Am pe rometric Tit rat io n of Uranium(IV) with Iron(III) R.
F. SYMPSON,’
R. P.
LARSEN,
R.
J. MEYER, and R. D. OLDHAM
Chemical Engineering Division, Argonne National Laboratory, 9700 South Cass Avenue, Argonne, 111. A method has been developed for the precise oxidimetric titration of uranium with iron(ll1) in which the end points are detected amperometrically with a rotating platinum electrode. After reducing the uranium to a mixture of the (Ill) and (IV) oxidation states with zinc, the titration i s carried out, first to the uranium(lll)-(lV) end point and then to the uranium(1V)-(VI) end point. For samples containing 200 and 20 mg. of uranium, precisions of 0.1 and 0.3% relative standard deviation are obtainable with no significant bias.
F
assay of uranium, the most common approach has been the oxidimetric titration of the uranium(1V-VI) couple. Although the study given this problem has been extensive and the number of papers published about it large, there is no generally agreed upon “best method.” Lead, amalgamated zinc, bismuth, chromium(II), and titanium(II1) are the most commonly used reductants; chromium(VI), cerium(IV), manganese (VII), and iron(II1) are the most commonly used titrants. Constant potential coulometric titration of uranium(V1) to (IV) eliminates the need for both chemical reductant and titrant. However, the method is sensitive to minor variations in technique and solution composition and the equipment is relatively expensive. Comprehensive reviews of the oxidimetric titration have been made by Rodden (22) and Booman and Rein (3)* The use of iron(II1) rather than chromium(VI), cerium(IV), or manganese(VI1) as the titrant for the uranium(1V-VI) couple eliminates iron as an interference in the analysis (12). This is of considerable importance since iron is probably the most common impurity in uranium metal, oxides, and salts, and is frequently used as an alloying constituent of the metal, Numerous methods of end point detection have been used for the uranium(1V)-iron(II1) titration. Auger (1) and Vortmann and Binder (16) used potassium thiocyanate as an indicator; Sagi and Rao (14) used Rhodamine 6G as a fluorescent indicator. Florence (6) carried out the titration spectroPresent address, Chemistry Department, Ohio University, Athens, Ohio. OR THE PRECISE
58
ANALYTICAL CHEMISTRY
photometrically by measuring the absorbance of uranium(1V). The most commonly used method of end point detection has been potentiometric (2, 4, 9, 17). hmperometric end point detection in the uranium(1V)-iron(II1) titration was investigated briefly by Kwiatkowski et al., (10) in the early days of the Manhattan Project but the results obtained, even on large samples, were high and erratic. After reduction with chromium(II), the chromium(I1)-(111) and uranium(1V)-(VI) end points were successively titrated. A dropping mercury indicator electrode was used. Florence (6) reports briefly on the use of the biamperometric, or “dead-stop,” method of end point detection for the iron(II1)-uranium(1V) titration but he found the end point to be sluggish. His results were also somewhat less accurate than those he obtained using spectrophotometric end point detection. Udal’tsova (15) used the deadstop method for the titration of milligram quantities of uranium(1V) with iron(II1). He reports errors of about 2%. Amperometric end point detection has also been used in other redox uranium titrations. Eskevich and Komarova (5) titrated uranium(1V) with vanadium(V), Gallai and Kalenchuk (7) titrated uranium(V1) with chromium(JI), and Zittel and Miller (18) titrated uranium(1V) with cerium (IV). In the method described here, uranium(V1) is reduced to a mixture of uranium(II1) and (IV) with a Jones reductor. The mixture is titrated in 0.3N sulfuric acid with standard iron (111) sulfate, first to the uranium(II1)(IV) end point and then to the uranium (1V)-(VI) end point. The rotating platinum indicator electrode (R.P.E.) is maintained a t -50 mv. us. the saturated calomel reference electrode. Prior to the first end point the current is anodic, between end points it is less than 1 pa. and nearly constant, and after the second end point it is cathodic. The titration is performed a t room temperature under an inert atmosphere. For samples containing 200 mg. of uranium, the relative standard deviation is 0.1% with no measurable bias. For samples containing 20 mg. of uranium the relative standard deviation is 0.3% with no measurable bias.
