The Determination of Plutonium by (Ethylene dinitrilo)tetraacetic Acid Titration W. 6. BROWN, D. R. ROGERS, E. A. MERSHAD, and W. R. AMOS Monsanto Research Corp., Mound laboratory, Miamisburg, Ohio
b A direct and an indirect (ethylene dinitri1o)tetraacetic acid (EDTA) titration for the determination of plutonium has been developed. A solution containing from 8 to 40 mg. of plutonium is buffered between pH 2.5 and 3.0, heated to boiling, and titrated with EDTA to a copper-l-(2pyrid yla zo)-2-na phthol (CU-PAN) end point. The indirect procedure is a continuation of the direct procedure. Ammonium fluoride solution, buffered at pH 3, is added to the titrated sample. Fluoride ions stoichiometrically demask EDTA from the plutonium chelate. The uncomplexed EDTA is titrated with standard copper solution to a PAN end point, For samples containing between 8 and 40 mg. of plutonium, the mean error of the direct method was less than f O . 1 mg., and the mean error of the indirect method was less than f0.2 mg. The standard deviation of 10 titrations performed on samples containing 14.0 mg. of plutonium was h0.08 for both the direct and the indirect methods. The indirect procedure is relatively free from interfering anions and cations.
not dependent upon electronic equipment is equally desirable. The use of (ethylene dinitri1o)tetraacetic acid (EDTA) as an analytical reagent has been previously discussed (18, 19). Milner and Woodhead determined plutonium(II1) by adding an excess of standard EDTA solution to a sample and back-titrating the excess reagent with standard thorium solution ( I S ) . Alizarin red S screened with methylene blue was the indicator for the titration. Cations which react with EDTA a t pII 2.5 and anions which compete with EDTA for plutonium ions interfere. Paley and Chang titrated plutonium (IV) directly with EDTA a t pH 1 using arsenazo as the indicator (15). After separation of plutonium by solvent extraction, Boase, Foreman, and Drummond added excess EDTA to the sample of plutonium(1V) and back-titrated the uncomplexed reagent with standard zinc chloride solution (1). This paper presents a direct and an indirect complexometric titration procedure for the determination of plutonium. EXPERIMENTAL
and Reagents. This work was performed in stainless steel glove boxes and the usual precautions for handling radioactivity were observed (11). The reagent grade chemicals used in this procedure were obtained from the J. T. Baker Chemical Co., Phillipsburg, N. J., with the exception of the plutonium metal, which was obtained from the National Bureau of Standards, Washington, D. C. A Beckman Zeromatic pH meter was used to make all p H measurements. Standard Copper Solution (0.0200M). Dissolve 1.2721 grams of oxide-free copper shot (99.9001,)in 20 ml. of 1 to 1 HN08. Dilute to 1 liter with deionized water. Standard Zinc Solution (0.0200M). Dissolve 1.3102 grams of oxide-free zinc shot (99.8%) in 10 ml. of 1 to 1 HNOa. Dilute to 1 liter with deionized water. EDTA (0.0200M). Dissolve 7.2290 grams of disodium (ethylene dinitri1o)tetraacetate dihydrate in 1 liter of deionized water and store in a polyethylene bottle. Standardize the EDTA solution against the standard zinc solution (4). PAN (0.1%). Ilissoive 0.1 gram of the indicator [ 1- (2-pyridylazo)-2-naphthol] in 100 ml. of ethanol. Apparatus
S"
techniques may be - - used for the determination of macroquantities of plutonium (11). However, only two methods are generally employed for the determination of semimicro amounts: amperometric titration of plutonium(V1) with iron(I1) (10) and controlled potential coulometry (17). These methods are xcurate and reliable, but a procedure '
i m A L ACCURATE
Table 1. Direct Titration of Standard Plutonium Solutions at pH 2.5 to 3.0 Using CU-PAN as Indicator
Pu
taken,
No. of
mg.
detns.
