!" " " " " " *** * 10 TYM FRA !1 Sir in '. * Liel 1 . .. .. . . . . . . . . ..2 - -- - 2 , UNCLASSIFIED JORNL . * 636 . . . 1 .::. bar . .2 12. x 7 . 21 . 49 . * W SP ANTIS . ".,. T! ':' . A '. .'. . . I " 12': . . . .. .. " ORNL-poloze CONF-805-ě тур. ? NOV 1 3 1964 . . - For Presentation at the Meeting of European-American Nuclear Data Committee January 18-22, 1965 Brussels, Belgium - : : - - . METHODS USED AT ORNL FOR THE PRECISE DETERMINATION OF URANIUM Paul F. Thomason t . .. ar . Analytical Chemistry Division Oak Ridge National Laboratory Oak Ridge, Tennessee MASTER •; s . •4 - the poor aw; unde Controlled-Potential Coulometric Titration of Uranium Son H go hoket Gede -LEGAL NOTICE my #yuma do naman Uranium has been determined routinely by the controlled-potential coulometric titration method for over five years at ORNL. This method is rapid, precise, accurate, and especially suited for the analysis of www ws ng themes and wat highly radioactive samples. While this titration is not a strictly absolute method, because it is not practical to titrate the last traces of uranium (requires infinite time), it nevertheless can be calibrated electrically in terms of the Faraday (96,487 + 1.2 coulombs). The electrical factor and chemical standardizations made with NBS 950-A U308 should check within 0.1% under the titration conditions specified. The method is based on the electrolytical reduction at -0.325 volts vs. S.C.E. of ute to ut* through the reduction step of ute to uts and disproportionation of u*s to ut* and ute in a 0.5 M H2SO4 electrolyte. If any ut* is present in the sample it must be oxidized chemically before titrating. The first slide shows a picture of the Q-2005 controlled-potential coulometric titrator which was developed by Kelley et al) and is in - - .. : routine use at ORNL. This instrument is capable of performing *Research sponsored by the U. S. Atomic Energy Commission under contract with the Union Carbide Corporation. oxidation or reduction coulometric titrations at controlled potential. Booman's instrumenti) uses mercury batteries for cell power supply, while the ORNL Q-2005 is alternating current line-operated with a full wave silicon diode voltage doubler circuit for cell power supply capable of delivering as much as 500 me. The digital voltmeter on top of the instrument continuously shows the readout voltage which is proportional to the amount of uranium titrated. The electrolysis cell assembly is at the left of the instrument and will be discussed in detail later. The panel of the instrument has the necessary controls to change the mode of titration from reduction to oxidation for reversal of polarity for change of a rrent ranges and cut-off switches. The O to 1.0 D.C. milliammeter contains contact points to actuate a relay to automatically terminate the titration at a preselected current cut-off. Several shunts are provided for the meter in order to measure various current ranges. The next slide shows a block diagram of the ORNL Q-2005 controlled- potential coulometric titrator. This instrument uses one modified Philbrick USA3-3M operational amplifier as a potentiostat which rigidly controls the voltage to within a few millivolts between the reference S.C.E. and the working electrode. The other USA3-3M opera- tional amplifier is used as an integrator with & resistor network available for measuring various fractions of the voltage accumulated upon the special low-leakage 10 ufa capacitor. The accumulated volt- age is proportional to Q. By proper divider network it is possible . to have the readout voltage expressed as the actual