of Teflon, but the last traces (0.02%) were tenaciously held. To determine whether such contamination might be specific with iridium, 20 other elements were tested with Teflon planchets (Table 11). Chemical rather than physical adsorption seems to be responsible for the contamination of Teflon. This is evidenced by the fact that only a few elements act as contaminants, and that thorough washing of even wettable Teflon will not remove the last traces.
It may be significant that the elements adsorbed are ones that form very stable fluoride complexes. LITERATURE CITED
(1) Boyd, G. E., Myers, L. S., Jr., Adamson, A. W., J . Am. Chem. Soc. 69, 2849
(1947).
( 2 ) Harshaw Chemical Co., Cleveland,
Ohio, “Harshaw Scintillation Phosphors,” 1958. (3) Ketelle, B. H., Boyd, G. E., J. Am. Chem. SOC.69, 2800 (1947).
(4) Nucleonics Corp. of America, Brooklyn 31, N. Y., Tech. Bull. 4.
(5) Renfrew, M. M., Lewis, E. E., Znd. Eng. Chem. 38,870 (1946). ( 6 ) Tracerlab, Inc., Waltham, Mass., Tracerlab Catalog E, 1958. (7) Troyanowsky, C., Ann. jals. et jraudes 51, 315 (1958). (8) Wayne, P. J., Bruner, W. M., SPE Journal 1,490-502 (1955). RECEIVED for review December 21, 1959. Accepted April 11,1960. Work supported by a fellowship from the Minnesota Mining and Manufacturing Co. and a research grant from the U. s. Atomic Energy Commission.
Spectrophotometric Determination of Uranium with Benzohydroxamic Acid in 1-HexanoI CLIFTON E. MELOAN,’ PAUL HOLKEBOER,* and WARREN W. BRANDT Department o f Chemistry, Purdoe University, lafayette, Ind.
i,‘ \
b
A spectrophotometric method for quantitatively determining small amounts of uranium i s based on extraction with 1-hexanol of the colored product formed when the uranyl ion reacts with benzohydroxamic acid (CCH~CONHOH)at pH 6.2. The system obeys Beer’s law between 7 X and 2 X mole of uranyl ion when extracted into 10 mi. of 1-hexanol. The effects of reagent-metal ratio, pH, diverse ions, extraction interferences, and reagent stabilities have been investigated. The stoichiometry of the colored species was determined to be 1 to 1 (uranium-benzohydroxamic acid) at pH 3.5 and 1 to 2 at pH values more basic than 5.5. The interference caused by thorium(1V) and cerium (111) i s eliminated.
S
metal ions react with hydroxamic acids to form soluble colored complexes or precipitates (1-3). Wise and Brandt ( 7 ) report the use of benzohydroxamic acid for the quantitative determination of vanadium. Dyrssen (4) reports the chelates of lanthanum(III), thorium(IV), and uranium (VI) with ,V-phenylbenzohydroxamic acid but does not propose them for spectrophotometricuse. The method developed here depends upon forming the soluble uranium(V1)benzohydroxamic acid chelate a t p H 6.2 in aqueous solution, extracting the EVERAL
I Present address, Kansas State University, Man1 attan, Kan. Present address, Western Michigan University, Kalamazoo, Mich.
rate problem and while the end results are the same it is much faster to do the addition as recommended. With larger reagent-metal ratios, such as 15 to 1, this effect is negligible. APPARATUS
00 400
450
500
550
W A V E LENGTH IN rnp
Figure 1. Effect of pH on color of the aqueous uranium(V1)-benzohydroxamic acid system
chelate into 1-hexanol, and measuring the absorbance a t 380 mM. The method is satisfactory and versatile for determining from 0.2 to 12 mg. of uranium. It is sensitive and precise and the system is stable for several days. The procedure does not require an exceedingly large excess of reagent. The volume of the aqueous layer can vary over wide limits and only one extraction is necessary. The only variable requiring careful control is p H and this is not difficult when standard instruments are used. The only other exact measurement is that of the volume of extractant added. It is necessary to stir the chelate for a t least 5 minutes after the p H is adjusted, as the rate of the chelate formation is slow. If a low reagent-metal ratio is used, the order of addition of reagents is important. The reagent solution should be added to the metal solution before any diluent is added. This again is a
Beckman Model B spectrophotometer equipped with matched 1.00-em. cells. All data reported are the average of a t least two independent measurements unless otherwise noted. Leeds & Northrup pH meter (lineoperated). REAGENTS
All organic extracting agents were distilled prior to use. Solutions of diverse ions were prepared from analytical reagent grade compounds and distilled water. CHARACTERISTICS
OF
THE SYSTEM
