A n Extractio n, Co nt ro Iled- Potentia I Co uIometric Method Specific for Uranium(V1) G. 1. BOOMAN and W. 6. HOLBROOK Atomic Energy Division, Phillips Petroleum Co., Idaho Falls, Idaho
b A method, specific for uranium, is based on methyl isobutyl ketone separation of tetrapropylammonium uranyl trinitrate from acid-deficient aluminum nitrate solution, followed by direct coulometric reduction of uranium(V1) a t a mercury cathode. Perchloric acid volatilizes ruthenium and technetium, which are the only known interfering species extracted with uranium. The uranium is determined by controlledpotential reduction of the citrate complex after a prereduction step to eliminate easily reducible material. The complete procedure requires approximately 2.5 hours. Results on samples from solutions of freshly dissolved aluminum-uranium fuel elements having about 20% burnup show excellent precision and no bias as compared to the isotope-dilution thermal mass spectrometric method. Experimental d a t a indicate a trimolecular uranylcitrate-aluminum complex, with each citrate molecule forming two six-membered rings with the uranyl ion.
T
determination of uranium in a high enrichment nuclear fuel element having undergone appreciable fission presents a most complex problem. Some 33 fission products are present a t concentrations sufficient for each to produce a significant bias. These are shown in Figure 1; data are taken from a recent compilation (8) based on simultaneous fission. The numbers on the chart indicate per cent bias directly, if the fission products require the same number of electrons per mole as does uranium and 100% uranium-235 is assumed. For lower enrichments, the molar ratios Jyould be correspondingly lower. Calculations for reactor fuels irradiated for any given lcngth of time can be made ( I ) , but the percentages given in Figure 1 will be applicable t o most high-flus uranium-235 reactors. Fuel-alloying constituents under consideration also cover a broad spectrum of the periodic chart, as indicated in Figure 1. In high enrichment reactors, the alloying constituents exist in up to a 300-fold molar excess over the uranium for major components. Alloying additions in the 1% range would give nearly a threefold molar excess over the uranium. A preliminary HE
10
@
ANALYTICAL CHEMISTRY
separation of uranium from this mixture is indicated on the basis of complexity and radiation level. The presence of organic material interferes with many methods of uranium determination. Tributyl phosphate-kerosine, methyl isobutyl ketone, and their degradation products are found in aqueous solutions as well as the organic phase in solvent extraction uranium recovery plants (6). A simple pretreatment scheme probably could not be developed which would assure separation of uranium from diverse organic material without complete destruction by wet digestion or ignition. The destruction of organic material is a common feature of all applicable separation schemes. For high precision uranium determination over a wide concentration range, the choice of methods would be limited to either a coulometric or potentiometric titration with sufficient preseparation to reduce the concentrations of interfering material to a negligible value. The controlled-potential coulometric method (4) n'as chosen rather than one of the potentiometric ($1) or constant-current coulo-, metric (6, 12, 16) methods because of better tolerance to many elements of various valence states and simplicity of performing the analysis. A suitable preseparation could be obtained by the double cupferron procedure (22), but this is lengthy and requires several transfers and phase separations. lIercury cathode electrolysis (7) is a definite possibility, but requires approximately an hour, involves quantitative transfer difficulties, and adequate separation data are not available. Liquid-liquid extraction is easily adapted to a high precision method if a single contact batch extraction gives high uranium extraction with good discrimination against other oxidizable or reducible material. Countercurrent extraction with sufficient extraction, scrubbing, and stripping stages is a powerful separation method, as shown by separation factors greater than 108 being obtained in uranium recovery plants ( 5 ) . While this is excellent for a continuous process, the mechanics are too involved for precise analysis on an individual sample basis. A single-contact liquid-
liquid extraction of the uranium from a 2.5M aluminum nitrate-0.9M ammonium hydroxide-0.005M tetrapropylammonium hydroxide salting solution into methyl isobutyl ketone had been studied a t this laboratory (17) and the data indicated that the uranium extraction was sufficient for a precision method with greater than 99.8% recovery and diverse ions were adequately separated, except ruthenium and technetium. Conditions were established for the extraction, simultaneous destruction of organic residue, and removal of ruthenium and technetium in a 600" C. sodium bisulfate-perchloric acid fusion, and the controlled potential coulometric t,itration using only one quantitative sample transfer. No bias was detected in using the recommended procedure. EXPERIMENTAL DETAILS