number of coulombs of electricity used in the titration, or similarly, the voltage may be expressed as milligrams of uranium titrated. This voltage is measurmd to 0.1 millivolt by means of a sensitive voltmeter (Rubicon) or a digital voltmeter. This slide shows a platinum gauze working electrode in the reduction mode of operation. A resistor should be shown in the anode circuit, as it hus been found necessary to limit the current to about 60 ma. If the initial current 18 allowed to go as high as 100-200 ma incorrect results are obtained, perhaps due to the electrolysis of some hydrogen. The next slide shows a diagram of the electrolysis cell. The most important consideration in the cell operation 18 the stirring. You will note the corrugated bottom of the disc stirrer. The object or this stirrer 18 to obtain vigorous stirring of the mercury pool at the interface of the aqueous solution. The stirring must, however, not break the mercury pool up into droplets of mercury which will disturb the current density. This cell is used to titrate 5 to 30 milligrams of uranium in 10 ml of 0.5 M H2SO4 with 7 ml of mercury as the cathode pool. The vycor porous glass tubes used as isolation membranes have an electrical resistance of approximately 100 ohms when filled with 0.5 M H2SO4. The aqueous solution is deaerated for at least 5 minutes with argon before the titration is started. Small amounts of iron are usually present in our samples, therefore a pre- titration in the reduction mode of operation at +0.085 volt vs. S.C.E. 18 carried out until a cut-off current of 50 ma is reached. The auto- matic cut-off feature 18 used for the pretitration. The integrator capacitor is then discharged by means of the switch. The instrument is allowed to stay in the reset position for at least two minutes while the reduction potential is adjusted to -0.325 volt vs. S.C.E. This permits the integrator capacitor to be fully discharged. The titration of ut° to u** 18 then initiated and allowed to continue until the cut-off current again reaches 50 wa when the automatic - - - device terminates the titration. The 50 wa cut-off current is on erbitrary termination point as it should represent 99.9+% completion of the reduction of ute to u**, assuming the initial current would be 100 ma or greater (although it was limited to ~60 ma by the limiting resistor). The current also can be limited by carefully controlling the potential during the initial part of the titration by gradually increasing the voltage from -0.1 to -0.325 volt vs. S.C.E. so the current never exceeds 60 ma. The titration of 5-8 mg of uranium should be completed in 12 to 15 minutes. Under these conditions no - - - - - - correction for background current 18 made. The cell must be leak tight except at the stirring rod opening in the teflon cap to prevent leakage of air into the cell, which will result in long titration times and incorrect results. The argon gas used for deaeration is bubbled through the aqueous solution throughout the titration. The next slide shows a picture of the new Q-2564 high-sensitivity coulometric titrator. This titrator is similar to the Q-2005 instru- ment except it is primarily designed to titrate 100 ueq or less of materials. The instrument uses three unmodified USA3-3M Philbrick operational amplifiers and is capable of doing constant current coulo- metry as well as controlled-potential coulometry. The current is limited to 8 ma by a resistor (any higher current will overload the operational amplifier) although when titrating ~50 ug of uranium the initial current is only about 500 ma. When titrating as little as . . 