Controlled Potential Coulometric Determination of Uranium and Copper in Homogeneous Reactor Fuels L. G. FARRAR,
P. F. THOMASON,
and M. T. KELLEY
Analyfical Chemisfry Division, Oak Ridge Nafional Laborafory, Oak Ridge, Tenn. In order to apply the principle of coulometric analysis to homogeneous reactor fuels, the interferences of a variety of fission and corrosion products were investigated. Essentially complete elimination of interferences is accomplished b y combining a simple extraction with a prereduction. This procedure prevents the interference of molybdenum(VI), chromium(VI), copper(II), and antimony(lll), and allows the subsequent quantitative determination of uranium. The method can tolerate all the fission elements that would b e present after approximately 10% burnup of the fuel. The relative standard deviation i s 1%. The concentration o f copper(1l) in the fuel can also b e determined, but this determination is less precise and i s subject to more interferences than that for uranium. This technique i s readily adaptable to remote manipulation and significantly decreases the time involved in the analysis of a complex system.
T
of the controlledpotential coulometer ( 2 ) suggests the possibility of a coulometric determination of uranium(V1) in the presence of the majority of the great variety of fission product elements and other elements that are found in homogeneous reactor fuel. Furman, Bricker, and Dilts titrated uranium(V1) coulometrically by use of cerous sulfate as an intermediate (6). Carson titrated uraiiium(V1) by using electrolytically generated bromine (3). The controlledpotential coulometric determination of uranium has recently been described by Booman, Holbrook, and Rein ( 2 ) . The fuel of the homogeneous reactor is a n aqueous solution that contains approximately 10 grams of enriched uranium(V1) as uranyl sulfate and 1.0 gram of copper(I1) as copper sulfate per liter, as well as excess sulfuric acid (0.05.T). The copper functions as a catalyst for the recombination of the hydrogen and oxygen that are produced by radiolysis during critical operation of the reactor and prevents the reduction of excess hydrogen of the uranium(V-I) in the fuel. The exact composition of the reactor fuel varies considerably during critical operation; the elements used in the construction HE SELECTIVITY
of the reactor are added to the fuel by corrosion processes, and fission products are formed by burnup of the uranium fuel. In order t o apply the procedure developed by Booman and coworkers ( 2 ) to the analysis of homogeneous reactor fuels, it was necessary t o investigate the possibility of interferences, especially by those elements that have a total fission yield greater than 1% ( 5 ) . At 1070 burnup the interference of elements with a yield of less than 2% should be less than the expected imprecision of the method. Polarographic waves indicate that many of the fission products are reducible a t more positive potentials, and some a t more negative potentials, than is uranium(J-I). Those which reduce a t more positive potentials than uranium(V1) interfere with the determination of uranium(V1) and must be removed; those which reduce a t more negative potentials than uranium(1-I) would not be expected to interfere. Chromium(VI), molybdenum(VI), and antimony(II1) are known interferences in the coulometric determination of uranium. ?tlolybdenum(VI) and antiniony(II1) cannot be removed by selective reduction from the electrolyte used for the controlled-potential coulometric determination of uranium(VI)-Le., I N sulfuric acid-because their reducing potentials coincide with that of uranir!im(VI). A quick, simple method t o remove these serious interferences without sacrifice of accuracy was desired. The removal of molybdenum(V1) and chromium(V1) was accomplished by means of an extraction with a saturated solution of a-benzoinoxime in chloroform. In addition, a study of the combined effects of the fission products on the determination of uranium(V1) was made.