Reagents and Solutions. The reagents for the extraction have been described (17). Preparation of the 2.8M aluminum nitrate solution is made somewhat simpler by adding water and concentrated ammonium hydroxide directly t o the aluminum nitrate nonahydrate crystals and mixing in well with a stirring rod before heating. The pH 4.6 citrate electrolytes were prepared either by mixing 100 ml. of 1.2M tripotassium citrate with 150 ml. of 1.2-44 citric acid or by mixing corresponding amounts of the salt and acid directly. Aluminum nitrate and aluminum sulfate solutions were made 0.5N in nitric and sulfuric acids, respectively. Apparatus. The controlled-potential coulometer used was basically as previously described (S),ivith the addition of an automatic background correction circuit and timers for controlling the deaeration, prereduction, reduction, and background correction times. K i t h these changes, only On and Re-Set pushbuttons are needed for instrument operation. Amplifier balance and the final integrated current value are checked manually. To enable the analysis to be carried out with only one quantitative transfer step required redesign of the electrolysis cell and electrode assembly. Borosilicate glass test tubes of 50-ml. capacity made a suitable vessel for the organic evaporation, fusion, and coulometric determination without transfers. In-
dentations in the test-tube walls just below the stirrer level were necessary. Tubes without indentations can be used, if a 16fold decrease in precision can be tolerated. Indented 50-ml. conical centrifuge tubes gave the same precision as indented 50-ml. test tubes, and showed a 30-fold decrease in precision when used without indentations. Apparently varying amounts of electrolyte are trapped between the glass walls and the mercury pool when the indentations are not present. With the better stirring in an indented tube the trapped solution is continually circulated back into the electrolysis region. Contact for the mercury pool was made by a platinum wire brought through the bottom of the Kel-F stirrer. The electrode assembly is shown in Figure 2. The two types of cells are shown in Figure 3, with the apparatus for evaporation of the organic extraction phase with hot air. A glass-tape heater wound around a 12-inch length of glass tubing heated the air sufficiently to remove 2 ml. of methyl isobutyl ketone in less than 5 minutes. The reference electrode is a standard Beckman Model G saturated calomel electrode with a fiber tip, lengthened to 5 inches, and having a potassium chloride crystal trap and reservoir connected to the side arm. Having clean electrolysis cells is very important. A negative bias of 0.25% was noticed after a few days when cells
-
the proper amount of freshly distilled mercury, the sample is ready to be placed in position under the electrode assembly. To make the actinic reduction effect constant, the coulometer timing cycle is started exactly 4 minutes after addition of the citrate electrolyte t o the sample. The timing cycle allotted 15 minutes to deaeration, 20 minutes to prereduction a t -0.25 volt, 14 minutes to uranium(V1) reduction a t -0.65 volt us. the saturated calomel electrode, and 1 minute to effect background correction. After the integrated voltage value was read, the instrument was reset for the next sample. For checking unextracted standards, the fusion was not used. The proper amount of mercury to be used in the coulometric reduction was determined by trial. Too high a mercury level produces erratic results caused by mercury splashing against the reference and anode electrodes, and too low a mercury level gives poor stirring efficiency. A range of about *0.3 ml. of mercury produced no noticeable difference in the optimum region. hleasuring the mercury added t o the cell with a graduated cylinder is recommended.
were rinsed with water. Cleaning the cells in concentrated nitric acid eliminated the bias. Procedure. For highly radioactive solutions, a 0.750-ml. sample aliquot was remotely pipetted into a 15 X 150 mm. test tube containing 6.0 ml. of 2.8M aluminum nitrate-l.0iV ammonium hydroxide-0.005M tetrapropylammonium hydroxide and 3.00 ml. of methyl isobutyl ketone. The test tube was stoppered with a polyethylene stopper and repeatedly inverted for 2 minutes. Approximately 2.5 ml. of the organic phase u.as then removed from the shielded facility. A 2.00-ml. aliquot of the organic phase was placed directly in the coulometric cell and evaporated to dryness, using the hot air stream. The fusion flux of 0.55 gram of sodium bisulfate was added to the cell, followed by 0.20 ml. of 72% perchloric acid. If the cells are placed in the furnace a t a 5- to 10-degree angle from the horizontal, no cover is needed, because spattering is confined to the bottom section of the tube. The muffle furnace temperature \vas raised from less than 200" C. up to 600" C. rapidly and then held a t 600" C. for 20 to 30 minutes. After dissolution of the melt in 2 ml. of water with gentle heating, cooling to near room temperature, addition of 0.50 ml. of 0.25M aluminum sulfate in 0.25M sulfuric acid, 4.0 ml. of 1.2M potassium citrate a t pH 4.6, and
RESULTS
Freedom from Bias. T o determine experimentally what trouble would
.