2.5 ug of uranium the initial current is about 40 to 50 wa, The titration of these quantities is carried out in a scaled-down cell similar to the one previously shown. The electrolyte must be deaerated with argon before addition of the mercury pool as any oxidation of Hg by the dissolvod oxygen causes high resw.ts. The micro cell uses 2 ml of 0.5 M H280, and 2 ml of mercury as the cathode. The titration 18 terminated at a cut-off current of 0.5 ma. It is necessary to correct for blanks ootained by electrolyzing the 0.5 M H280, electrolyte to a cut-off curront of 0.5 ua. This correction 18 for trace Impurities in the electrolyte as well as for the charging current and 1s equi- valent to about 0.25 mg of uranium. The background current which 18 largely faraduic current is also corrected and is equivalent to 0.36 ug of uranium (0.5 ma x 600 sec = 300 u coulombs = 0.36 ug uto, when titrating 50 mg of uranium for 10 minutes. When titrating 2.5 mg of uraniun it is possible to complete the titration in 5 minutes. Thus the faradaic current correction is equivalent to only 0.18 ug in this case. It was also necessary to use copper tubing to conduct the argon deaeration gas into the cell as tygon tubing allowed enough oxygen to diffuse through to interfere with the titration. Results of the micro titration of uranium are shown in Table I. Table I. Controlled-Potential Coulometric Titration of Microgram Quantities of Uranium Pretitration 0.0 yolt vs. S.C.E. Reduction of u* to u**. 0.025 volt vs. S.C.E. Blank correction - 0.25 $ 0.0l ug of uranium Number of Determinations Uranium, ug Error % Taken Found Rel. Std. Dev. % 1:12: 48.32 2.48 48.32 2.47 < 0.1 < 0.5 0.2 0.8 . . .. 10 The next slide shows control charts for the routine determination of 5-milligram quantities of uraniu with the Q-2005 instrunent. You will note the method shows a relative standard deviation of 0.1% with 0.2% precision at the 95% confidence level. This precision has proved to be adequate for most of our work. In special cases we have obtained 0.05% relative standard deviation by titrating 30-50 mg of uranium. It is necessary to control the current so that it does not exceed 60 ma by careful adjustment of the potential during the initial part of the titration. Schraid et al 5,49 obtained a relative standard devia- tion of 0.22% when titrating 5 mg of uranium with an instrument built in the Eurochemic Instrumentation Section from ORNL drawings of the Q-2005 controlled-potential coulometric &ftrator. Automatic Potentiometric Titration of Uranium Another precise and accurate method for the determination of uranium routinely used at ORNL 18 the automatic potentiometric titra- tion of about 30 mg of uranium with ferric sulfate as titrant. The next slide shows a picture of the ORNL Automatic Servo Titrator Q-17281" which is capable of drawing accurate titration curves depicting volume of titrant (length of chart) versus potential of an electrode system (width of chart). The drive for the chart paper is electrically locked to the drive of the syringe delivery buret by means tof synchro motors, thus the length of the chart is proportional to volume of titrant. The instrument contains an anticipation gateing circuit which automatically slows the delivery of the titrant during potential change, thus preventing the addition of too much titrant at the equivalence point. If the potential change is great enough, the delivery of titrant will stop entirely until equilibrium 18 again established. You will note in the slide the tall form 180-ml titration beaker 18 heated to 90~95° C in order for the