Uranium(V1) and benzohydroxamic acid (C6H5CONHOH)react to form a colored species which, a t concentrations less than 1.6 X 10-3M uranium, is soluble a t all p H values. The particular colored species formed is dependent on p H (Figure 1). At p H 3 to 4 a definite red color is formed. As the p H is increased, this changes to a red-orange a t p H 5 and to an orange a t p H 7. ilt any p H more basic than 11 the color diminishes very rapidly. Continuous Variations Studies. The nature of the aqueous species of the uranium(V1)-benzohydroxamic acid chelate was investigated by Job’s method of continuous variations (6). The results showed that the absorbing species a t pH 4.0 is a 1 to 1 complex of uranium(V1) and benzohydroxamic acid. VOL. 32, NO. 7, JUNE 1960
791
3 e
RATIO OF REAGENT TO METAL
0.0 -.4
Figure 2. Dependence of absorbance on ratio of b&ohydroxamic acid to uranium(V1)
At p H 9.0 the absorbance of each solution was measured a t several wave lengths to determine if another absorbing species were present. Each sample studied had a maximum absorbance a t a mole fraction of uranium(V1) of 0.3 (Table I), indicating the presence of a 1 to 2 complex. However, instead of a steady decrease in absorbance for solutions of higher mole fractions, the absorbance began to increase again a t a mole fraction of 0.6. From the change in color in solutions of higher mole fractions of uranium(VI), it was apparent that another species of high uranium content was forming. A precipitate slowly settled out of the solutions containing 0.6 and 0.7 mole fraction of uranium(V1). The results obtained in the continuous variations study a t p H 9 were difficult to interpret exactly, because more than one absorbing species was present in basic solution. However, they indicate the possible presence of a 1 to 2 chelate of uranium(V1) and benzohydroxamic acid. The other species observed in solutions which contained an excess of uranium(V1) may be due to some reaction between uranium(V1) and the base added. When uranium (VI) solutions are made basic, the
Table 1.
0.1
0.2 0.3 0.4 0.5 0.6 0.7
792
0.720
0.592 1.043 1.053
365mp 0.075 0.368 0.812 0.685
0.560 0.855
0.800
ANALYTICAL CHEMISTRY
9
PH
yellow color becomes slightly more intense. Tridot (6) explains this intensity as the formation of NaU05, WazKO6,and Na4U08, successively. The experimental results indicate that uranium(V1) and benzohydroxamic acid react to form a t least two species in aqueous media. In acid solution, in n-hich the ionization of benzohydroxamic acid is repressed, the formation of the 1 to 1 complex is favored. However, in basic media, the concentration of the benzohydroxamate ion favors the formation of a 1to 2 complex. Nature of Precipitate Formed. In a series of tests it vias observed that if a uranium concentration of 1.6 X 10-3Mwere exceeded and if a large excess of benzohydroxamic acid n-ere present, a precipitate started to form a t pH 6. This precipitate mas orange-brown after it had been filtered from the solution and dried. Its composition is U02(C6H5CONH0)2. H20 based on micro-analyses in which the carbon and nitrogen values were identical with theory and hydrogen was 2.88 instead of 3.1070.
Continuous Variations Study of the Uranium(V1)-Benzohydroxamic Acid System at pH 9.0
360mp 0.075 0.380 0.840
7
6
Figure 3. Effect of pH on extraction of the uranium(V1)benzohydroxamic acid system with 1 -hexanol
(Total No. of moles, 1 nu‘VI’
5
x
-4bsorbance 370mp 380mp 0.053 0,060 0.328 0.358 0.733 0.798 0.590 0.650 0.460 0.525 0.580 0.728 0.450 0.640
VARIABLES AFFECTING THE METHOD
Ratio. Figure 2 shows that a 20 to 1 or greater ratio of benzohydroxamic acid t o uranium (VI) should be used to obtain maximum absorbance but is not necessary to obtain good analytical results. If a lesser ratio is to be satisfactory, it must be rather carefully controlled. Effect of pH and Temperature on Extraction. 1-Hexanol was used as the extracting agent in preference t o other alcohols because it is cheap and easily purified, and separates from water very rapidly. Figure 3 shows the results of an extraction study. This extraction shows a marked dependence upon p H with a maximum absorbance a t 6.2. Control of pH to 6.2 =t 0.1 keeps the error within 1%. Beyond these limits the error increases more rapidly, as shown in the figure. It is the 1 to 2 species which extracts. Normal room temperature variations of a few degrees had no effect on the results. Conformity to Beer’s Law. The ability of the uranium(V1)-benzohydroxamic acid system to follow Beer’s law in an organic extractant such as 1-hexanol was demonstrated within the usual 1 to 5% error of photometric measurements. Reagent-Metal
STABILITY STUDIES
400mp 0.084 0.243 0.594
420mi 0.018
0.165 0.440
0.450
0.345
0.330 0.242
0.425
0.345 0.270
0.310
Stability of Alcohols. 1-Hexanol as well as other immiscible alcohols will not extract the uranium(V1)benzohydroxamic acid chelate quantitatively if the alcohol is used several days after it has been purified by distillation, because of traces of hydrogen peroxide which form in the
'g
E' 0 aD In
I-hexanol
W
0 2
sU
8 m a
1
o'8
0.71 0
I
I
I
I
8
16
24
32
.
DAYS
Figure 4.
