil-free air for samples containing more than 50 y of plutonium; oxygen-free helium, carbon dioxide, or nitrogen for smaller samples) to give smooth gas stirring. Heat a t approximately 75" C. until permanganate decolorizes; the permanganate color should persist for a t least 2 minutes. Turn off heat, allow gas to stir, and cool for 10 to 15 seconds, turn off gas stirring, remove safety, add 25 pl. of 37% formaldehyde solution, and replace safety immediately. Turn gas stirring back on, and cautiously heat until manganese dioxide is destroyed. Increase temperature cautiously until brown fumes of nitrogen dioxide appear (look in the air sweep across the top of the vessel). Turn off the heat but continue to gas stir and cool for a t least 1 minute before transfer. The carrier precipitation of plutonium fluoride on lanthanum fluoride used to remove interferences is done as follows. Place sample in a 3-ml. centrifuge cone and dilute with 1 ml. of 231 hydrochloric acid Add 100 p l . of O.1M hydroxylamine hydrochloride solution t o reduce any plutonium(V1) ; stir and allow to digest for 10 minutes. Add 100 pl. of lanthanum nitrate solution, stir, then add 150 p l . of concentrated hydrofluoric acid. Stir, then allow the precipitate to digest for 5 minutes. Centrifuge 5 minutes, discard the supernatant liquid, and wash twice with 1-ml, portions of wash solution. Dissolve the precipitate in 500 ~ 1 of. lanthanum fluoride solvent solution and transfer to the pretreatment vessel. Proceed as for direct samples, using the dilute perchloric acid as rinse for the centrifuge cone and transfer pipet. Use additional perchloric acid as needed to ensure complete transfer of the plutonium. Several modifications of the direct pretreatment procedure are useful to avoid making a separation in the case of certain interferences. For samples lorn in nitrate, 25 ~ 1 of . concentrated hydrochloric acid or saturated sodium azide solution may be used in place of formaldehyde. The solution must be heated to near the boiling point for a few minutes to remove the reaction products. No nitric acid should be added. This modification is not suitable for samples containing less than 30 y of plutonium or large amounts of nitrates. Kitrates, if present, cause severe etching of the indicator electrodes and generator cathode in the presence of chlorides. This generally is shown by erratic behavior of the indicator system. For solutions containing sulfate, the use of hydrochloric acid or sodium azide is mandatory if no separation is made. Nitrates may be removed prior to oxidation in these samples by adding perchloric acid and ferrous perchlorate VOL. 2 9 , NO. 10, OCTOBER 1 9 5 7
1419
solution and heating until there is an evolution of nitrogen oxides. Sulfuric acid should never be added to a sample. Fuming with sulfuric acid or perchloric acid is highly unsatisfactory, and is dangerous from the spattering viewpoint. For samples containing small amounts of reducing agents that reduce the permanganate too quickly for quantitative oxidation of the plutonium, the separation step can be avoided by adding sufficient sodium bismuthate along with the permanganate to reoxidize the manganese to permanganate. The solution is heated until the permanganate decolorizes as before. The use of bismuthate alone, more permanganate, or other oxidizing agents such as persulfate, is not satisfactory. This modification is not suitable for samples containing large amounts of phosphate, because of the precipitation of bismuth phosphate in the vessel. Titration Procedures. Pretitration of the electrolyte is necessary to remove reducing or Oxidizing impurities. Add a slight excess of ferrous sulfate to a stock electrolyte, if oxidizing impurities are present. For samples that contain 50 y or more of plutonium, use 20 t o 50 ~ 1 of. 0.2M potassium dichromate and titrate to an end point, using the same current to be used in titrating the sample. Use sufficient dichromate so that the indicator potential starts a t about 300 mv. This potential will increase during the titration to values above 600 mv.; a t the end point there is a sudden drop to almost zero. The end point should be approached in small increments, since the indicator potential tends to lag. The pretitration should require 2 minutes (or more) of titration time.