H
IDA
IPA
XA
AI
Si
P
UPPER NUMBERS REFER TO FEED SAMPLES. LOWER NUMBERS REFER TO RAFFINATE SAMPLES. FOR 25% BURNUP: DIVIDE BY THREE. FOR 10% BURNUP: D I V I D E BY TEN. METALS USED I N REACTOR F U U
Li
Na
pm
-
+
4
#
0 0 3 2 7 00010
K -
Fe
3.7
12
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1.4
0.011
0080
0.043
Ru
Rh
Pd
In
Sn
Sb
1400
11
80
43
Zr
Rb 3700
Nb
4600 30000
~
t
Mo
Tc
1500 21000
I
I
6000 12000
Co
I 2800
Ni
Go
Ge
As
2.7
1.0
*
sc
I
18
Os
cs
Ir
Pt
TI
Pb
Bi
800(
SEE Fr
-
Figure 1.
Per cent mole ratio of fission product to uranium 50% burnup, 1 00-day cooling VOL. 31, NO. 1, JANUARY 1959
Figure 3. Tubes used for fusion and electrolysis
Figure 2. assembly
Coulometer electrode
be encounterrd with fission product mixtures using thc citrate elcctrolyte a n d no separation, analysis was performed on a series of 13 solutions of 20% burnnp aluminum-uranium fuel elements. This rrsulted in an arcrage bias of +X.X7, and a coefficient of variation of 3.60j0. Evrn though the precision on five pairs of this group resulted in an accrptablr corfficient of variation of 0.24y0, thc large and variable bias could not be tolerated. High background currents wcre noticrd with thesc sampks. Following the extraction method given in the procedure, the bias was redwed to an insignificant value. The coinparison of six concentration values determined by the eontrollcd-potential coulometric method gave an avcragr difference of +0.46% as compared to the isotope dilution mass sprctmmrtric (1.9) method. The resultant T trst value of 0.35 means that if the same e.xperiment were repeated 11 times, the experimental difference of 0.467, or larger would be cxpected eight timrs, because of random sampling of a single distribution. Sixt,y-one degrees of freedom were associated with this test. Another experiment of higher discriminatory power was performed, using first cycle aqueous raflinatc from the tributyl phosphatekerosine process ( 5 ) . These samples contained less than 5 X mg. of uranium per ml. as determined by a pellet fluorophotometric method. Three samples were extracted, a uranium spike was added, then the samples were fused, redissolved, and coulometrically titrated.
12
ANALYTICAL CHEMISTRY
Recovery of 100.02, 99.95, and 99.92% resulted. No significant bias is present, comparing the experimental T of 1.64 with the T value at the 95% confidence level of 2.23. This was a more critical test than the first, because the coulometric coefficient of variation of 0.028% was the comparison standard in place of the mass spectrometric value of 1.9%, and organic material resulting from column ope.ration was present. Ten degrees of freedom were associated with this test. With the uranium sample mixed with citrate electrolyte, the laboratory lights caused approximately 0.1% reduction of a 750-7 uranium(V1) sample to uranium(1V) per minute. The reduction rate is apparently less with the cell positioned under the electrodes, because the over-all effect due to light was 1.9% when the standard procedure was used. Results from a previous coulometric study (4)and extraction data (17) on the methyl isobutyl ketone system indicated possible interference from only a few elements. The fission products, ruthenium, technetium, molybdennm, cerium, and telluriuin, were
0.0 , 350
, I I
375
,
I
,
, I ,
of special concern. A 110-7 addition of ruthenium, as a mixture of nitroso nitrate complexes, gave no bias when added either before or after the extraction step. The effectiveness of the perchloric acid-sodium bisulfate fusion in removing ruthenium was clearly shown by the disappearance of the 0.51-m e.v. ruthenium gamma peak when gamma spectrometer scans were compared before and after fusion of extraction residue from a radioactive sample. Another indication of lack of bias from ruthenium was the background current at -0.65 volt. Sufficient ruthenium to cause a 1% negative bias would show a final background current of about 20 Fa., twice the normal value of 10 pa. This performance was characteristic of 110-7 ruthenium-750-7 uranium samples extracted or not extracted and with or without addition of ammonium persulfate to a bisulfate fusion (no perchloric acid). Technetium would be expected to volatilize easier than ruthenium. A 1.5-mg. sample of molybdenum as ammonium molybdate extracted with 750 7 of uranium showed no bias. When 150 y of cerium(1V) as the nitrate were extracted with uranium, a 3.2% negative bias resulted, probably due to occlusion of uranium in the hydrolysed cerium(1V) precipitate in the acid-deficient, 2.5M aluminum solution. As fission product cerium in a solution resulting from dissolving reactor fuel elements in nitric acid would be present predominantly as cerium(III), a 150-7 sample of the latter was extracted and found to produce no bias. With 7.5 y of tellurium as tellurium dichloride added to the fusion mixture, a negative bias of 0.4% was observed. No bias could he detected when 15 y of tellurium and 750 y of uranium were taken through the extraction-fusion procedure. These individual studies along with the results on plant process samples
,
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400 425 4% 475 WAVELENGTH, MlLLlMiCRONS
5W
525
Figure 4. Effect of alurninum(ll1) on uraniurn(V1)-citrate absorption spectra 1. 2.