EXPERIMENTAL
Reagents. Iron(II1) ammonium sulfate solutions, approximately 0.2, 0.1, and 0.025, were prepared by dissolving the appropriate amount of FeNH4(S04)2.12 HtO in 0.1Jf H,SO,. These solutions were standarized by reducing an aliquot with a Jones reductor and titrating with standard Ce(SO& using ferrous 1,lO-phenanthroline as the indicator. Standard uranium(V1) solutions were prepared by dissolving accurately weighed standard samples of National Bureau of Standards U308 in nitric acid. Before weighing, the U,Os was heated in a muffle furnace for 1 hour a t 900’ C. Conversion of the nitric acid solutions to a sulfate medium was accomplished by fuming with sulfuric acid. Other solutions were prepared from reagent grade chemicals and were used without further purification. Apparatus. The potentials were applied and currents were measured with a Sargent “-4mpot.” -4rotating platinum microelectrode was used as the polarizable electrode and a saturated calomel electrode with a potassium chloride-agar salt bridge was the reference electrode. The rotating electrode was driven by a Sargent (5-76485) constant-speed, synchronous rotator a t a speed of 600 r. p. m. The titration vessel was a 250-ml., tall-form, Berzelius beaker fitted with a rubber stopper. Holes were drilled in the stopper to accommodate the salt bridge, the R.P.E., a nitrogen inlet tube, and the buret delivery tip. The buret delivery tip was kept below the surface of the solution. Sitrogen was bubbled through the solution continuously throughout the titration. Fiveand ten-ml. microburets were used to deliver the titrant. ;iliquots of the uranium standard solutions were taken with micropipets. N o aliquots smaller than 250 p l . were taken; the aliquots at the 200-mg. level were taken with a 2000 p1. pipet. Electrode Treatment. The R.P.E. requires a daily pretreatment to function properly. When the last titration of the day is completed the electrode is rinsed with concentrated nitric acid, rinsed with water, and stored overnight in deionized water. Before the first titration of the day the electrode is preconditioned by the following treatment. 17Tith the electrode immersed in a previously titrated solution containing U(T’I), Fe(II), and Fe(III), apply a potential of -50 mv. vs. S.C.E. for 30 minutes. During this treatment pass nitrogen
through the solutioin. Without this treatment the times required to establish steady currents are very protracted, particularly in the first titration. -lfter a month or :.o of use an electrode may not function properly following the above treatment. This malfunctioning of the electrode is evidenced by the occurrence of low results and nonlinearity of the current-volume plot beyond the second end point, cf. Procedure. To restore the electrode to a satisfactory operating condition the following treatment is used. Heat the electrode in concentrated nitric acid for one hour. -1fter riming with water, immerse the electrode in 0 . 2 5 H2S04 and apply a potential sufficient to evolve oxygen. Continue this treatment for one hour. Follow this with the daily pretreatment. Procedure. Pipet a sample containing 15 to 200 mg. of uranium into a 50-ml. beaker. JLdd 1 ml. of concentrated sulfuric ailid and heat on a sand bath to dense fumes. (Samples which contain large amounts of nitrate, particularly as salts, may have to be fumed twice.) Cool and take up with 30 nil. of water. (It may be necessary to reheat the sample to dissolve sulfate salts.) Pass the solution through a Jones reductor dirrctly into a Berzelius, tall-form, 250-nil. beaker. Wash the sample through the column with five 20-1111. portions of 0.2N sulfuric acid. Insert the electrodes, buret tip, and nitrogen inlet tube, and apply a potential of - 5 0 niv. us. S.C.E. K i t h nitrogen passing through the solution titrate with standard iron(II1) solution. (The concentration of titrant and the buret size are commensurate 11ith the amount of uranium titrated.) *!bout 8 current zs. titrant volume mea-uenients are sufficient i o give a good titration. Prior to the first end point readinqs should be taken a t about -10 and -2 pa.; between end points, fereral readings should be taken to determine the slope of the residual current line (the current is usually less than 1 pa. and the dope nearly zero); and after the second end point, readings should be taken a t about f 2 and +10 pa. d number of leadings should be taken after the second end point in the first titration of the day to determine if the R.P.E. iq functioning properly. If the current us. titrant volume plot is not linear, the electrode must be reconditioned by the monthly treatment described above. Prior to the first end point the current achieve5 a steady d u e within a few seconds; just prior to the second end point 1 to 2 minutes are required before achieving a steady value; after the second end point the response is again quite rapid> cf. Discusiion. For samples containing 15 mg. of uranium or lesq thr solution must be blanketed at all times with nitrogen, even a\ it emerges from the Jor~es reductor. For this size sample there is generally no initial anodic current and, therefore, no uraniuni(II1)-(IT’) end point.