For samples containing less than 50 y of plutonium, care must be used in adding the excess of dichromate, to prevent excessively long pretitrations. A useful expedient was to determine the amount of reducing impurity in the electrolyte by adding a known amount of dichromate to an accurately measured volume of electrolyte, and finding the decrease in the titer of the dichromate. A calculated amount of dichromatetobring the electrolyte almost to the end point was added to 1 or 2 liters of stock electrolyte; the final adjustment was made by adding dilute dichromate to the electrolyte in the titration vessel until the indicator potential rose above 600 mv. The potential will decrease upon the addition of dichromate, if more than a slight excess is used. Titrate continuously with the same current as is to be used for the sample until the indicator potential falls 20 mv. from its highest value. Titrate in 0.1-minute increments added a t the rate of one increment every 3 minutes
until the potential falls to 580 mv. Add 0.03-minute increments a t the rate of one increment every 3 minutes until the potential decreases below 520 mv. Record final millivolt reading, allowing a few minutes for the indicator potential to stop drifting. To titrate the sample, transfer the oxidized sample to the titration vessel, and titrate as for the pretreatment step. The indicator system is much more sluggish for plutonium than for dichromate, and care must be taken not to overshoot the end point. For samples of 3 to 20 y of plutonium, interpolation to find the exact time of the end point is needed, as a significant part of the titration time may be only part of a 0.03-minute increment. Linear interpolation has been found satisfactory. To transfer the oxidized sample from the pretreatment vessel, first remove 2 ml. of pretitrated electrolyte with a elean dry transfer pipet; then pull the sample into the titration vessel by vacuum. Rinse the pretreatment vessel with the pretreated electrolyte held back. Resume gas sweep and fill the pretreatment vessel partly full with distilled water. Make automatic titrations in a similar fashion with about 10% of the electrode held back in the anticipation sidearm. For low values of plutonium, set the automatic titrator to deliver 0.1-minute increments every 3 minutes after the preliminary end point is reached. For the high levels, make the first part of the titration continuously, followed by use of 0.05-minute increments every 2 minutes after the preliminary end point.
the destruction of manganese dioxide is the method recommended for use if possible. With the exception of low level samples run with hydrochloric acid destruction of the manganese dioxide, the accuracy and precision are good. No automatic titration studies using the formaldehyde procedure are shown; their accuracy and precision are comparable to those of manual titrations reported in this paper. Most of the effort on the method using formaldehyde was concentrated on the titration of low level samples. The lower limit found was about 2 y of plutonium in most samples of interest other than those in which the plutonium is present in tracer amounts only. The standard solutions were made up by dissolving weighed amounts of metallic plutonium of high purity in hydrochloric acid and fuming with nitric acid to remove chlorides, or by using weighed aliquots of such solutions to make diluted standards. The plutonium fluoride samples were made by dissolving weighed amounts of plutonium trifluoride in dilute nitric acid.
Calculations. Grams per liter of plutonium = 0.618 itw/S
1M perchloric acid Pu(V1) 1.04 volts
where
i
= current in milliamperes
,/
=
w
=
S=
time of ferrous ion generation in minutes equivalent weight of plutonium in grams sample size, microliters
The equivalent weight of plutonium can be taken as 239/2 for ordinary work. S o chemical determination of the atomic weight of plutonium has been made; the value calculated from mass spectrographic and packing fraction data is 239.06. The influence of isotopic composition on the atomic weight is very small-for example, each per cent of plutonium-240 content will increase the weight by only 0.01 unit. The refined value of the atomic weight can, of course, be used if desired. RESULTS
Table I gives the results of a series of titrations made in the investigation. A variety of pretreatment methods is shown; the use of formaldehyde for I
1420
ANALYTICAL CHEMISTRY
DISCUSSION
Chemistry of Pretreatment and Titration. The method is dependent upon the quantitative oxidation of plutonium to plutonium(V1). Connick (6)has reviewed the literature on plutonium chemistry, and gives the following values for the couples: f------)