N o olurninum, pH 4.5 potosium citrate 0.1M ~lurninum.pH 4.5 p o t a s i m Citroto
nere thought to prove that the diverse ions in a nitric acid solution of a 20y0 burnup aluminum-uranium fuel element would produce no significant bias. A day-to-day variation in results obtained with standard uranium(V1) nitrate solutions was traced to a n aging effect in the mixed aluminum citrate electrolj te. This variation was a function of aluminum citrate batches, time of storage. and storage temperature. Adding the aluminum(II1) separately, as given in the procedure, eliminated this aging effect, although a batch effect n a s still present. The presence of carbonate in the potassium citrate gave no b b s . Precision. The precision of the method as reported previouslv (4) was confiinied: a coefficient of variation of 0.02S70 \yas obtained with eight pairs of i 5 0 y uranium(V1) samples detei mined directly n-ithout extraction or fusion. Six individual 750-7 aliquots of a standard uranium solution were processed through the extraction procedure. The coefficient of variation was 0.14iYc. Later two factors were found \$hirh coulrl account for the larger variance. Besides depending on pipet measurements of initial sample, total methyl isobutyl ketone, and the methyl isobutyl ketone aliquot transferred to the cell. these results were obtained before the best electrolyte and fusion conditions \wre established. Use of the separate aluminum(II1) and citrate solutions as stated in the procedure should give even better precision. By using the data from three pairs of dissoh-er solution samples having 20y0 burnup and a mean concentration of 1.66 mg. of uranium(V1) per ml., a coefficient of variation of 0.143% was obtained. K i t h the extraction procedure, greater than tenfold increase in precision is possible as compared to the isotope-dilution, thermal mass spectrometer method. The recovery data on the three raffinate samples, spiked with i 5 0 y of uranium(V1) in the cell before fusion, give a coefficient of variation of 0.051~c.
Figure
5.
Uranyl
trinitrate
ion
be carefully chosen. The simple onecontact extraction filled these requirements. A further advantage of the extraction is that radioactive samples having a gross gamma activity of as much as 1O1O disintegrations per minute could be handled in an open laboratory after the initial remote separation. With a 1/2-inch thickness of poly(methy1 methacrylate) plastic next to the sample, the contact activity reading was 6 milliroentgens per hour. After fusion treatment, the activity read in the same manner dropped to 2 milliroentgens per hour, because of ruthenium volatilization. K i t h this small amount of activity, the acceptable radiation dosage limits (19) would be exceeded only with long exposure a t close range. Equilibration of the 2.8M aluminum nitrate salting solution with methyl isobutyl ketone n a s difficult. To circumvent a n elaborate equilibration procedure, unknown samples are compared to standards, both are extracted ivith aliquots of the same reagent batch, and the same volumetric apparatus is used. The unknown sample concentration is calculated using the volts per unit concentration value obtained for the extracted standard corrected for the blank. Fusion. The fusion a t GOOo C. proved t o be a n effective method of removing organic material and the fission products, ruthenium and technetium. Small amounts of organic materials can interfere n ith the coulometric method by being reduced a t the same potential as uranium(V1). Separating from organic material by extraction would involve consideration of com-
DISCUSSION
Extraction. The high uranium recovery associated n-ith the extraction is necessary in a high precision method. as the various species n hich can change uraniuni(V1) extractability could lead to significant bias unless the increase or decrease in recovery is small. T o nullify the effect of sample solution constituents which increase recovery requires an initially high recovery without the presence of these constituents. To nullify the effect of other material which would decrease uranium extraction or extract itself, salting solution conditions must
0
Figure 6. complex
0
Uranium-aluminum citrate
*indicates aluminum bonding position Uranyl oxygen atoms a r e situated perpendicular above and below the plane of the p a p e r