Stability of aqueous benzohydroxamic
alcohol. They can be removed by distillation. Stability of Reagent. It mas reported (7) that benzohydroxamic acid rapidly lost its strength in basic solutions, but no quantitative data were available on its stability in acid or neutral solutions. An aqueous 0.01M benzohydroxamic acid solution was prepared a t p H 6.; and 2.2. These solutions were used periodically to form the uranium chelate (Figure 4). There is a rather rapid loss in strength initially, but it tapers off. The acidified solution is more stable, as expected. Stability of Extracted Chelate. The stability of the extracted system was determined in cells sealed with wax t o prevent evaporation (Figure 5 ) . The color of the m-ater solution of uranium(Y1)-benzohydroxamic acid was used as a comparison. The aqueous solution faded very rapidly, but a t 380 mp the absorbance did not appear to change very much. The 1hexanol system is the more stable. Effect of Diverse Ions. The effect of diverse ions was investigated by adding the desired compound to the system before the reagent was introduced. The experimental procedures were not altered in cases in which precipitate formation resulted. Because the uranium-benzohydroxamic acid system is very similar to the vanadium- b e n z o h y d r o x a m ic acid system of Wise and Brandt (Y), it was not necessary to test the effects of all the diverse ions, as those which interfere by reaction with the reagent or precipitate formation will behave in a similar manner. The data in Table I1 show the ions of primary concern for this problem. Averages of two or three values are reported. Agreement was within the usual 1 to 1.5y0of qpectrophotometric data. On the basis of this study and that of Wise and Brandt ('i'), the following generalizations can be made. The compounds which interfere with the method are strong oxidizing agents that attack the reagent, strong reducing
acid
0.21
0.01 0
1
I
8
I
1
32
24
16
DAYS
Figure 5.
Stability of the extracted chelate of uranium(V1)
Table II.
Effects of Diverse Ions
Added Substance Added Th(NO3)4 Ce(NO3)3 ( N H ~ ) B M O 14H20 ~OZ~. Na2WO4.2H20 SaC2H3O2 HZO4 HClOa NaF SapS04 KaC1
2 3 4 2 4 2 2 4 2 2
38 38 76 38 76 36 38 T6 38 38
agents that reduce uraniuni(V1) to uranium(IV), or ions that react with the reagent to deprive the uranium(V1) of sufficient reagent for complete color development, Aluminum(II1), tin(IV), zirconium(IV), and titanium(1V) would appear in this latter category even a t the higher p H (7).
Uranium, Mg. Found Diverse Ions Present, MoIe 0.1 3 38 2 36 4 76 2 35 4 72 2 38 2 38 4 72 2 38 ? 38
1.0 2 34 2 31 4 72 2 32 4 48 2 38 2 38 4 66 2 38 2 38
5
10.0 2 25 2 34 4 66 2 16 4 30 2 32 2 38 4 15 2 38 2 38
droxamic acid-thorium, molybdenum, tungsten-an be reduced or eliminated. Cerium(1V) reacts d h the reagent but cerium(II1) has no effect. Therefore excess reagent reduces cerium(1V) to cerium(II1) and reduces the interference. LITERATURE CITED
SUGGESTED PROCEDURE FOR DETERMINING URANIUM(V1)
To a solution containing not more than 5 x 10-6 mole of uranium(V1) add 10 ml. of 0.1M benzohydroxamic acid. Dilute the solution to 50 to 100 ml. with distilled water and adjust the pH to 6.2 =k 0.1 with 0.3M sodium hydroxide. Extract the solution with a pipetted 25-ml. portion of 1-hexanol. Centrifuge if separation of the layers is not complete in 5 minutes. Measure the absorbance of the extract a t 380 ._ mp against a 1-hexanol blank. Convert the absorbance to the amount of uranium(V1) with the aid of a calibration curve. ' The range of application can be controlled by varying the amount of extractant. By increasing the concentration of reagent the interference of several diverse ions that react with benzohy-
(1) Bhaduri, A. S., Ray, P., Science and Culture (India)18,97 (1952). ( 2 ) Das Gupta, A. K., Gupta, J., J. Sei. Znd. Research (India) 9B, 237 (1a.m)
(3) Dhar, S. K., Das Gupta, A. K., Zbid., 1 lB, 500 (1952). (4) Dyfssen, David, Acta. Chem. Scand. IO,333-9 (1956). ( 5 ) Job, P., Ann. chim. 10 (9), 113 (1928). (6) Tridot, Gabriel, Zbid., 10, 225-70 (1955). ( 7 ) Wise, W. M., Brandt, W. W., ANAL. CHEM.27,1392 (1955). RECEIVEDfor review July 27, 1959. Accepted February 8, 1960. Abstracted
from theses submitted by Clifton Meloan (January 1959) and Paul Holkeboer (August 1956) to the Graduate School, Purdue University, in partial fulfillment of the requirements of the degree of doctor of PM"SoPhY* Work supported by the Purdue Research Foundation and the officeof Ordnance Research, contract KO,D.4-33-008-ORD-607. VOL. 32, N O . 7, JUNE 1960
0
793