Pu(IV) 0.982 volt Pu(II1) t-+ 1M nitric acid Pu(V1) 1.10 volts t ) Pu(1V) 0.92 volt Pu(II1) c-----f 1M sulfuric acid Pu(V1) 1.2-1.4 volts Pu(1V) 0.75 volt, Pu('II1) f----f
Reagents such as permanganate and bismuthate will oxidize plutonium(II1) and plutonium(1V) to plutonium(V1) in all three media; in practice, only perchloric acid is suitable as a medium for the preliminary oxidation for a titration, as the presence of large amounts of nitric acid is undesirable for a reductimetric titration, and the presence of large amounts of sulfate makes the rate of oxidation of the plutonium very slow. The rate of oxidation of plutonium decreases with increasing acid concentrations, but this effect is not large, and relatively high acid concentrations can be used to aid in keeping the plutonium and other constituents in solution. A more important effect on the oxidation rate is the concentration of plutonium. The rapid oxidation of plutonium to plutonium(V1) is largely through disproportionation of the intermediate plutonium(V) species; this is concentration-dependent. Thus,
moderate to high concentrations of plutonium can be oxidized quantitatively in a short time; low concentrations take much longer for the oxidation, since the sloryer direct oxidation of the plutonium is required. The presence of any chromium in the samples will result in the presence of chromium(V1) a t the end of the preliminary oxidation step. I n the sulfate system used for the titration, chromium(V1) and plutonium(V1) have about the same oxidation potential; hence chromium is a potential interference in the methods. However, in perchloric acid and in the absence of sulfate, chromium(V1) can be preferentially reduced n-ith formaldehyde. leaving the plutonium in the hexavalent state. The exes6 of formaldehyde must be removed prior to transfer of the oxidized sample to the sulfate titration medium. If it is not removed, the plutonium will be reduced becaube of the enhancement of the oxidation potential of plutonium(VI) by sulfate complexing of plutonium(1V). Formaldehyde is readily removed by reaction with nitric acid a t an elevated temperature. Any excess nitric acid left in the sample does not interfere in the titration step. The formaldehyde will react with the excess permanganate, the manganese dioxide formed in the oxidation step, and any chroniiuni(V1) before the reaction with nitric acid occurs. Ferrous ion is generated by the electrolysis of ferric sulfate in dilute sulfuric acid in the manner of Cooke and Furman (6). The generation a t 1 0 0 ~ o current efficiency requires a large concentration of ferric ion, which interferes in the indicator system. By complexing the bulk of the ferric ion with phosphate, the free ferric ion concentration is lowered to a value that does not interfere with the indicator system. The concentration requirement for the electrolysis is met by the buffering action of the phosphate complex, which prevents any significant change in the free ferric ion concentration. Fluoride ion has the same effect as phosphate. The residual nitric acid left in the sample that has been oxidized by the regular procedure will react with ferrous ion, if traces of nitrite are present. If these traces are removed by the addition of urea, no measurable reaction occurs at room temperature. Hence the titration medium is made about 1M in urea. Permanganate was the only satisfactory oxidizing agent for the preliminary oxidation, in spite of the nuisance of formation of manganese dioxide. Bismuthate gave erratic results unless a small amount of manganese was present to act as a carrier for the oxidation; this still left manganese dioxide to be removed a t the
end of the pretreatment. Persulfate with or without silver present as catalyst gave very high results, indicating that the excess was not being destroyed by the formaldehyde, hydrochloric acid, or sodium azide solutions tried a s reductants. Halogen oxysalts, such as periodate, are not suitable for this application because of the nature of the reaction products, or because they do not have sufficient oxidizing power. The use of dilute hydrochloric acid or a solution of sodium azide for the removal of excess permanganate and manganese dioxide was studied. Neither of these reagents reduces chromium(V1) or plutonium(V1) in sulfate media, so the excess need not be removed prior to titration. The use of hydrochloric acid results in severe corrosion of the generator electrodes with samples containing nitrates (even with urea present). With nitrate-free samples, hydrochloric acid gives good titrations. The use of sodium azide offers a n advantage over hydrochloric acid in that it does not cause corrosion in the presence of nitrates, but its action is considerably slower than hydrochloric
Table 1.
Sample Solution Plutonium nitrate 0.2409 g./l.