instrument was not included because high current capacity was not needed in this investigation. A 1-volt, Rubicon potentiometer was used to measure the potential on the integrating capacitor. CALIBRATION OF COULOMETER. The bias of the coulometer, measured as voltage, was determined by the following method. The anode and the reference electrode were shorted, and a constant-voltage source (a 1.345-volt mercury cell) was connected between the anode and cathode. The source of the controlled potential for the instrument was turned off, and the current from the mercury cell was observed for a definite time interval by measurement of the charge of the integrating capacitor. A switch was used to operate the timer and coulometer simultaneously. The number of coulombs that should be represented by the potential reading on the integrating capacitor can be calculated frcm the following relationship: Coulombs
=
volts - X time (seconds) (1) ohms
The voltage of the constant-voltage source is known (1.345 volts). The coulometer contains a built-in, 100-ohm resistor, and the duration of the current flow can be read directly in seconds from the timer. Ten coulomb values, representative of various time intervals, were obtained by the above method. These data were then used to prepare a plot of coulombs fed into the coulometer versus coulombs calculated from the potential reading on the integrating capacitor. The same resistance (100 ohms) and the same 10 values for the duration of current flow were used in each case. The difference between the measured and calculated coulomb values represents instrument bias. Extrapolation of the curve obtained by the plot indicated that the coulometer had a bias of +0.0003 volt. I n order to establish the relationshir, between the voltage recorded on thk integrating capacitor and the quantity of uranium(N) reduced in the electrolysis cell, 10 determinations of uranium(VI) in various concentrations were made, When the concentration of EXPERIMENTAL DETAILS uranium(V1) is known, the reading on Apparatus. CONTROLLED-POTEN-the integrating capacitor, which integrates the total current passed through TIAL COULOMETER.The controlledthe cell during reduction, can be related potential coulometer used in this work to the total quantity of uranium(V1) was constructed in this laboratory; i t reduced. The equation obtained from is essentially the same as the original the above method of calibration is: instrument described by Booman ( I ) . However, the circuitry that provides the high current capacity of the original Uranium, mg. = 48.92 ( E - 0.0003)( 2 ) VOL. 30, NO. 9, SEPTEMBER 1958
1511
where
E
=
48.92
=
+ O , 0003 =
potential reading on integrating capacitor, volts standard factor for determination coulometer bias, volts
In order to verify the empirical calibration with uranium standards, a n instrumental method of calibration was used. The potential recorded on the integrating capacitor is a direct measurement of the total current consumed during any reduction. If the coulometer is operated with a constant current, the coulomb equivalent of the potential accumulated on the integrating capacitor can be calculated from the product of current and time. Theoretically the number of equivalents of any ion reduced by the coulometer can be calculated directly without any other calibration. The anode and cathode were shorted, and a constant-voltage source (a 1.345volt mercury cell) was connected between the anode and the reference electrode. The source of the controlled potential was turned off, and the current from the mercury cell was observed for a period of 1000 seconds by measuring the charge of the integrating capacitor (0.331 volt). The milligrams of uranium represented by the potential reading on the integrating capacitor can be calculated from the following relationships: 96,500 coulombs = 1 gram-equiralent weight of uranium 1 coulomb = 1.233 mg. of uranium Also
I
= E / R = 1.345 volts/lOl.2 0.01329 ampere
=
where
R
= resistance of load resistor potentiometer = 101.2 ohms (measured with Leeds 8: Korthrup test set 5430-A)
and coulombs = time (seconds) X current (amperes) = 13.29 0.331 volt = 13.29 coulombs 1 volt = 40.27 coulombs = 49.65 mg. of uranium Analysis of solutions that contained known amounts of uranium showed that 1 volt of the integrating amplifier is equivalent to 48.92 mg. of uranium when 5-mg. samples are reduced in the coulometer. Therefore, at this low concentration of uranium a bias of unknown origin exists in the method, and the empirical value of 48.92 was found to give the best results (1.11% relative standard deviation), A possible explanation of the bias is the induced reduction of uranium(V1) by mercury during the electrolysis. This has been observed in both sulfate and chloride media during controlled-cathode reduction studies at Hanford ( 4 ) . ELECTROLYSIS CELL. The electrolysis cell, similar to that described by 15 1 2