DISCUSSION
Kinetics of the Uranium(1V)-Iron (111) Reaction. The uranium(1V)iron(II1) titration as practiced in many laboratories incorporates potentiometric end point detection, particularly when the amount of uranium being determined is small, 10 mg. or less. The titration must be carried out a t 85” to 90” C. since a t room temperature and in sulfuric acid solutions of convenient acidity, 1 to 2-V, it has been impossible to achieve steady potential readings in the vicinity of the end point. The drifting potentials encountered a t room temperature have been attributed to a slow chemical reaction, slow electrode response, or both. Sagi and Rao (14) in a recent publication concluded that the uranium(1V)iron(II1) reaction is “very slow a t concentrations of uraniuni(1V) and iron (111) encountered near the equivalence point in a volumetric titration.” Florence (6),however, in another recent paper states that in the potentiometric titration “high temperatures are necessary only for the rapid establishment of equilibrium potentials a t the indicator electrode, since in dilute sulfuric acid a t room temperature the oxidation of uranous ions by ferric ions is very rapid.” Observations which were made during the development of the amperometric titration lead us to concur with Sagi and Rao. In the amperometric titration the rotating platinum electrode responds exclusively to iron(II1) and the response time of the electrode is within the time required t o mix the contents of titration vessel after the addition of an increment of titrant. Any current in excess of the 0.1 to 0.2 pa. of residual current is, therefore, evidence for the presence of unreacted iron(II1). In the “Procedure” it was pointed out that just prior to the uranium(1V)-iron(II1) end point, 1 to 2 minutes must pass after each addition of titrant before the current returns to the residual value. If the titration (10 mg. of uranium) is carried out in 2.V sulfuric acid rather than the 0.3A’ acid called for in the “Procedure,” the current rises to about 5 pa. within a few seconds after each increment (0.01 meq.) of iron(II1) titrant is added, and returns to the residual value more and more slowly as the titration progresses. When the titration is 90% complete, the current decay time is in excess of an hour. Beyond the end point, a constant current is again established as soon as the titrant and solution are mixed. If 0.01 meq. of uranium(1V) is added to the titration vessel a t this point, the currrnt decreases very slowly, in a manner wholly comparable to that observed prior to the end point. From these observations it is concluded that
at room temperature the uranium(1V)iron(II1) reaction in 1 to 2 N sulfuric acid is so slow that a potentiometric titration is not feasible. The 10 or so volume us. potential readings which are required to establish an end point could not be made within a workday. Comparison with Other Uranium(1V)-Iron(II1) Titrations. By carrying out the titration amperometrically rather than potentiometrically or with an indicator-e.g., thiocyanatethe need for complete chemical reaction in the vicinity of the end point is obviated, and the titration can be carried out a t room temperature. These features are also common to Florence’s (6) spectrometric end point detection. However, a unique advantage of the amperometric titration is the fact that milligram amounts of uranium can be determined more precisely. Cellini and Lopez (4) demonstrated that the potentiometric titration could be carried out a t room temperature by reducing the acidity of the sulfuric acid medium from 1.V to 0.W. However, equilibrium potentials in the vicinity of the end point were not rapidly established and several minutes were required to obtain each point in the titration curve. Effect of Oxygen on Titration. The importance of excluding oxygen in this titration should be