Quantity,
acid or formaldehyde in the destruction of manganese dioxide. It was difficult to remove manganese dioxide routinely when this reagent was used. For some samples, a preliminary separation of the plutonium from a n interference is necessary prior to oxidation. When applicable, the use of the carrier precipitation on lanthanum fluoride is recommended. Extraction of the plutonium into an organic solvent, followed by back-extraction into a nitric acid solution, is a possibility. Samples containing organic matter should have the organic matter removed by wet combustion with a mixture of concentrated perchloric and nitric acids prior to oxidation, unless it is known that the organic matter will not reduce permanganate. Interferences. Although the method is highly specific for plutonium, a number of interferences in the preliminary oxidation and t h e titration exist. Substances t h a t readily reduce permanganate, such as chloride and oxalate, interfere in the preliminary oxidation by forming large amounts of manganese dioxide
Coulometric Titration of Plutonium
Av. G./L. Pu
Pre-
&iona,
25 (6 y)
KO. e/c 6 0.2284 & l ,72
25 (6 7)
10
PI.
Remarks 1. Nitrates removed by ferrous
perchlorate method. Sodium bismuthate added in oxidation; hydrochloric acid used to remove MnOs 2. Automatic low level titration
0.2378 f 5 . 5 1 SeeRemarks 1 and 2 3. 25 pl. 2M u02(x03)2 added 0.0950 g./L 100 (9.5 y ) 10 0,0946 f 0 . 6 1 4. Formaldehyde pretreatment 5. Manual titration 20 (1.9 y ) 7 0.0953 f 2 . 9 2 Seeremarks4and5 lOO(9.5 y ) 9 0.0945 h 1 . 2 1 Seeremarks4and5 6. 100 pl. 2 . 0 N UOz(N03)2 added t o samples; plutonium separated by LaF3 method prior to titration 20 (1.9 y ) 6 0.0953 f 5 . 2 1 See remarks 4, 5, and 6 lOO(1mg.) ISh 10.38 f 1 . 6 5 Seeremarks4and5 10.31 g./L f 0 . 8 9 Seeremark 1 25 (625 y ) 10 2 4 . 7 24.85 g./l. 7. Automatic high level titration 25 (625 y ) 5 24.7 f 0 . 8 7 Seeremarks 1 and 7 C 100 ( 1 mg.) 55b 11.Gd 1 0 . 1 0 8. Samples contained fluoride, gram lanthanum, iron, and chro/litere mium impurities See remarks 4 and 5 Plutonium fluoride 50 (500 y ) 10 9.70 0.80 See remarks 1 and 7 9.86 g./L Solution 4.79 g./l. 50 (250 y ) 10 4 . 8 4 f1. 87 See remarks 1 and 7 1.81 f l .li See remarks 1 and 7 1.77 g./L 5? (90 y ) LO 8 1.73 z k i . 1 See remarks 1 and 7 20 (45 y ) 9. Air blanket on cell 25 (45 y ) 8 1.77 i.1 .4 See remarks 1 and 7 10. COZblanket on cell Given in terms of standard deviation of single value. b Pairs of duplicates, each pair run by different chemists or at different times. 6 Standard value not known, each pair of duplicates run on different solutions. Median value of sample group. Range was 14 to 10 g./l. Calculated from differences in duplicate values. VOL. 29, NO. 10, OCTOBER 1957
1421
Table 0.