8.1 13.1 14.0 16.2 24.3 40.4 13.1"
4 4 10 12 4 4 2
Pu found (mean value), mg. 8.1 13.1 14.0 16.2 24.3 40.3 13.2
Mean error,
mg. 0.0 0.0 0.0 0.0 0.0 -0.1 +0.1
Plutonium(1V) oxidized to plutonium (VI) by divalent silver. 5
ANALYTICAL CHEMISTRY
Cu-EDTA Solution. Mix equivalent quantities of the standard copper solution and the standard EDTA solution (4). Store in a dropping bottle. Standard Plutonium Solution (0.004184M). Polish the plutonium metal (99.97 f 0.03%) with a metal polishing wheel in a dry argon atmosphere to remove the oxide. Dissolve 1.0000 gram of the metal in 2N HC1. Add 15 ml. of 3N HzS04 and heat to fumes of SOs. Continue fumlng to inciuient drvness. cool. and dilute to 1 liter' with O.$NH$04 (1%'). Buffered Fluoride Solution (pH 3). Dissolve 10 grams of ammonium fluoride in 100 ml. Gf deionized water and add reagent grade hydrofluoric acid (48%) until the solution is buflered at pH 3.0. Dilute to 250 ml. with deionized water. Use polyethylene vessels for preparation and storage of the solution. Recommended Procedure. Quantitatively transfer the sample solution, containing from 8 t o 40 mg. of plutonium, to a 150-ml. beaker. Add 2 ml. of 1 to 1 HzS04 and heat to fumes of SOa. T o destroy any organic matter which may be present, carefully add concentrated "03, dropwise, to the fuming sample. Cool and wash down the cover glass and beaker walls with a minimum amount of 1N HzS04. Heat to incipient dryness, cool, and dissolve the salts in approximately 10 m!. of 0.25N &Soh. Add 3 to 5 ml. of glacial acetic acid to the sample and adjust the pH to between 2.5 and 3.0 by the dropwise addition of 10% sodium acetate. DO not allow the pH of the solution to exceed 3.0. Heat the sample to boiling. Add 3 drops of PAN, 1 or 2 drops of Cu-EDTA solution, and titrate the sample with Table II. Indirect Titration of Standard Plutonium Solutions at pH 2.5 to 3.0 Using PAN as Indicator
Pu taken, mg. 8.1 13.1 14.0 16.2 24.3 40.4 13.1" 5
Pu found (mean value), mg. detns. No. of 4 4 10 16 3 3 2
8.2 13.1 14.0 16.2 24.2 40.3 13.0
Mean error, mg. +0.1 0.0 0.0 0.0 -0.1 -0.2 -0.1
Plutonium(1V) oxidized to plutonium
(VI) by divalent silver.
standard EDTA solution to a lemonyellow end point. As the end point color of PAN indica1;or is approached, allow 1 minute between additions of EDTA. After the end point has been attained, add 1 to 2 ml. of buffered fluoride solution to the sample. Titrate the released EDTA with standard copper solution. As the end point is approached, add 1 or :2 additional drops each of buffered fluoride solution and of PAN indicator solution. Titrate the sample to the faint pink end point color of the Cu-PAS complex.