pounds soluble in both phases, and require a multistep process. Destruction of all organic material seemed a direct approach. \Yet oxidations with perchloric acid and perchloric acid followed by sulfuric acid proved troublesome because of the long digestions and lengthy boiling necessary to remove the last traces of chlorine liberated during oxidation. Several modifications of the gravimetric ignition procedure at 1000" C. mere tried. Six 750-7 uranium samples were ignited in platinum crucibles at 1000° C. for 1 hour, and the residue was dissolved in 1 to 1 nitric acid, the excess nitric acid evaporated on the water bath, and the uranyl nitrate dissolved in 0.5 ml. of LM nitric acid. A coefficient of variation of 0.07% was obtained, with a n average recovery loss of 0.08%. Sixteen standards run directly without ignition gave a coefficient of variation of 0.055%. This loss of uranium and decrease in precision were believed due to uranium entering small cracks and crevices in the platinum crucible and not recovered during dissolution. However, changing to quartz crucibles resulted in approximately 3y0 loss of the uranium. Lanthanum and magnesium nitrates were investigated as possible carriers, to act as a n easily dissolvable surface on which the uranium oxide could be formed. TT'ith a 500 to 1 excess of lanthanum results were 0.2 to l.8y0 high, with magnesium 2% low, and a n increase in background current at -0.65 volt from the usual 10 to about 20 pa. was noted. This increase in background current occurred only when the salt was used in the ignition procedure; no interference was noted when the lanthanum or magnesium nitrates were added directly to the coulometer cell. 1-arious methods used to purify the spectrographic grade lanthanum nitrate produced little change in the interference. Ignition at 650" C. for 30 minutes also gave low results. Khen a small crystal of aluminum nitrate was added to the uranium sample in a quartz crucible, a 6% loss \vas noted. Because small amounts of aluminum(II1) might be present by mechanical entrainment after extraction from aluminum nitrate media, a n ignition additive was sought to enable quantitative uranium recovery. The sodium bisulfate fusion procedure was found to have the following advantages: an initial release of water, which washes down the inside of the ignition tube; a n oxidizing atmosphere, which helps destroy organic material without a 1000° C. ignition temperature; good solvent properties for metal oxides, allo~ving no adsorption or occlusion loss; formation of stable uranyl sulfate, VOL. 31, NO. 1 , JANUARY 1 9 5 9
13
-
easily water-soluble and not requiring an oxidizing redissolution solvent; high solubility of the resulting melt in water, permitting a small electrolysis volume with a consequent fast reduction time; and high reagent purity, giving no detectable increase in background current. Fusion with sodium bisulfate is a straightforward procedure requiring no special precautions, but variations can cause trouble. Incomplete evaporation of the organic liquid causes a charry mass to form which is not all decomposed during the ignition. Adding concentrated sulfuric acid to the solution of uranium in methyl isobutyl ketone and heating to fumes, followed by bisulfate fusion, are not recornmended, because a 6% high answer resulted. Adding a solution of sodium bisulfate produced about a 0.5% negative bias, probably due to spattering loss. Sodium pyrosulfate can be used with equally good results, but the bisulfate salt is more readily available. The sodium salt has greater solubility of the resulting sulfate than potassium bisulfate or pyrosulfate fusion. lT7ith lower melting and decomposition temperatures, sulfur trioxide is removed faster with the sodium salt. The loss of uranium on the quartz surface was 90 to 95% when the uranium and organic residue were ignited a t 650" C. before a bisulfate fusion. Uranium recovery was found to be a function of ignition temperature, when 0.56 gram of sodium bisulfate and no perchloric acid were used. A 1-hour ignition cycle, from room temperature to 600°, 550°, and 500" C., gave, respectively, 0.0, -0.3, and -1.8% bias. This could be explained by the excess sulfuric acid present a t lower temperatures, which could cause reduction of some uranium a t the prereduction potential. Nature of Citrate Complex. Spectrophotometric absorption curves are shown in Figure 4 for the uranium(V1) citrate complex and the aluminum(111)-uranium(V1)-citrate complex. The different absorption curve in the presence of aluminum shows that uranium(V1) forms a trimolecular complex with citrate and aluminum. The increased fine structure and sharpening of the peaks with aluminum present show a change toward the very sharp absorption bands exhibited by the uranyl trinitrate complex in organic solvents (16). Without the aluminum, the gen~ral shape of the curves is similar to those given by Feldman, H a d , and Keuman (10 11) for complexes of uranium with citric, tartaric, and malic acids. These authors suggested that the unique spectra exhibited by uranium(1-I) with hydroxy-, di-, and tricarboxylic acids r e r e due t o 14
ANALYTICAL CHEMISTRY
IO
a 0
f
06
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a
2
04
z
2
02
00
I
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I
-03 -0.3
I
-04 -0.4 OW 0 KE
Figure 7.