Interferences in Coulometric Titration of Plutonium
Do S o t Interfere
Not Tested Alkalies Alkaline earths, except barium Rare earths, except cerium Beryllium Magnesium Zinc Cadmium Aluminum Scandium Yttrium Gallium Indium Thallium
Interfere Tested
Hydrogen (ion) Copper Boron Phosphorus Arsenic Antimony Bismuth Chromium Molybdenum Tungsten Ammonium Sitrate Iron Manganese Fluoride Lanthanum a Does not interfere in trace amounts, up to lOy0of plutonium present. \t70uld interfere by precipitation as sulfate in titration step. e Interferes in retreatment step only. d Interferes in Tow level titrations only e Would interfere by precipitation as phosphate in titration step. Titanium Actinium Silicon Germanium Siobium Tantalum Polonium Cobalt Yickel -4mericium Curium Californium Berkelium Thorium
prior to oxidation of the plutonium. The addition of more permanganate is not satisfactory, as freshly precipitated manganese dioxide catalyzes the decomposition of permanganate in the hot dilute acid; the additional permanganate decomposes before the plutonium is quantitatively oxidized. This difficulty is overcome either by separating the plutonium from the reducible ion, or by adding bismuthate to the oxidation mixture to reoxidize the manganese. The last expedient is suitable for small amounts of reductant only. An interference that caused much trouble before it was found and eliminated is the presence of inert (to formaldehyde or hydrochloric acid) mangsnese dioxide in the permanganate solution used for the oxidant. This forms if the solution is left standing for more than a feu- hours, and causes high results, because ferrous ion readily reduces it. The only way found to eliminate this interference was to use freshly prepared permanganate solutions, not more than 3 to 4 hours old. The destruction of manganese dioxide by formaldehyde makes sulfate an interference in the pretreatment. as in the presence of sulfate the plutonium is also reduced. The use of hydrochloric acid aroids this interference, but nitrates must be absent or removed prior to the oxidation with permanganate. Uranium in large excess interferes with the nitric acid oxidation of the excess formaldehyde. The interference is probably caused by uranium complexing of the traces of nitrite needed to catalyze the nitric acid oxidation. This interference can be overcome by adding a large excess of nitric acid, except nhere the amount of plutonium is small. This case requires separation of the plutonium. 1422
ANALYTICAL CHEMISTRY
A few interferences were studied by making titrations with trace amounts (up t o 10% of the amount of plutonium present) and large amounts (10 to 100 times the amount of plutonium) of the substance present. Table I1 gives a summary of interferences. Substances listed under “ S o t tested, do not interfere” are not expected to interfere in the method. Substances listed in the “Not tested, interfere” column are expected to interfere, but may not do so. The precipitation interferences would not be expected to occur with trace amounts of the substance. Of the substances that interfere, only cerium and oxygen are not eliminated by the lathanum fluoride carrier precipitation. Reprecipitation of the plutonium should not be necessary. Cerium tracks with the plutonium in this separation, so another means of separation for the plutonium from cerium must be used. Separation from cerium was not studied in this work. Oxygen interferes in the titration step. However, only a t low levels is there a noticeable error if oxygen is not excluded during the titration. General. Although the pietreatment of the sample depends t o a large degree on balancing of the oxidizing and reducing potentials 3f the reactants. the method is not particularly sensitive t o variations in amounts of reagents, times of manipulations. and techniques. The only troublesome manipulative inteiference was caused by permanganate solutions t h a t were too old. Unless all electrodes in the titration cell are made of the same platinum alloy wire, a galvanic couple is formed which may seriously interfere in either the generation of the reagent or the indicator
Tested
Not tested
Silvera Mercury Sulfatea,< Ch1orine“tC Bromine Iodine ’ Rutheniume Palladium“ Osmiuma Uraniuma,c,d Cerium Oxygen4 Gold Tin
Bariumb Radium* Leadb Vanadium Selenium Tellurium Rhodium Iridium Platinum Zirconiume Hafniume Protactinium
system. Pure platinum wire is attacked slightly by the titration solution, especially if chloride is present. The platinum-iridium alloy wire gsve good service for more than 6 months before replacement was necessary.
ACKNOWLEDGMENT
The authors are indebted to M7. W. Mills for preparation of the plutonium standard solutions, to L. T. Blouin, Mary D. Money, and Mirium XI. Jones for making analyses on standard solutions, and to G. J. Alkire for his help in preparing this paper.
LITERATURE CITED
Carson, W. N., Jr., ANAL. CHEM. 22, 1565-8(1950). Zbid., 25, 226-30 (1953). Ibid., pp. 1733-5. Zbid., 26, 1673-4 (1954). Connick, R. E., Chap. 8, ‘[Actinide Elements.” Sational Nuclear Ene r w Series. Division IV. Part 14a.
(1951 j. (8) Koch, C. IT., Paper 17.4, “Transu-
ranium Elements,’’ National Nuclear Energy Series, Division IV, Part 14B, McGraw-Hill, Yew Tork, 1949. ( 9 ) Reilley, C. N., Cooke, JT. D., Furman, ?;. H., ANAL. CHEM. 23, 1223-6 (1951). (10) Seaborg, G. T., Katz, J . J., Manning, IT. N., “Transuranium Elements,” Sational Kuclear Energy Series, Division IV. Part 14B. McGrawHill, New York, 1949. RECEIVEDfor review December 8, 1956. Accepted -4pril 30, 1957.