Table 111. Pu taken, 15.6 15.6 15.6 15.6 12.9 12.9 12.9 12.9 13.1 13.1
13.1 13.1 14.0 13.1 13.1 13.1
RESULTS AND DISCUSSION
EDTA forms stab13 1 to 1 complexes with plutonium(III), (IV), (V), and (VI) ( 6 - 8 , l l ) . Because of the stability of these complexes, plutonium in any valence state may be titrated with EDTA by using a suitable end point indicator. This eliminittes the need for a valence adjustment i3tep in the determination. An indicator generally does not form a colored complex with all the valence states of plutonium; therefore, the indirect indicator, Cu-PAS, was investigated for end point detection. Flaschka and Abdine introduced the copper complex of PAS as an indicator for those cations with which PAN does not form a colored complex. They suggested that titrations be performed in hot solutions because of the sluggish reactivity of the indicator complex (6). Direct Titration. The results of the direct E D T A titration of samples containing from 8 to 40 mg. of plutonium are shown in Table I. I n this concentration range, the mean error of the direct procedupe was less than 1 0 . 1 mg. The s t m d a r d deviation, calculated from 1 3 titrations performed on 14.0-mg plutonium samples, was *0.08. The optimum pH range for the titration is between 2.5 and 3.0. At lower pH values the Cu-PiiN end point reaction is sluggish and difficult to detect; st higher values, the plutonium(1V) ions may hydrolyze. Small quantities of sulfate ions are sufficient to inhibit the hydrolysis of plutonium(1V) (a); therefore, samples are converted to the sulfate prior to titration. Even in the presence of sulfate ions, hydrolysis of plutonium(1V) may occur when the hydrogen ion concentration drops below Because the hydrolysis reaction is irrcversible, low rwults may be obtained if the pH of the sample is raised above 3.0. The rate of chelation of plutonium with EDTA is dependent upon the sulfate concentration of the sample. The reactioii is rapid if the excessIi&Oc is expelled during the fuming of thc sample as recommended in the proccdrire. Indirect Titration. The results obtained by the indirect titration of
13 I
13.1 13.1 13.1 13.1 14.n
13.1 a
Interference Study of the Direct and Indirect Determination of Plutonium" Direct Indirect Interference __ pu Mean Pu Mean error, found, error, Added, found, mg. mg. mg. 0.1 15.7 15.7 0.1 Li(1) 3.0 0.0 15.7 0.1 15.6 3 .O Na(1) +14.0 15.7 29.6 0.1 Ni(I1) 3.4 31.1 - 0.1 f15.5 15.5 3.6 Fe(111) 12.9 - 0.1 0.0 12.8 3.0 U(V1)
+
+
+
%3) Ce(1V)b
Ba(I1) Ca(I1) Mg(I1) Cr(II1) Al(II1) Cu(I1) Bi(II1) Au(II1) COfII) ~~iIV ) Cd(I1) Mn(11) Zn(I1) Th(IV)
Ago)
'
3.0 3 .O 3.0 3.0 3.0 3.0 3.0 1.7 3.7 3.6 3 .O 3.0 5 .O 0.5 0.5 1.5 5.2 3 .O
12.9 13.6 13.3 13.1 13.1 13.0 13.4 29 , 2 25.5 18.5
+
++ 0.0 0.7 0.4 -
0.0 0.0 0.1
+ 0.3 ,
+15.2 $12.4 5.4
12.8 13.5 13.2 13.1 13.1 13.0 13.0 ~. . 39.1 13 .O 13.1
+Destroys indicator
-
0.1
++ 00 .. 63 0.0 0.0
- 0.1 - 0.1 +15.1 - 0.1 0.0
Destrovs indicator
13.1 13.7 13.4 16.5 19.2 13.1
0.0"
++ 00 .. 63 ++ 3.4 5.2 0.0
13.1 13 .o 13.1 13.7 19.1 13.1
-
+ +
0.0 0.1 0.0 0.6 5.1 0.0
Analyses performed in duplicate.
* Cerium(1V)hydrolyzed during direct titration.
samples containing from 8 to 40 nig. of plutonium are shown in Table 11. At this concentration range, the mean error of the indirect procedure was less than k0.2 mg. The standard deviation, calculated from 10 titrations performed on 14.0-mg. Plutonium samples was hO.08. A comparison of the data presented in Tables I and 11shows that fluoride ions stoichiometrically release EDTA from the Pu-EDTA complex. Samples containing more than 40 mg. of plutonium cannot be titrated by the indirect procedure because the titration of the released EDTA with standard copper solution leads to such an intensely blue-colored solution of the Cu-EDTA complex that the end point rolor of the PAN indicator is obscured
(4).