ANALYTICAL CHEMISTRY
Booman ( I ) , consisted of a 5-cm. length of 3.6-cm. (outside diameter) borosilicate glass tubing. A three-way stopcock of borosilicate glass was sealed to one end of the tubing in order to form the bottom of the cell, and a short length of platinum wire was sealed into the tubing a t a position adjacent to the stopcock. One arm of the stopcock was connected to a mercury reservoir and the other to a waste container. The top of the cell was covered with a Teflon plug having openings for a stirrer, an anode, a silver-silver chloride reference electrode, and a nitrogen inlet; a n opening was also provided for the introduction of electrolyte and sample. The anode consisted of a length of platinum wire inserted into a small filter stick. In all cases, the stick was filled with 1N sulfuric acid. An 1800-r.p.m. synchronous motor was used for stirring. Reagents. All the readily available chemicals used weie reagent grade. The standard solution of uranium was prepared from uranyl nitrate and was checked by gravimetric analysis. With the exception of europium and technetium, nonradioactive fission products were obtained from reagent grade chemicals t h a t had been shown by spectrographic analysis to be free of uranium, Kitrogen n-as purified b y passage through two towers t h a t contained chromous sulfate solution and then through a toner t h a t contained distilled water. I n the homogeneous reactor fuel, uranium is present as uranyl sulfate; therefore, it is conl-enimt to use 1N sulfuric acid as an electrolyte. As far as possible, compounds which were used as the sources of the synthetic fission product elements were dissolved in 1N sulfuric acid. The fission product elements, palladium. rubidium. rhodium, and ruthenium, were dissolved in 2N hydrochloric acid, and iodine n-as dissolved in ethyl alcohol. Procedure. To the cell is added 5 ml. of 1 N sulFuric acid electrolyte. The electrolyte is dcaernted for 15 minutes and is then electrolyzed a t the highest potential that v-ill be used (in this case -0.30 volt) until the current reaches the lefel t h a t will give 99.9% reduction of 5 mg. of uranium(V1) (-0.05 ma.). The current is turned off, and the integrating capacitor is discharged a t t h e same time. The sample that contains uranium(V1) plus simulated corrosion and fission products is added to the cell and is extracted for 20 minutes with a satuiated solution of a-benzoinoxime in chloroform. The chloroform phase is discarded, and the aqueous phase, which remains in the cell, is n-aslied with chloroform to remove the organic phase. It is then purged with nitrogen for 5 minutes, is prereduced a t 0.0 volt to a final current of 0.05 ma., and then reduced a t -0.30 volt until background current (-0.05 ma.) is obtained. The time consumed in the prereduction step varies from 3 to 7 minutes, depending on the concentration of those elements reducible a t 0.0 volt and the efficiency of the chloroform wash. No
correction is necessary for the background current a t -0.30 volt because the analysis is complete when background current (0.05 ma.) is obtained; the charge on the integrating capacitor is read at this point. However, if the reduction is continued, the current will drop to zero in approximately 2 minutes. If the charge on the integrating capacitor is not read vvhen background current is obtained, a bias of about 0.1% would be introduced. RESULTS A N D DISCUSSION
Preliminary Experiments. The controlled-potential coulometer can be used for the quantitative determination of copper in the absence of elements that are reducible a t 0.0 volt. Copper, which is added to homogeneous reactor fuel as a recombining catalyst, is reduced a t 0.0 volt in 1N sulfuric acid. Therefore, it is a n interferenee in the determination of uranium(V1) and must be removed. The elimination of copper interference and the quantitative determination of copper(I1) can be accomplished simultaneously by reduction a t 0.0 volt in the absence of fission products. Several of the fission products are also reduced a t 0.0-volt potential and would cause a high copper(I1) result. The amount of copper(I1) present in the electrolyte can be calculated by means of the following equation: Copper, meq. where
