acid medium, expecially at low concentrations of the dye, uranium does not form strong complexes with the dye. Therefore, uranium does not affect the complexometric titration of other metals when the p H of the solution is below 7 . Preliminary results have shown t h a t the dye is a selective and sensitive reagent for uranium. No attempt has been made to make any practical analysis, but further studies should be made so that detailed procedures may be worked out for the analysis of practical samples.
Bailar, Jr., for helpful discussion and to the Utica Metals Division, KelseyHayes Co., for the use of facilities in checking part of results presented,
(8) Liu, J. C. I., Ph.D. thesis, University of Illinois, 1951. (9) Martell, A. E., Calvin, M., “Chemistry of the Metal Chelate Com-
pounds,” Prentice-Hall, New York, 1952.
LITERATURE CITED
Bailar, J. C., Jr., University of Illinois, 1957, private communication. Blanquet, P., Anal. Chim. Acta 16, 44 (19571. Cheng, K.’L., Bray, R. H., ANAL. CHEM.27, 782 (1955). Cheng, K. L., Lott, P. R., Zbid., 28, 462 (1956). Forman, J. K., Riley, C. J., Smith, T. D., Analyst 82, 89-95 (1957). Job, P., Ann. chim. 9, 113 (1928). Krishen, A,, Freiser, H., ANAL. CHEY.29, 288 (1957).
ACKNOWLEDGMENT
The author is grateful to John C.
Paley, P. N., Proc. Intern. Conf. Peaceful Uses Atomic Energy, Geneva, 1965 8 , 225 (1956). Pribil, R., Jelinek, M., Chem. listy 47, 1326 (1953). Rodden, C. J., ANAL. CHEU. 25, 1598 (1953). Sen Sarma, R. N., Mallik, A. K., Sci. and Culture (Calcutta) 20, 135 (1954), RECEIVEDfor review March 12, 1957. Accepted January 18, 1958. Pittsburgh Conference on Analytical Chemistry and A plied Spectroscopy, Pittsburgh, Pa., d a r c h 1957.
Determination of Total Fluoride Content in Uranium Tetrafluoride Using Ion Exchange Columns K. F. SPOREK’ Research and Development Division, Eldorado Mining and Refining, lid., Ottawa, Ont., Canada
,A simple and rapid method for the determination of total fluoride content in uranium tetrafluoride is based on separation of the fluoride ion by means of a cationic ion exchange resin and titration of the liberated hydrofluoric acid with sodium hydroxide. The solid sample of uranium tetrafluoride is dissolved in a mixture of sodium hydroxide and hydrogen peroxide and after dilution the solution is passed through the ion exchange column. The above mixture of reagents is also suitable for dissolving UO,, U308, UOs, and U02F,,all of which may b e present in lesser or larger amounts in technical uranium tetrafluoride. A determination requires only 15 to 20 minutes and no complicated or expensive apparatus is needed. The standard deviation for 15 determinations was 1 0 . 1 3 on a value of 23.91 % fluorine.
D
of the total fluoride in uranium tetrafluoride is necessary for the proper control of hydrofluorination of uranium dioxide processes. The methods described by Rodden ( 2 ) for the determination of fluoride in uranium hexafluoride and in uranium tetrafluoride use the gravimetric lead chlorofluoride technique for the former comETERMINATIOX
1 Present address, Department of Chemistry, University of Ottawa, Ottawa, Ont., Canada.
1030
ANALYTICAL CHEMISTRY
pound and the distillation of fluosilicic acid followed by thorium nitrate titration technique for the latter. Neither method is sufficiently rapid or accurate for routine testing of materials with high fluoride contents. Gillies et al. (1) used (ethylenedinitri1o)-tetraacetic acid (EDTA) for the dissolution of uranium fluoride and uranyl fluoride. For the determination of fluoride, they used pyrohydrolysis of the sample in a platinum boat 1Tith steam a t 900’ C. t o hydrofluoric acid v-hich was continuously titrated with sodium hydroxide to a phenolphthalein end point. Special pyrohydrolysis apparatus n-as required and the determination time 11-as slightly over an hour. A modification of this method using a nickel reaction tube is described by Susano, Khite, and Lee
(4).