I
-05 -0.5
I
I
COTrL'T C OI5VT:& biI . -- VVC CLLTTSS
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-06 -0.6
-07 -0.7
I
I
-09 -0.9
-IO -1.0
Effect of aluminum on uranium-citrate reduction wave
tridentate chelation, as a different spectrum was observed with acids structurally unable to form tridentate bonds. Using electron density data and interatomic distances from x-ray crystal studies on rubidium uranyl trinitrate (14) results in the structure shown in Figure 5. A uranium(V1)citrate molecule of similar structure with bonding to citrate oxygen on one plane would appear probable. Aluminum(II1) could make this structure more stable by neutralizing the negative charge on the citrate molecule. This neutralization of excess negative charge could occur by the aluminum being common to a five- and a seven-membered ring, as indicated in Figure 6. The polarographic waves in Figure 7 clearly show the reduction of two species and indicate a slow equilibrium process. A similar type of wave was reported by Willard and Dean ($4) involving the change in reduction potential of di-o-hydroxyazo dyes when aluminuni(II1) was added. As suggested by the work of these authors, a wave of this type could be used as Lhe basis for an electrometric aluminum method. Unfortunately, this effect is not peculiar to aluminum. These polarographic m-aves are different from the dimer waves observed by Neuman, Havill, and Feldman 120) A log ( i l l d - i) us. E plot gives a slope of 64 mv. for the aluminum-uranium citrate wave and good linearity. hpparently the complex does not involve dimerization of uranium with the large excess of aluminum and citrate present. Without aluminum present, dimerizstion may occur to enable the uranium to fill seven or eight coordination positions without the complex requiring a high negative charge. A double wave of much less stability is obtained
B,+
+
AI+'+
I+++
E.+
+
+tt
Fo
+++
C3
zn+ 'Figure 8. Change in diffusion current (microamperes) between 0.5 and 0.7 volt due t o added ion Uranyl concentration, 0.3 1 mM Added ion concontration 0.1 5mM
in the iron(II1)-aluminum(1II)-citrate system. Because this is not the case with copper(II), a coordination number of at least 6 is indicated for the central ion in the complex. As shown in Figure 8 the interaction effect of metal ions with the uranium
citrate complex follows closely the expected order of metal-organic chelate bonding strengths (18). This figure explains the lorn tolerance of the citrate electrolyte coulometric method to cerium(IY), mercury(I), and the less than expected tolerance to iron(II1) As good coulometric reduction behavior was observed with uranyl ion in 2M sodium tartrate a t pH 4.3, using the same potentials as with the aluminum citrate media. this difficulty can be avoided. Ions appearing below the heavy horizontal line inscribed a t a caurrent difference of 1.5 pa. cause no difficulty fcr uranium determinations in the citrate electrolyte. An aniperometric titration of 2.5 X l O - 4 M uraniiim(T'1) nith aluniinum(111) is shown in Figure 9. Even though the curvature indicates nonstoichionietric addition of aluminum to the uranium citrate, a 1 to 1 ratio of alriminum to uranium is indicated for the complex. The horizontal line of 0.025 pa. corresponds to the current value with 100-fold excess of aluminum. The low rate of uranium(V) disproportionation noticed previously ( 4 ) a t aluminum t o uranium ratios of 60 to 1 or less could be caused by presence of a significantly smaller proportion of uranium-aluminum-citrate species. Increasing the aluminum concentration should allow larger uranium samples to be handled using the citrate electrolyte. Because aluminum(II1) ions are preferentially combining with uraniumcitrate molecules, an increase in stability is indicated for both aluminum and uranium in the trimolecular form compared to the separate complexes. The large amount of curvature would lessen the value of this titration for aluminum or uranium determinations but the possibility of more nearly optimum conI
3 4 MOLE RATIO
ditions for p H and citrate concentration has not been explored. A 4400 molar ratio of citrate to uranium(V1) was present in this titration. Considering the spectrophotometric, polarographic. and amperometric data, along with a study of molecular models, makes plausible a trimolecular uranylcitrate-aluminum complex, with each citrate molecule forming two six-membered rings with the uranyl ion. In analogy, to the uranyl trinitrate molecule, the existence of the three bonding citrate oxygen atoms in a plane perpendicular to the uranium-uranyl oxygen axis would form a consistent picture. This agrees with the conclusions of Comyns in a recent review of uranium coordination (8). Postulation of two seven-membered rings could be rejected on the basis of steric hinderance. The position of the aluminum ion must be close to a t least one of the oxygens bonded to the uranium in order to produce the 0.15-volt shift in half-wave potential. If the aluminum bonding was separated by a -CH,member from a uranium bonding group, the effect on the polarographic wave would not be expected t o be as large as is observed. Thus the aluminum is probably bonded to the remaining carboxylic acid group, and perhaps chelate bonding to the hydroxylic oxygen and a second carboxylic oxygen could explain why aluminum prefers the uranium-citrate molecules to uncomplexed citrate molecules. The resulting structure allowing tridentate bonding of both aluminum and uranium is shown in Figure 6. CONCLUSION