Interferences. Cations which titrate partially or completely between p H 2.5 and 3.0 interfere in the direct E D T A titration of plutonium. Twenty-three cations, listed in Table 111, were added t o solutions of known plutonium content. After the Samples were treated according to the recommended procedure, they were analyzed by the direct and indirect EDTA titration methods. Cations which caused a mean error greater than twice the standard deviation of the methods were considered interferences. Fourteen cations listed in Table 111 interfered in the direct titration. Cerium(II1) and zinc partially interfered in the direct titration and also interfered in the indirect titration. These cations are incompletely complexed by EDTA below pH 4.5. Consequently,
any small pH change which may occur after completion of the direct titration will cause a change in the amount of EDTA complexed with these cations. The error contributed by this phenomenon will depend upon the magnitude and the direction of the pH shift. Cerium(1V) hydrolyzed during the direct procedure and did not stoichiometrically interfere in the direct or the indirect titration. Thorium and aluminum quantitatively interfered in the direct and in the indirect procedures. Both cations react with stoichiometric amounts of EDTA a t pH 2.5 to 3.0. Fluoride ions subsequently demask stoichiometric quantities of EDTA from these cations. Phosphate ions precipitate plutonium and interfere with the method. Prior to the conversion to plutonium sulfate, plutonium may be quantitatively separated from interfering cations by either ion exchange (3, 9, 11, 16) or solvent extraction ( I , 1 1 , l i ) . Xylenol Orange Indicator. Xyleiiol { 3,3'-bis [N,N-di(carboxyorange methyl) - aminomethyl] - o - cresolsulfonphthalein) was found to be equivalent to Cu-PAN as a n end point indicator for the direct titration of plutonium with EDTA. However, in the indirect titration no color change of xylenol orange was observed a t the end point. LITERATURE CITED
(1) Bnase, D. G., Foreman, J. IC.,Drumniond, J. L., Tulantu 9, 53 (1962). ( 2 ) Drummond, J. I,., Welch, G. A., J . Chem. SOC.1958,3218. (3) Faris, J. P., Buchanan, R. F., U. s. VOL 35, NO. 8, JULY 1963
1001
Atomic Energy Comm. Rept. TID 7606 (1960). (4) Flaschka, H., Abdine, H., ChemistAnalyst 45,58 (1956). (5) Flaschka, H., Abdine, H., Mikiochiin. Acta 1956, 770. (6) Foreman, J. li., Smith, T. D., J. Chem. SOC.1957, 1758. (7) Gel’man, A. D., Artyukhin, P. I., Moskvin, A. I., Zh. Neorgan. Khim. 4, 1332 (1959); Nucl. Sci. Abstr. 13, 18993 (1959). (8) Kabanova, 0. L., Danuschenkova, hl. A., Paley, P. N., Anal. Chim. Acta 22,66 (1960).
(9) Kressin, I. K., Waterbury, G. R., ANAL.CHEM. 34,1958 (1962). (10) Larson, R. P., Seils, C. A., Meyer, R. J., U. S. Atomic Energy Comm. Rept. TID 7606 (1960). (11) Metz, C. F., ANAL.CHEM.29, 1748 (1957). (12) Metz, C. F., Waterbury, G. R., Ibid.. 31.1144 (1959). (13) Mil&, G. W. C.‘, Woodhead, J. L., Analyst 81,427 (1956). (14) Moore, F. L., Hudgens, J. E., Jr., ANAL.CHEM.29,1767 (1957). (15) Paley, P: N., Chang, W. C., Zh. Analzt. Khzm. 15, 598 (1960); Nucl.
Sci. Abstr. 15, 7305 (1961). (16) Ryan, J. L., Wheelwright, E. J., Ind. and Eng. Chem. 51,60 (1959). (17) Scott, F. A., Peekema, R. M., Proc. U.N . Intern. Conf. Peaceful Uses At. Energy, Geneva, AIConf. 28, 914 (1958). (18) Welcher, F. J., “The Analytical Uses of Ethylenediaminetetraacetic Acid,” Van Nostrand, New York, 1957. (19) Wilkins, D. H., Hibbs, L. E., Anal. Chzm. Acta 18, 374 (1958).
RECEIVEDfor review August 10, 1962. Accepted March 11, 1963.
A Continuous Photometric Fluorine Analyzer C. W. WEBER and 0.