Because a simple and accurate method was required for the determination of fluoride in uranium tetrafluoride prepared by hydrofluorination of uranium dioxide, without costly and complicated apparatus, and no existing method was satisfactory in accuracy or simplicity, experimental work was carried out. APPARATUS AND REAGENTS
Apparatus. Jones reductor tubes, with stems reduced in length t o about 2 inches, were suitable for ion exchange columns. Reagents. Amberlite IR-120 cation exchange resin (16 t o 50 mesh
U.S. standard screen), analytical grade. SODIUM FLUORIDE SOLUTION, 0.2.v. Dissolve exactly 8.400 grams of sodium fluoride (fluorometric grade) dried overnight at 130’ C. in about 200 ml. of water. Transfer the solution to a 1-liter volumetric flask, and dilute to the mark. SODIUMHYDROXIDE SOLUTION, 0.2N. Dissolve 8.0 grams of sodium hydroxide pellets (analytical grade) in about 100 ml. of water. Cool the solution, transfer it to a 1-liter volumetric flask, and dilute to the mark. T o standardize the solution, pipet 25.0 nil. of the standard 0.2N sodium fluoride solution into a 250-ml. conical flask, and add 5 ml. of 25% sodium hydroxide solution and 70 ml. of water. Pass the mixture through an ion exchange column exactly as described in the procedure and titrate the eluent with 0 . 2 3 sodium hydroxide. Correct the volume €or the blank and calculate the factor for the sodium hydroxide solution in the usual way. Methyl red indicator, 0.5% solution in alcohol. Hydrochloric acid regeneration solution, 1 part of concentrated hydrochloric acid and 2 parts of water. PROCEDURE
Dissolution of Samples. Weigh accurately into a 250-ml. conical flask from 0.4 t o 0.7 gram of uranium tetrafluoride, and add 5 ml. of 25% sodium hydroxide solution and 10 ml. of 30% hydrogen peroxide solution. Attach an air condenser ( 2 feet in length) to the flask a t once. Mix the contents by gently swirling the flask for about 5
minutes (if the sample has not completely dissolved, as with low grade material, heat slightly over a Bunsen burner). When all the sample has dissolved, wash the condenser with a little water, allow the washings to run into the flask, and disconnect the condenser. Dilute the solution to about 100 ml. Determination of Fluoride. Prepare a n ion exchange column b y filling a Jones reductor tube (stoppered at t h e bottom with a pad of glass wool) with enough Amberlite IR-120 ion exchange resin to give a bed 11 to 12 cm. high. Fill the column nearly to the top with 1 t o 2 hydrochloric acid solution, agitate the resin with a glass plunger to expel trapped air bubbles, and allow the acid to pass through the column a t a rate of about 5 drops per second. Wash the column with water until 250 ml. of the eluent requires not more than 2 drops of 0.2X sodium hydroxide for neutralization with methyl red indicator. Drain the liquid in the column to the top of the resin. Transfer the sample solution into the freshly prepared ion exchange column and elute it a t the rate of about 2 drops per second. Wash the original flask and the column with small volumes of water, using in all 250 ml. Add to the eluent 3 drops of methyl red and titrate the free hydrofluoric acid with 0.2N sodium hydroxide until the color changes from pink to lemonyellow. T o run a blank determination, pass 100 ml. of water containing 5 ml. of 25% sodium hydroxide through a freshly prepared column and wash the column with 250 nil. of water in exactly the same way as above. Titrate the eluent n ith 0 . 2 s sodium hydroxide and subtract the value from that obtained for the sample solution, Calculate the fluoride content by the following equation: Volume -~of S a O H X S X 0.019 X 100 ~~
IVPight of w n p l e taken
EXPERIMENTAL
Dissolution of Sample. T h e insolubility of uranium tetrafluoride has made t h e determination of its fluoride t o n t e n t difficult. Of t h e systems tested, a inivture of sodium hydroxide and hydrogen peroxide was the most satisfactory solvent. It completely dissolved uranium tetrafluoride without heating. Uranium trioxide and dioxide were also dissolved rapidly in the cold; uranium octoxide dissolved rapidly n hen the reaction mixture was slightly heated. This mixture was particularly valuable because sodium was the only ion present in the final solution other than uranium and fluoride. Preliminary Experiments. T h e accurate determination of fluoride in t h e solution prepared b y t h e above procedure still presented a rather difficult problem. T h e thorium nitrate titration, lead chlorofluoride gravimetric,