The method presented is very specific, sensitive, precise, and easy to use. Although the amount of the analyst's time
5 6 7 OF ALUMINUM TO URANIUM
8
9
10
Figure 9. Amperometric titration of 0.25mM uranium(V1)-citrate with aluminum(lll) 1.1 M potassium citrate, p H 4.6
required is small, methods allowing less elapsed time from sampling to reporting of results are in demand for process streams and desired for less complex mixtures than the example cited. An obviously simpler case would be a similar solution without organic material. A recent report (9) gives a study of a high enrichment, uranyl sulfate-fission product system. Even though a 20minute tu-benzoinoxime chloroform extraction is made to remove molybdenum(VI) and technetium(T'II), organic destruction is not required and the analysis could be performed within 1 hour. Ruthenium is also tolerated in a sulfate system using a prereduction step. Another possibility in the absence of reducible organic impurities would be extraction of the uranium followed by direct addition of the organic phase to the electrolyte without the destruction step. Finding a suitable solvent-electrolyte pair may be difficult. The direct determination, using a prereduction step with citrate or sulfuric acid electrolytes, is an excellent, fast method for many samples. More sophisticated approaches are necessary where a preliminary separation is awkward or undesired. An interesting possibility is a scheme developed a t Hanford (23) for plutonium analysis. The same type of controlled potential coulometer is used with alternate oxidation and reduction of the plutonium through the (1V)-(111) cycle. Because many interfering materials are reduced or oxidized irreversibly, this technique gives accurate results in many complex mixtures. For the case of the irreversible uranium(IT7)-(VI) cycle, an intermediate electrolysis step may be possible which would increase specificity. The use of platinum electrodes allows a wide choice of oxidation potentials and may not cause undue difficulty, as preliminary experiments indicated that a uranium(V1) polarographic wave can be obtained using platinum electrodes in strongly acid solution. Changing electrolyte pH or composition b e h e e n steps should be another useful tool. The repeat cycling procedure is helpful when high precision is desired, because many determinations can be easily done on one sample aliquot, and the precision increases proportionally to the square root of the number of determinations. Using separate platinum or mercury electrodes for oxidation and reduction would allow discrimination against metals which plate on or dissolve in the electrode material. Cleaning or chnnging the electrode material betyeen steps would also be effective. Fusion time could be shortened if perchloric acid T V ~ R not added and a sulfuric acid medium was used for the coulometric reduction. A platinum electrode for the prereduction would be a definite advantage in the sulfuric acid system, as oxidation of VOL. 31, NO. 1, JANUARY 1959
e
15
small amounts of mercury is a problem at the positive potentials required. Submicrogram amounts of uranium may be determined coulometrically with existing technique to &0.3-y (95% confidence limits). This is comparable to sensitive spectrophotometric methods and well in the fluorophotometric range. Improvements in the submicrogram range are sure to develop with cell geometry optimized for steady background currents, electrolytes chosen and purified for low absolute background current value, and selection of optimum electrolysis time. Electrode concentration methods, which have been very successful in submicrogram polarography, could well be applicable to uranium using techniques developed by radiochemists for counting plate preparation by electrolytic deposition of the hydroxide or oxalate precipitates. LITERATURE CITED
(1) Blomeke, J. O., Todd, AI. F., “Uranium-235 Fission Product Production as a Function of Thermal Neutral Flux, Irradiation Time, and Decay Time,” IT S. Atomic . Energy Conim., Rept. ORNL-2127, Part 1, Vols. 1 and 2 (1957). 12) Bolles. R. C.. Ballou. S . E.. “Calcu‘ lated h i v i t i i s and ’hbundances of Uranium-235 Fission Products,” U. S. Navy, Rept. USNRDL-456 (1956).
(3) Booman, G. L., ~ A L CHEY. . 29. 221 (1957). ’ (4) Booman, G. L., Holbrook, IF‘. B., Rein, J. E., Ibid., 29, 219 (1957). (5) Bruce, F. R., Fletcher, J. AI., Hyman, H. H., Isatz, J. J., “Progress in Nuclear Energy. 111. Process Chemistry,” 11IcGraw-Hill. Sew York. 1956. (6) Carson, IT. S . , ASAL. CHEM.2 5 , 466 (1953). (7) Casto, C. C., “Electrolytic Separation Methods,” in Rodden, C. J., “hnalytical Chemistry of the Manhattan Project,” pp. 511-22, McGraw-Hill, New York, 1950. (8) Comyns, A. E., “Co-ordination Chemistrv of Uranium.” United Kingdom AtGmic Energy aukhority, Rept. &RE C/R 2320 (1957). (9) Farrar, L. G., Thomason, P. F., Kelley, M. T., AXAL. CHEX 30, 1511 (i958). (10) Feldman, I., Havill, J., “Spectro-
photometric Studies of the UranylLactate, -Malate, and -Tartrate Systems in Acid Solution,” L-niversity of Rochester, Rept. UR-293 (1953). (11) Feldman, I., Havill, J., Xeuman, W., “Polvmerization of Uranyl-Citrate, -Mafat,e, -Tartrate, and -Lactate Complexes,” Ibid., Rept. UR-315 (1954). (12) Furman, ?;. H., Rricker, C. E., Dilts, R. V.. AXAL.CHEX.25, 482 (1953). 3) Goris, P., Duffy, If-. E., Tingey, F. H., Zbid., 29, 1590 (1957). 4) Hoard, J. L., Stroupe, J. E., “X-Ray Analysis of the Crystal Structure of Grand Comnounds.” in Uieke. G. H.. Duncan, A. h. F., “Spertroscopic Prop: erties of Uranium Compounds,” pp. 1335, McGraw-Hill, Sew York, 1949.