reaction 2 Fe*" + u** utº + 2 Fetc to proceed rapidly. Ferric sulfate 18 used as the titrant because small amounts of iron in the samples do not interfere. The titration 18 carried out by pipetting 25 to 35 mg of uranium into the titration beaker containing 60 ml of 0.5 M H2SO4. The solu- tion 18 heated to 90-95° C and 10% w/v chromous sulfate solution 18 added drop by drop until the potential of the gold wire indicating electrode is about -0.295 vs. the high temperature S.C.E. reference electrode. The chromous sulfate solution 18 made by passing a 10% w/v solution of potassium chrome alum (violet form) through a Jones reductor. The crte solution 18 stored in a glass syringe fitted with a long delivery glass needle. The Cr* solution 18 stable for weeks in the syringe and only a few drops at the end of the needle must be expelled to waste before addition to the VO2S04 solution. The next slide shows a typical titration curve as recorded on the ORNL Q-1728 potentiometric titrator. The excess Cr+2 reduces the uranium to u** and the first break in potential on addition of the ferric sulfate solution represents the oxidation of excess Cr* and any ut" that may have been formed. The titration is continued and the next break in potential indicates the oxidation of u** to uto, The titer between the two potential breaks is equivalent to the amount of uranium titrated. The number of divisions of chart travel is carefully measured and related to the chart divisions for titration of standard uranium solutions made from NBS 950-A U30g. Specially purified uranium metal from the Metals and Ceramics Division at ORNL is also used as -8- a primary standard. The precision of the method 18 excellent as the titration curves can be reproduced to the width of the pen line which 18 quite small when compared to a titration of approximately 13 inches of chart between the two breaks for 30 mg of uranium. The potential breaks are of the order of 200 millivolts and are quite sharp. The indicating gold electrode must be kept clean of surface contamination by periodically cleaning in K2Cr207-H2SO4 cleaning solution, followed by distilled water rinsing. Sometimes it is necessary to flame the gold wire to remove any organic material that passivates the surface causing drawn-out, 111-defined potentiometric breaks. The last slide shows tine control charts that have been obtained by submitting standard solutions interspersed with actual samples. The relative standard deviation 18 0.1% with t 0.2% at the 95% confidence limit. This precision is readily obtained routinely by technicians as shown on the slide. If weighed samples are used and the titration is performed carefully by an analytical chemist greater precision may he attained. ACKNOWLEDGEMENTS The author would like to thank M. T. Kelley and J. C. White for their many suggestions when developing these methods. Credit is due the following persons who are responsible for developing most of these methods: W. D. Shults, J. M. Dale, H. E. Zittel, Louise Dunlap, B. B. Hobbs, F. J. Miller and W. R. Mountcastle, Jr. M. T. Kelley, D. J. Fisher, H. C. Jones, E. B. Wagner and Robert Stelzner were responsible for development of these electroanalytical instruments. REFERENCES 1. Kelley, M. T., Jones, H. C., Fisher, D. J., J. Anal. Chem. 31, - 488, 956 (1959). 2. Booman, Glenn L., J. Anal. Chem. 29, 213 (1957). 3. Schmid, E., Humblet, L., and Eschrich, H., Application of Controlled-Potential Coulometry to the Determination of Uranium in Sulfuric Acid Solutions, NP-14007, ETR-162, January 1964. 