a n d calcium fluoride gravimetric methods are considered t h e most satisfactory. T h e lead chlorofluoride method appeared the most promising, as it could be applied after certain modifications to solutions containing uranium and could be developed into a volumetric method similar to the sulfate ion deterniination (9). However, preliminary experiments shoI+-ed that the precipitation stage would require considerable research to arrive a t optimum conditions. Attempts to isolate the fluoride from uranium by distillation with perchloric acid lvere unsuccessful. As uranium tetrafluoride v a s dissolved without using an acid, fluoride lvas the only anion present in the ensuing solution. Therefore, it n-as thought possible either to adsorb the acid on an anion exchange resin and then elute it rrith a known quantity of sodium hydroxide solution, or to adsorb the sodium and uranium ions on a cation exchange resin and titrate the liberated acid in the eluent. The latter version appea,red more promising. The sample, after dissolut'ion in the sodium hydroxide and hydrogen peroxide mixture, ivas diluted with water and the solution was passed through a column cont.aining a strongly acidic cation exchange resin of the sulfonic acid type (dmberlite IR - 120) in its free hydrogen form. The eluent \vas collected and tested by titration with a standard sodium hydroxide solution. Cranium and sodiuiii ions were strongly adsorbed by the rwin and the freed hydrofluoric I with the eluent. The optiniuni contlitioiia for operation of the columns and titration of the hydrofluoric acid in the eluent w r e tested. Operation of Ion Exchange Columns. I n esperiments o n the dissolution of uranium tetrafluoride samples from 0.4 t o 0.7 gram of niaterial required about 5 nil. of 2 3 7 , sodium hydroxide solution and about 10 ml. of 30YG hydrogen peroxide solution. Several esperiiiients showed that a bed about 25 mm. in diameter and about 12 cm. in height was sufficient to retain the quantity of sodium hydroxide present in about, 10 ml. of the 25% solution and the quantity of uranium present in about 1 gram of uranium tetrafluoride. These dimensions Tvere adopted in Jones reduct'or tubes for use as columns. The optimum speed of elution and the 1-olume of wash n-ater required for hydrofluoric acid were tested: The column could be eluted a t a coniparatively fast rate equivalent to about 20 ml. of solution per minute, and complete elution !vas effected by n-ashing with 200 to 250 nil. of water. The regeneration of the ion exchange resin (removal of adsorbed sodium and uranium ions) was easily and quantitatively effected by passing about 150
ml. of 1 to 2 hydrochloric acid through the column and washing with water. The washing was continued until about 250 ml. of the eluent contained no more acid than the equivalent of 1 to 2 drops of 0.2hr sodium hydroxide. Titration of Hydrofluoric Acid. As hydrofluoric acid is comparatively weak (degree of ionization of normal solutions at 18' C. is 0.07 for hydrofluoric acid and 0.78 for hydrochloric acid), its end point with alkali is less satisfactory than that of strong mineral acids. The hydrofluoric acid liberated during the pyrolysis with steam was titrated by Gillies et al. (1) to a phenolphthalein end point. However, because the solutions in the present work were exposed to atmospheric carbon dioxide, the above indicator could not be used without prior removal of carbonic acid. When methyl red was substituted for this indicator, the titration could be carried out in the presence of carbon dioxide that dissolved during elution. The end point could be detected with l drop of 0.2 S sodium hydroxide. The blank value obtained by passing 250 to 300 ml. of x a t e r containing 5 ml. of 25% sodium hydroxide solution through a column was about 0.1 ml. of the standard solution and was very reproducible. The end point of the titration mas reached when the indicator remained yellow on further addition of a few drops of sodium hydroxide. I n a few control experiments the solutions after titration with sodium hydroxide were reacidified with a measured quantity of 0 . 1 S hydrochloric acid, boiled for a few minutes to expel carbon dioxide, and cooled in flasks stoppered with rubber bungs carrying absorption tubes filled with soda-lime (bscarite). The results in the presence of small quantities of carbon dioxide and under carbon dioxide-free conditions n-ere identical, proridpd elution of the sample, determination of the blank d u e , and standardization of the sodium hydroxide solution mere carried out under the same conditions. The final procedure, employing direct titration of the liberated hydrofluoric acid, was tested 17-ith a U. S. Atomic Energy Commission uranium tetrafluoride standard (analyzed sample KO. 17, New Brunswick Laboratory) and with a number of samples of experimental uranium tetrafluoride obtained by hydrofluorination of uranium dioxide. The procedure was simple and rapid; the time required for a determination varied from 15 t o 25 minutes, depending on the speed of elution. DISCUSSION
OF RESULTS
The results obtained with the analyzed sample of uranium tetrafluoride (Table I) were satisfactory in precision and accuracy. The estimate of standVOL. 30, NO. 6, JUNE 1958
* 1031
Table I.