(15) Iiaplan, L., Hildebrand, R. -I., .%der, M.,J . Inorg. Ce. S u c l e a r C‘hevi. 2, 153 ( 1 956). (16) Lingane, J. J., Iwamoto, R . T., Anal. Chim. Acta 13, 465 (1955). (17) llaeck, W.J., Booman, G. L., Elliott, 31. C., Rein, J. E., ASSL. CHEW 30, 1902 (1958). 18) Martell, A . E., “Metal Chelate Compounds,” in Rollefson, C;. K., Powell, R. E., “Annual Reviews of Physical Chemistry,” pp. 339-60, .innun1 Reviews, Inc., Stanford. Calif., 1955. 19) Xatl. Bur. Standards Handhook 59, “Permissible Dose from Esternal Sources of Ionizing Radiation.’ 1954. 20) Seuman, IT. F., Havill, J. R,, Feldman, I., J . Ani. Chem. Soc. 73, 3593 (1951). (21) Rodden, C. J., Warf, J. C., “Uraniiiin
in
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A‘”uu“II,
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...,
“ A n n l ~llrlY.~ t i o ~ l
Chemistry of the Manhattan Project,” pp. 51-77, McGran -Hill, S e n - York, (22) kulfs, C. L., De, A. I- Controlled Potential Coulometry,” Second United Nations International Conference on Peaceful Cses of .Itomic Energy, A/CONF. 15/P/914, U.S. -4, June 1958 (24) Willard, H. H., Dean, J. -4.,AXAL. CHERI. 22, 1264 (1950). RECEIYED for review 1Iay 31, 1958. h c cepted Septem-ber 2, 1958. The Idaho Chemical Processing Plant IS operated by Phillips Petroleum Co. for the U. S. Atomic Energy Commission under Contract S o -4T(10-1)-205.
Constant-Potential Coulometric Reduction of Organic Nitro and Halogen Compounds VIVIAN
B.
EHLERS and JOHN
W. SEASE
Department of Chemistry, Wesleyan University, Middletown, Conn.
b Constant-potential coulometric reduction a t a mercury pool cathode has been tested with 18 organic nitro compounds and eight organic halogen compounds. Methanol containing lithium chloride or tetraalkylammonium halide was the solvent. In most cases errors were well under 1% for sample concentrations of 5 X 10-4M or greater. Several binary mixtures, whose components had reduction waves separated by at least 0.35 volt, were analyzed successfully by using consecutive electrolyses a t different cathode potentials.
A
LTHOUGH a moderate amount of
work has been done in applving the technique of constant-potential electrolysis to the coulometric determination of inorganic ions, relatively little has been done analytically with organic compounds (12, 1‘7) despite the widespread interest in the polarography 16
ANALYTICAL CHEMISTRY
of such substances (5,19). Experiments were undertaken to determine the feasibility of using constant-potential coulometry as a general method for the quantitative determination of organic compounds which can be reduced electrolytically, both individually and in mixtures. Nitro compounds were selected for the initial work because their polarographic properties have been extensively studied (5, 19) and their reduction could be expected to proceed smoothly. Furthermore, their half-wave potentials are such that it is possible to reduce them without going to large negative cathode potentials; this allows considerable freedom in the choice of solvent and electrolyte. The second portion of the work mas concerned with the reduction of halogen compounds, which require potentials that are considerably more negative. Although aqueous ethanol has been used extensively in organic polarography
and preliminary experiments showed that quantitative reduction could be achieved in 504, ethanol, it seemed desirable to go to completely nonaqueous solvents to obtain better solvent properties for compounds which are insoluble in water. Commercial 99% methanol was selected for routine use, bccause it dissolved sufficient lithium chloride or tetraalkylammonium halide to give reasonable conductivity, was cheap, and could be used without preliminary purification. APPARATUS A N D REAGENTS
Cells. The cells were of the conventional H-type, the larger patterned after t h a t described by Lingane, Swain, and Fields ( 1 0 ) . The cathode compartment n a s made from a widemouthed Erlenmeyer flask 7 5 mm. in diameter and 135 mm. high and the anode compartment from an rlectrolytic beaker 55 mm. in diamcter and 135 mm. high. The connection betneen