4. Schmid E., and Eschrich, H., Determination of Uranium( VI) in the Presence of Molybdenum by Controlled-Potential Coulometric Reduction, NP-14008, ETR-164, February 1964 5. Kelley, M. T., Fisher, D. J., and Wagner, E. B., J. Anal. Chem..! 32, 61 (1960). -10. LIST OF SLIDES Slide No. ORNL No. Title Controlled-Potential Coulometric Titrator ORNL Q-2005 P hoto 46178 Electronic Controlled-Potential Coulometric Titrator Block Diagram: Switched for Reduction LR-DWG. 41212 LR-DWG. 68010R Coulometric Titration Cell High-Sensitivity Controlled-Potential Coulo- metric Titrator Photo 60246 LR-DWG. 69686 Control Charts, ORNL Q-2005 Controlled-Potential Coulometric Titrator, Determination of Uranium U(VI) + 2 + U(IV) E = 0.345 vs. E (SCE) ORNL Automatic Servo Titrator ORNL Q-1728 Photo 17304 LR-DWG. 17321 Typical Titration of ~ 32 mg of Uranlim by 0.1 N Fe(III) by Means of the ORNL Model Q-1728 Automatic Recording Velocity-Servo Potentio- metric Titrator LR-DWG. 29771 Control Charts, Determination of Uranium with the Q-1728 Automatic Titrator Using Ferric Sulfate as Titrant ' T ... . ... S h ih t on An zile LATIT TODASIE. . Y # 1 .73 "LAD . - 24- VAT 1997 *: 14 * IR KRUL. - Ki. * . SA . * * ** Yil in de :** 33 . . . . vit :: HY? . US NXX m . * ** N . N . 'i 12 S., . . !! ! , , In .... W:nir-nexum mensen i "TI . ii 25 *.* " . : ! . ;wi ' ' ! . 2 . VU 14 . ' . .' A . .:: ! ; NI . I PILI. ''. . 1 Y . ; MA ..... - "T . . 1 . ... . . ► UJI...... ini. OL . I. . it'.:.. ; t - - - 4 - . | : It 's • . . 2.4: ' ' iP. ' . . . - . E 1 . M . . . . A . WAT S. .. . " 2-' . .., . . , . . . P 4 y 21 , . v . 1 + * ' WY . - r - - -::- hi.• -,. . - 1 . VE M : A . P . . .. sve pravice in many way SLIDE 1 LT. . M.LVO 1. . . . . . C T RW. . N NOIN -MTV. I UNCLASSIFIED ORNL - LR-OWG, 41212 P-300v Ev> 'coo ISOLATED ELECTRODE R.E. "POLARITY CELL POWER SUPPLY "CELL C.E. ve R.E." CURRENT" + "CONTROL CELL POTENTIAL") CONTROLLED CATHODE (P+ GAUZE) ZENER DIODE CONTROL POTENTIAL SOURCE -USA-3-M3 TRANSISTOR CURRENT AMPLIFIER CHOPPER. STABILIZED DIFFERENCE AMP. CONTROL AMPLIFIER SENSITIVITY X1 X 50 Loka 400 K "opt USA-3 - M3 ions son X40 LIMIT LIGHT - -- -- INTEGRAL READOUT DEVICE -- - 1-300 v - --- eto +300v "INTEGRATOR BIAS" - - o to i v -- CURRENT INTEGRATOR ANALYTICAL INSTRUMENTATION GROUP QAK RIDGE NATIONAL LABORATORY ELECTRONIC CONTROLLED-POTENTIAL COULOMETRIC TITRATOR BLOCK DIAGRAM: SWITCHED FOR REDUCTION SLIDE 2 . . 1 KT . UNCLASSIFIED ORNL.LR-DWG. 680108 GAS INLET S.C.E. P+ WIRE- ***** VINTIP ANTOVA turi - STIRRER GVAAVAT - VYCOR TUBES CORRUGATED DISC - fo 40/12 JOINT K15 mm HO CONTACT 80-ml WEIGHING BOTTLE 80 mm HG POOL 40mmy PA GAUZE - Coulometric Titration Vessel SLIDE 3 UNCLASSIFIED PHOTO 60246 . . I .. ' 1 . NYT T. . . . - . . . 2 ** * ( • • • • • • : TA ' Wein WLED * ..: ::. . : S ., . o . 10 I. . . 3 . . . . Ti ** " J. , 7 - A LL . .. - - IM * VAN " 2 . .. MR. A * 7 LES IS 9 .3 '! ! " 11 # :12 in .. . 14 L ' !" . I TIL .. . : 1 'N . .. . . L I. i . 11 . .. . . 22.1 iv .. . H . .., KAOS WW SLIDE 4 UNCLASSIFIED ORNL-LR-OWG. 69686 CONTROL CHARTS ORNL Q-2005 CONTROLLED-POTENTIAL COULOMETRIC TITRATOR DETERMINATION OF URANIUM UIWI ) + 26' U(K), E: -0.345 vs E(SCE) URANIUM: COULOMETER NO. 2 $.0.1 % 1940 anywa taman ini wam e URANIUM: GOULOMETER NO. 3 S.0.1% * * *vir was when your DEVIATION FROM KNOWN VALUE (%) URANIUM: COULOMETER NO. 1 S. 0.1% 1961 w ymawiano makam bere to URANIUM : COULOMETER NO. 3 S: 0.1% - 1961 -0.2 it hanam JAN. amma FEB. MAR. APR. MAY JUNE JULY AUG. - SEPT.--- OCT.-- NOV. DEC. SLIDE 5 UNCLASSIFIED PHOTO 17304 .. * . ; * '' ... . A'S . " ? ? . 2 CITRU .* ' . 3 , 1 "Ti . h i !.! .. " . : . :1. ".. ! ! ' S 1 . ' c * :, , i siti . . 