stressed. I n our first attempts to titrate uranium(1V) with iron(III), the uranium (111)formed in the Jones reductor was oxidized t o uranium(1V) by passing air through the solution for a few minutes. I n the subsequent titration, the end points were sharp but the precision was poor and only 98 to 99% of the uranium was recovered. Although it has been demonstrated (13) that prolonged aeration of uranium(1V) in 1.8s sulfuric acid a t 25’ C. causes no measurable oxidation to uranium(VI), the acidity a t which the amperometric titration is carried out, 0.3&Y, is apparently too low to prevent oxidation. This effect of acidity has been studied by Halpern and Smith (8). They measured the rate of oxidation of uranium(1V) by oxygen a t various acidities and showed that the rate increases markedly with decreasing acidity. INTERFERENCES
Since the problems of cationic interference in the uranium(1V)-iron(II1) titration is thoroughly discussed in previous papers (6, 14) and reviews ( 3 , lI), there was no need for a thorough interference check. However, vanadium and molybdenum which were of particular interest to the authors were checked out. Both interfere. Vanadium was added to the uranium sample as sodium vanadate. d sharp first end point was obtained, but no second VOL. 37, NO. 1, JANUARY 1965
59
Table I. Results in Amperometric Titration of Uranium(1V) with Iron(ll1)
U KO. of recovered, titraU taken, mg. c/o tions 172.8-209 2 16 97-67.88 5,231
99.98 99.99 99.10
17
39 6
Rel. std. dev., “;c 0.1 0.41 0.28
end point had been reached when iron was in 100% excess of the equivalence point. The first end point apparently corresponds to the point at which vanadium(I1) and uranium(II1) formed in the reductor have been converted to vanadiuni(II1) and uranium(1V). The vanadium(II1) however, is titrated along with the uranium(1V) causing high results for uranium. Molybdenum was added to the uranium solution in the forni of sodium molybdate solution. It interferes with the titration in a manner similar to vanadium. I n the case of molybdenum the reaction between iron(II1) and the loTTer oxidation states of molybdenum is very slow as evidenced by the very slow attainment of steady-state currents throughout the titration. The titration was discontinued when iron(II1) in 100% evccss of the equivalence point had been added without reaching the end point. The effects of excess iron and chromium on the titration were also investigated. Yo difficulty was en-
countered from iron at iron to uranium weight ratios as high as 8 to 1. Chromium at chromium-to-uranium weight ratios of one or less does not interfere providing the chromium is oxidized to chromium(V1) prior to the sulfuric acid fuming step and providing the fuming is discontinued a t the first appearance of heavy fumes. When dichromate samples were fumed for a prolonged period, crystals of anydrous Crz07 apparently formed and they could not be readily redissolved. With increasing concentrations of chromium the linearity of the anodic current prior to the first end point is affected and it is therefore necessary to accumulate not only more data, but data closer to the end point, if the accuracy of the titration is to be maintained. RESULTS
The results of a series of titrations of 5 to 209 mg. of uranium are given in
Table I. The low bias in the 5-mg. series is thought to be due to atmospheric oxygen despite the fact that stringent precautions were taken to exclude it, cf. Procedure. The titration can be carried out much more rapidly with little loss in precision by obtaining just two points prior to the first end point and two after the second end point and assuming that the residual current between the end points is zero. Using this approach on the data we gathered at the 200-ing. level, the relative standard deviation is 0.2% instead of 0.1% (Table I). ,Igain there is no apparent bias.