H. HOWARD
Technical Division, Oak Ridge Gaseous Diffusion Plant, Union Carbide Nuclear Co., Oak Ridge, Tenn.
b A sensitive automatic analyzer has been developed for the continuous determination of elemental fluorine in gas mixtures. As the sample stream i s passed through hot sodium chloride, the fluorine quantitatively liberates chlorine, which i s measured at 360 mp in a pressure-controlled flow colorimeter cell. The range of the analyzer, calibrated with standard fluorine-nitrogen mixtures, i s 0.05 to 15 mole of fluorine, using a 10-cm. cell at 500 mm. of Hg. The relative standard deviation of the fluorine analyzer i s about h1.5% above O.8Y0 fluorine, and the standard deviation i s about h0.04 (absolute) at lower levels. Initial response lag i s 6 seconds, with full response in about 1 minute. The instrument i s applicable in the presence of hydrogen fluoride and other gases which do not absorb radiation near 360 rnp or liberate chlorine from sodium chloride.
for the low concentration range of interest. Staple and Grilly (3) determined fluorine continuously by measurement of the thermal conductivity of chlorine displaced from sodium chloride by the fluorine; this method also lacked
A
Figure 1 , Ultraviolet absorption spectra of fluorine and chlorine
70
determination of fluorine in a mixture of fluorine and nitrogen was needed for satisfactory control of a fluorination process. The operating range of concentrations was 0.3 to 10 mole % fluorine, to be controlled within 10.05 to *0.57,, respectively, throughout this range. Previous analytical control consisted of intermittent chemical analyses, which required constant manual effort to obtain an acceptable degree of process control. An automatic continuous analyzer was developed to reduce analytical costs and provide smoother process control. A few continuous fluorine analyzers have been described in the literature. Weber ( 5 ) developed an automatic, fluorine analyzer based upon pncumatic detection of the reduction of molar flow as fluorine reacts with sulfur dioxide; this analyzer \vas not sensitive enough CONTINUOUS
1002
ANALYTICAL CHEMISTRY
t
I CM. C E L L AT S.T.P. 330 m p
PRINCIPLE
Fluorine absorbs radiant energy only weakly in the ultraviolet and visible regions of the spectrum. Davis (1) reports a n absorbance of 0.3, for a depth of 1 em. at 0” C., 760 mm. of Hg, at the maximum absorption wavelength of 285 mp (Figure 1). Chlorine, on the other hand, gives a n absorbance of 3.0 at its maximum absorption wavelength of 330 mp. Therefore, to achieve the desired sensitivity in the photometric fluorine analyzer, the fluorine sample is passed through hot sodium chloride, where it displaces an equivalent amount of chlorine : Fz(g)
+ 2T\’aCl(s)
+
Clz(g)
220
260
300 340 WAVELENGTH, ~ ? p
380
the required sensitivity and precision. Staple, Schaffner, and Wiggin (4) utilized the liberation of bromine from sodium bromide by fluorine and the subsequent measurement of light absorption by the bromine; this method was not attractive because of reported bromine adsorption-memory effects. This report describes an automatic, continuous, fluorine analyzer which is basically similar to the bromine-displacement analyzer, except that the light-absorption measurement is made on chlorine displaced from sodium chloride by the fluorine. .Idequate scnsitivity :tnd prccision arc obtained and no adsorption difficulties are encountered under the operating conditions established.
+ 2XaF(s)
The resulting gas then flows through a chlorine-sensitive colorimeter, which generates a n electrical signal directly related to the concentration of fluorine in the sample. CONSTRUCTION AND OPERATION
Thc analyzer (Figure 2 ) consists basically of a heated sodium chloride reactor, flow colorimeter, pressure-control system, flow limiter, a vacuum pump for drawing a continuous sample through the analyzer, and a recorder. Sodium Chloride Reactor. The sodium chloride reactor is a nickel cylinder, 1-inch i.d. by 8 inches long. Approxiniately 100 grams of reagent grade, crystalline, sodium chloride is retained in the cylinder by nickel wool filters. It is designed so t h a t i t can be readily disconnected and one end removed for replacing the sodium chloride as needed. I n operation the reactor hangs vertically within a cylindrical heater which maintains i t a t 250’ (3. The gas stream passes upwart1 through the reactor. The sodium chloride is heated because fluorine, at low concentrations (< lye),