1. . .. 2 '. '. . . . pla ' . . . y . . . 1.49 . . ! 1 . . . . , . . : 21, P . WA 7 'I" . Y .' 1.V .' . : . . ! .! '. ' " ... T " . . ..: . ) . . " .':* ! . EX .' R L 11 F :: : ' . XX . Y . '. t. . Y .. 1 . '-.* AITA - . . ... 1 d ' ¿.-. . 3 - - . . PIRTI UP . h ' *4 I - : . ON - 2 . X X ! . Anh . , 11. N * ... - . . : . 20. L'O 14 . . 17 NET . 1. . in - Zirnir . . . : min SS . . . . : * ?. V si . ' .. it',*. I ... 's . in,' ''2.. : 1-1 . W . .' X. . *.. * ... " . iyil 477 * Jy ki : X . * ... . Lyr * * . L i. : PE . . + 1 - LATE, .. AV . : . -. Eliyir". .. . *NOVO. ily.. . . . L 5 + 1. * i - i lumtuulmırlumhur 2 0 12. i 100. . * " . ..... ! . . . id T ICCEK? ' . . . 1. " ! 1.1. . .' . , 1 1 . 4 -. . : . iii- II. :: . LA . * ' . . . WS . 4 . * . T L 3 *R7 " * IL I . .' . ". NIK '- X 19 ** . 6 . . . . t $". H E . . 1 R .. . 12 . ** 1 I . 1 A 2 . ..." SAL, . i . , . * en . ' . ' . 1. . V 2 ., V tay . 1 . in ,17 . . TYS . 1,1** -. 7. . 1. . - . ..!! ' MI ' 14 vel . . . ! *** ' . LU 1. ** ii O R d . . N ' :' !; ! ' . T . : ME! he . A . . . . . . . - . . 2 * F - " - - ' Til . K * . - . . - I S aw 1 2 '. .. .. ? w * . . 74 . .7 4 . ' . In . 2 . .. : SLIDE 6 UNCLASSIFIED ORNL-LR-DWG. 17324 UM+Few → + Fet CONDITIONS : 180 ml vessel Jones Reductor for Preparing Crt: (ZnHg) 5-cc/in. Syringe 90°C CO2 Atmosphere Q-4728 Serial No. 2 Titrator Au Indicator Electrode S.C.E. Reference Electrode Janie T. Gass, General Analysis Laboratory CHART TRAVEL 1 in. PONT (xsCpItu + Fem → UV. Fe I + Cap II LUX+UM+ cl+UW+UH+ cầ+ xsGE START OF ADDITION OF FEW AS TITRANT TYPICAL TITRATION OF ~32 mg OF URANIUM BY 0.4 N Fe" BY MEANS OF THE ORNL MODEL Q-4728 AUTOMATIC RECORDING VELOCITY- SERVO POTENTIOMETRIC TITRATOR SLIDE 7 w morrendoronarson the brother reason monteret hun arbre now there are bio prender alla m omento nt r e A P KROVA ATT hu *** 173V urgentemente comprimento do the wes paths parts ar pas pour la company Wym *; iany na nad passion is in my 14, B - mong itanon., bravou the UNCLASSIFIED ORNL-LR-OWG. 29771 URANIUM: POTENTIOMETRIC, FERRIC SULFATE METHOD de UCL DEVIATION FROM KNOWN VALUE (%) E NEW CONTROL -0.45 ,-9-87 NEW CONTROL 4-28-37 01-10111-20121-31 -10 11-20121-29-1011-20121-30-10 11-20 21-30 1-10111-20 | 21-31 -10 11-21 121-30 K JANUARY- FEBRUARY MARCH APRIL MAY JUNE * UCL DEVIATION FROM KNOWN VALUE (%) lululululu LCL NEW CONTROL 8-13-57 1-10 11-20121-31 1-10111-20121-31 1-10 11-20121-301-10 111-20121-31 1-10 (11-20121-30-10 11-20121-31 K JULY - AUGUST SEPTEMBER-* OCTOBER NOVEMBER DECEMBER SLIDE 8 . .. DATE FILMED 12/28/64 LEGAL NOTICE This roport was prepared as an account of Govoramont sponsored work. Nolthor the United Statos, nor the Commission, nor any person aoting on bubull of the Commission: A. Makes any warranty or reprosentation, oxpressed or implied, with respect to the acou- racy, complotonous, or usofulness of the Information contained in this roport, or that the uso of any information, apparatus, mothod, or procons disclosod in this roport may not infringo privatoly ownod righto; or B. Assumos any liabilities with rospoot to the uso of, or for damagou rosulting from tho uso of any information, apparatus, method, or procour disclosed in this report. As used in the abova, "person soting on behall of the Commission" includes any om- ployoo or contractor of the Commission, or employee of such contractor, to the oxtont that suoh omployoo or contractor of the Commission, or omployoe of such contractor proparos, disseminatos, or provides access to, any information pursuant to his omploymont or contract with the Commission, or his omploymont with such contractor. END