Determination of Fluoride in a Standardized Uranium Tetrafluoride Sample
Sample Taken, Gram 0.1930 0.2713 0.4189 0.2940 0,6999 0 5665 0,4053 0.459i
0.5331 0.5376 0 5666 0,4498 0 5785 0.4653 0.6218
(Calculated fluoride content 23.91%) Fluoride Found Gram c /O 7 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
0459 0645 1005 0703 1664 1364 0968 1096 1267 1295 1362 1065 1383 1117 1479
Mean Std. dev.
ard de\-iation was 10.130 on a value of 23.9%, which was satisfactory for the purpose in hand. The mean value obtained from 15 experiments for the fluoride content of the sample was 23.89% against the theoretical value of 23.91%. This was calculated from the uranium tetrafluoride content (97.93%) and the uranyl fluoride content (1.69%) which n-ere determined in the S e w Brunsn-ick Laboratories of the U. S. Atomic Energy Commission. The results on the expcrimental materials were in agreement n ith the expected values. Operation of the columns was simple. The time required for elution of a sample was about 15 minutes and for regeneration about 10 minutes. The end point of the titration n-as also
23.78 23.77 23,99 23 91 23.77 24.08 23 88 23 84 23 7 i 24 09 21.04 23 68) 23 931 24 011 23 i 9 J 23.89 10.13
Remarks Titration finished after removal of CO?
Titration carried out in presence of dissOl\TPd CO:
satisfactory, though not a5 sharp as when a strong mineral acid is titrated. Complete color change from pink (excess acid) to lemon-yellow was effected by adding about 0.5 nil. of 0 . 2 s sodium hydroxide. Although the change was gradual and went through the intermediate shades, the end point could be determined to within one drop of the standard solution. S o detectable losses of hydrofluoric acid resulting from interaction with the glass apparatus 11-ere observed. The use of plastic equipment. n-hich was not then obtainable, might lead to even better results. This analytical method is suitable for determining the total fluoride content in uranium tetrafluoride uranium hexa-
fluoride, and u r a n ~ - fluoride. l Uranium oxides and nietallic elements generally cause no interference but acids (except carbonic acid) must be absent. I n this respect, the method resembles that using pyrohydrolysis ( 1 ) . As all the uranium aiitl an!. other metal ions in the original sample \Till be removed from the column by hydrochloric acid, it should be possible to cletermine uranium in the same sanipk ns used for the fluoride determination. However, because of the ease with n-hich uranium fluoride can be dissolvetl and the uranium content can be determined rrhen the socliuin hydroxide-hydrogen peroxide reagent is used, this is usc,ful only when the sample is limited. ACKNOWLEDGMENT
The author n-ishes to thank Eltlorado Mining and Refining, Ltd., for permission to publish this paper. LITERATURE CITED
( 1 ) Gillies, G . AI., Keen, 1.J., Lister, B. -%. J., Rees, D., "Determination by Pyohydrolysis of Uranium,
Fluorine, and Oxide Impurity in Uranium Tetrafluoride," Brit. Atomic Energy Research Establishment C/M 225 (October 1954). (2) Rodden, J., "Ahalytical Cheniistry of the llanhattan Project," pp. 226-70, lIcGrav+Hill, S e n Tork,
e.
1950. (3) Sporek, I