E
=
0.4110 ( E - 0.0003) (3)
potential reading on integrating capacitor, volts 0 4110 = standard milliequivalent factor for any element (from Equation 2) +O 0003 = bias of coulometer, volts (obtained from Equation 2) The concentration of copper in the reactor fuel can be determined prior to critical operation of thc reactor, as interfering fission products will not be present. Critical operation of the reactor would not be expected to alter the concentration of copper in the fuel to any extent. B series of 10 determinations of 0.200 mg. of copper(I1) in the presence of 5.00 mg. of uranium(J71) in 0.5 to 2.0 ml. of dilute sulfuric acid gave a relative standard deviation of 1.2%. The supporting electrolyte was 5.0 ml. of 1s sulfuric acid; copper was reduced at 0.0 volt to a final current of 0.05 ma, When copper(I1) and uranium(V1) are both present, they can be determined consecutively in the same electrol>-te by the reduction of the copper(I1) to metallic copper a t 0.0 volt and the reduction of the uraniuni(V1) to uranium(1V) at -0.30 volt. Results so obtained in the absence of fission =
products gave recoveries of 99.0% of the copper and 99.5% of the uranium known t o be present. Attempts to reduce more than one test portion of a solution that contained both copper(I1) and uranium(V1) in the same electrolyte resulted in high values for copper and low values for uranium and indicated that the deposition of copper was changing the nature of the mercury cathode. This effect on the cathode was also observed by Booman and coworkers ( 2 ) . A fresh mercury cathode was used for each test portion when determinations of both copper(I1) and uranium(V1) were desired. The presence of as much as 50 mg. of chloride in the electrolyte does not cause interference with the determination of uranium(V1) ; however, more than 0.10 mg. of fluoride cannot be tolerated because fluoride reacts with the mercury cathode and the electrodes. As the number of determinations of uranium made in the same electrolyte solution is increased, the speed of reduction and the initial starting current both decrease. Separate polarograms of uraniuni(VI), copper(II), and molybdenuni(VI), each in 50% diammonium citrate electrolyte having a p H of 7.0, indicated that uranium(T’1) can be reduced a t -0.60 volt, that copper(I1) can be removed by prereduction a t -0.30 volt, and that molybdrnum(V1) is not reduced in this medium. Ten aliyuots of a standard solution of uranium(V1) that contained copper(I1) and molybdenum(V1) were analyzed a t the above potmtials. Reduction of the uraniuni(V1) was very s l o ~ .and calculation of uranium present (Equation 2) showed that the reduction n-as incomplrtc. The formation of a brown solution was noted; the brow1 color probably indicated the presencv of uraninrn(7.’), which did not disproportionate rapidly in the citrate medium, and therefore resulted in incomplete reduction of the uranium(V1) present Repeatrd attempts t o reduce the brom-n solution a t a potential of -0.iO volt failed. httempts to determine uranium(VI) in a 10% solution of diammonium citrate also resulted in thc formation of the brown solution. In each case a prereduction a t 0.0 volt was made. Use of the citrate electrolyte was discontinued because of the erratic reduction of uranium(V1). Development of Method. Preliminary experiments on the controlledpotential coulometric determination of uranium(VI), chromium(T-I), and antimony(II1) are the most serious interferences. The interference of copper(I1) can be eliminated by prereduction a t 0.0 volt; the corrosion products, nickel and iron(I1). do not electrolyze a t the reducing potential of uranium(V1) (-0.30 rolt) in 1 N
was discarded, and the aqueous phase was washed with a 10-ml. portion of chloroform to wash out the residual organic phase. The aqueous phase that contained the uranium(V1) was degassed and reduced a t -0.30 volt after a prereduction a t 0.0 volt. For a series of 13 samples extracted and electrolyzed in the above manner, the relative standard deviation was 1%; qualitative tests showed that the separation of molybdenum was complete. The extraction is not complete in less than 20 minutes. The results of these determinations are given in Table I. In the case of the chromium(V1) interference, a series of five determinations of uranium(V1) was made on solutions that also contained the additives listed in Table I. I n each