LITERATURE CITED
(1) Auger, V., Compt. Rend., 155, 647 (1912). (2) Belcher, R., Gibbons, D., West, T. S., ANAL. CHEM.26, 1025 (1954). (3) Booman, G. H., Rein, J. E., “Treatise on Anal. Chem.” Part 11, Vol. 9, p. 89, Interscience, New York, 1962. (4) Cellini, R. F., Lopez, J. A,, Anales Real SOC.Espan. Fis. Quina. (Madrid) Ser. B , 52, 163 (1956); Anal. Abstr. 4, 97 (1957). (5) Eskevich, V. F., Komarova, L. A., Zhur. Anal. Khim. 15, 84 (1960); C. A . 54, 13981i (1960). (6) Florence, T. ill., Anal. Chim. Acta 23, 282 (1960). (7) Gallai, Z. A., Kalenchuk, G. E., Zhur. Anal. Khim. 16, 63 (1961). (8) Halpern, J., Smith, J. G., Can. J . Chem. 34, 1419 (1956). (9) Issa, I. &I., Elsherif, I. &I., Anal. Chim. Acta 14, 466 (1956). (10) Kwiatkowski, S., Owens, J., Friess, S.,Grimes, W. R., Casto, C. C., U , S. At. Energy Comm. Rept. C-4.100.23, p. 10, Dec. 1, 1945. (1:) Rodden, C. J., “Analytical Chemistry of the Manhattan Project,” p. 71, ilIcGraw-Hill, Ken, York, 1950. (12) Ibid., p. 70. (13) Ibid., p. 65. (14) Sagi, S., Rao, G. G., Talanta 5, 154 (1960). (15) Udal’tsova, T‘I. I., Zhur. Anal. Khim. 17. 476 11962’1. (16) ’Vortmann,’G., Binder, F., Z. Anal. Chem. 67, 2169 (1926). (17) Weiss, (i., Blum, P., Bull. SOC. Chim. France 1947. 735. (18) CHEIII. Zittel, 36. H. 45 (1964) E., ihller, F. J., ANAL.
RECEIVEDfor review July 6, 1964. Accepted November 6, 1964. Work performed under the auspices of the U. S. Atomic Energy Commission. Argonne National Laboratory is operated by the University of Chicago under Contract W-3 1-109 eng-38.
Comparison of Theoretical Limit of Separating Speed in Gas and Liquid Chromatography J. CALVIN GlDDlNGS Departmenf o f Chemistry, University o f Utah, Salt lake Cify, Utah
b An investigation is made of the principal factors affecting separating speed in gas and liquid chromatography. Following an earlier proposal by Knox, it i s assumed that the maximum inlet pressure is one of the basic limitations on analysis speed. This assumption is fully justified b y this work. It i s further shown that the comparative speed of separation depends, to a large extent, on the relative viscosity and diffusivity of liquids and gases. For moderately difficult separations gas chromatography is superior because of its small C term. For extremely difficult separations liquid chromatography is superior because of its low critical inlet pressure, the 60
ANALYTICAL CHEMISTRY
latter mainly resulting from the slow diffusivity of liquid systems. The approach to further increases in speed is discussed for both methods.
I
(4) a comparison was made between the ultimate separating power of gas chromatography (GC) and liquid chromatography (LC). In this work we shall compare the theoretical limit of separating speed between the two methods. If we are given a pair of solutes with a certain degree of molecular similarity and we wish to separate them at a fixed level of resolution, it is desirable to know how rapidly this resolution can be achieved using one method as compared to the N A PREVIOUS PAPER
other. We shall investigate some of the factors involved in this comparison. It is usually valid to assume that the speed of any given separation can be increased by imposing a larger pressure drop (and thus flow rate) across the column. The argument for this was first presented by Knox (6) in connection with GC. The reasons for this assumption will be made clear later. In any case we shall assume that one of the limitations imposed on any chromatographic system is the inlet pressure, p , (this concept does not apply directly to paper and thin layer chromatography, although the capillary pressure has an analogous role). The magnitude of the maximum p , may be comparable in GC