case, chromium(V1) was extracted into the chloroform phase and its interference in the drtermination of uranium was thereby eliminated. The extraction time was increased t o 30 minutes in order to allow complete extraction of both chroniium(V1) and molybdenum(V1). After each extraction the contents of the cell were deaerated and prereduced a t 0.0 volt, and the uranium(V1) was reduced at -0.301 volt. Antimony(II1) is reducible at -0.32 volt ( 8 ) in IN sulfuric acid and interferes in the determination of uranium(V1). Although antiniony(II1) does not extract into the chloroform phase, it can be oxidizrd rvith nitric acid to antimony trtroxide, which is insoluble in dilute acids. Calculations from fuel burnup data show that the concrntration of antimony in the homogeneous reactor fuel will not a t any time be greater than 10 y per ml. Xo interferenee could be detected in five determinations of uraniuni(V1) made with this concentration of antiniony(II1) present; there-
sulfuric acid. Iron(II1) is reduced t o iron(I1) a t 0.0 volt. The possibility of extracting the molybdenum(VI), chromium(VI), and antimony was investigated. In order t o minimize the number of manipulations in the extraction step, it Fas desirable t o perform the extraction in the electrolysis cell provided the extraction did not interfere with the reducing capacity of the cell. Because the electrolysis cell can be drained from the bottom, an extractant heavier than 1K sulfuric acid-namely, chloroform saturated Tvith a-benzoinoximewas used. A t r s t a:tinple of &iiulated honiogeneous r e x t o r fuel was added to 5 ml. of 1 S sulfuric wid in the cc4l and was extracted for 20 minutes with 10 ml. of a snturated solution of a-benzoinoxinie in chloroform. The chloroform phase Table I. Results of Controlled-Potential Coulometric Determination of Uranium(VI) [Sample composition in milligrams: Cu(II), 0.300; ?vlo(VI), 0.500; Cr(VI), 0.500; Si(II), 1.00; U(VI), 5.00; in 0.5 to 2.0 ml. of dilute sulfuric acid. Other con-
ditions as described in text] Measured L-r ani uni Potential Found, ( E ) ,Volt 3lg.a 4.977 4 980 4.980 4.989 5.011 4.989 4.999 5 104
0.1017 0.1018 0,1018 0.1020 0.1025 0.1020 0.1022 0 1025 0 1020 0 1025 0.1028 0 1025 0 1028
a
4 980 5 01.1
5 034 5.014 5 034 Av. 5 009 Relative std. dev. 1 11‘; From Eqiintion 2,
Results of Controlled-Potential Coulometric Determination of Uranium(V1) in Presence of Fission Products [Uranium(YI) present, 5.00 mg. in 0.5 to 2.0 ml. of test sample. Other conditions as
Table II.
Present Tc .4g Eu Pd Ce(1T-/ I Rb
Te Rh
Kb
La CS a
described in text] Fission Product Element Amount, Reduction potential 7”
50 500 500 500 200 500 100 200 100 100 100 100
S o . of
in 112- H2S04,volt -0.5 0 -0.80 0
rktnp.
6 5
8 5 8
Ob
0 ;\lore negative than
-0 70 0
Av. Error in Detn. of z;(VI), 5;
a -1
0
Does not reduce More negative than - 1 0 Does not reduce
1 5 0
3 3
3
Amounts are greater than will be present after 105( burnup of See ( 7 ) .
0 18 0.15 0 20 0 10 0.16 0 10 0 18
0 0 0 0 0
12 10 10 10 12
f11t.1
VOL. 30, NO. 9 , SEPTEMBER 1958
1513
fore, no antimony removal step was required. However, the presence of 200 y of antimony(II1) in the solution electrolyzed caused an error of 4%. A polarogram of ruthenium(1V) in IN sulfuric acid indicates that the reduction potential of ruthenium(1V) is -0.85 volt; hence, ruthenium(1V) will not interfere in the determination of uranium(V1). A series of 10 determinations of uranium(V1) made with 200 y of ruthenium(1V) present in the solution electrolyzed showed no interference. A solution of ruthenium(VI1) and (VIII) was prepared from a solution of ruthenium(1V) by oxidation with periodic acid. Five dcteriiiinations of uranium(V1) in the presence of rutheniuni(VI1) and (VIII) showed no deviation from the known uranium value. The low reduction potential of ruthenium(VII1) indicated that it is reduced immediately t o ruthenium(1V) in the presence of mercury at zero applied potential. A list of additional fission product elements that were checked for interference in the controlled-potential coulcmetric determination of uranium(V1) is given in Table 11. After approximately 1070 burnup of the fuel, the
minor fission product elements-i.e., those that have less than 1% fission yield-were calculated to be chemically insignificant. Cerium(1V) in 1N sulfuric acid has an oxidation potential of +1.40 volts and is capable of oxidizing ursnium(1V) to uranium(V1) ; this oxidation mould give a high value for the reductometric titration of uranium(V1). However, cerium(1V) is readily reduced t o cerium(II1) at zero applied potential and causes no inter. ference with the determination of uranium(V1). Polarograms of neodymium and ytterbium, representative rare earths, indicate that the rare earths are reduced at very negative potentials (more negative than - 1.0 volt) in 1N sulfuric acid and do not interfere in this method. Barium sulfate and some rare earth sulfates are removed during cleanup operations on the homogeneous reactor fuel. As a final check, five determinations of uranium(V1) were made on test solutions that contained all the fission product elements listed in Table I1 and the corrosion products listed in Table I in amounts designated in those tables. The recovery of the uranium known to have been present was 99.6%;
the relative standard deviation of the results mas 1%. ACKNOWLEDGMENT
The authors wish to thank H. C. Jones, Analytical Instrumentation Group, who constructed the coulometer; C. F. Leitten, Solid State Division, who furnished the europium oxide; and R. H. Busey, Chemistry Division, who furnished a pure solution of potassium pertechnetate. LITERATURE CITED
(1) Booman, G. L., ANAL.CHEM.29, 213 (195T’I. , \ -
~
(2) Booman, G. L., Holbrook, W. B., Rein, J. E., Ibid., 29, 219 (1957). (3) Carson, W.N., Ibid., 25, 466 (1953). (4) Carson, W. N:, personal communication. (5) Friedlander, G., Kennedy, L. W., “Nuclear and Radiochemistry,” rev. ed., pp. 74-6, Wiley, New York, 1955. (6) Furman, N. H., Bricker, C. E., Dilts, R. V., ANAL.CHEM.25,.482 (1953). (7) Kolthoff, I. M., Lingane, L. L., “Polarography,” Vol. 11,2nd ed., p. 436, Interscience, Yew York, 1952. (8) Ibid., pp. 546, 548. RECEIVEDfor review January 16, 1958. ilccepted May 16, 1958.
A Direct Flame Photometric Determination of Boron in Organic Compounds BRUCE E. BUELL Research Department, Union Oil Co. of California, Brea, Calif.
p A rapid, flame spectrophotometric method has been developed for determining boron directly in organic compounds. The method eliminates extractions or distillations of boron usually required b y chemical procedures and is suitable for boron Accuracy contents as low as 0.1%. and precision are on the order of 1 to 2y0of the amount present.
atomizing organic solutions directly into the flame. Numerous workers have applied flame photometry to various determinations directly in organic solutions (2-6, 7-12). The technique presented for boron determination is similar to that developed by Conrad and Johnson ( 2 ) for barium and calcium in lubricating oils. APPARATUS A N D REAGENTS
instrumentation has been applied to the flame photometric determination of boron by Bricker, Dippel, and Furman ( I ) and by Dean and Thompson (5). The latter authors reviewed determinations of boron by flame photometry and used acetone and alcohol mixed with water as solvents to enhance boron emission in inorganic compounds. The purpose of this investigation was to extend this application to the analysis of organic boron compounds and their concentrates in lubricating oils by ODERN
1514
ANALYTICAL CHEMISTRY
A Beckman Model D U spectrophotometer was used with the following Beckman accessories: a 9200 flame attachment, a 4300 photomultiplier, a 92300 spectral energy recording adapter, and a 4020 oxy-hydrogen atomizerburner. A Leeds &. Northrup Speedomax Type G recorder with a 10-mv. range and a chart drive of 2 inches per minute was used to record all measurements. Solvent mixture A was prepared by mixing equal volumes of 2-propanol and cleaners, naphtha. Solvent mixture B was prepared by
diluting 50 grams of 300 neutral oil to 500 ml. with solvent A. Boron standard solution, 400 mg. per liter. This solution was prepared by gently warming 0.571 gram of analytical reagent grade boric acid in 100 ml. of technical grade 2-propanol under a reflux condenser. When cool, the solution \vas transferred to a 250-ml. volumetric flask and 100 nil. of technical grade cleaners’ naphtha and 25 grams of neutral oil were added, The mixture was diluted to the mark with solvent A. All other calibration standards were prepared by quantitatively diluting this standard solution with solvent B. Sodium standard solution, 1000 mg. per liter. This solution was prepared by diluting 8.62 grams of Petronate H sodium sulfonate (2.9% sodium, L. Sonneborn Sons, Inc., New York, N. Y.) and 16.4 grams of neutral oil to 250 ml. with solvent A. All other sodium standards were prepared by diluting this concentrate with solvent B. INSTRUMENT SETTINGS A N D ADJUSTMENTS
The instrument settings used for
