kept low enough by redistilling all acids except perchloric. DISCUSSION
Standards are most easily prepared by precipitation of mixed standard solutions in the usual way. Calibration curves should be made for all exposure steps to be used. Data on recovery obtained spectrographically agree well with those of the radioisotope studies but are less precise and need not be prcsented. The calibration curves for the above lines of copper, cobalt, and zinc have correct slope and are straight. The Mo 3170.3 line gives a lower slope (toward log concentration axis), probably because of reversal. Useful concentration ranges, expressed in parts per million in the alumina ash, are: 200 to 10,000 for copper, 10 to 500 for cobalt, 400 to 20,000 for zinc, and 20 to 1000 for molybdenum. Samples containing much iron, such as feces or oxalic acid-ammonium oxalate extracts of soils, give arcing mixtures rich in iron, with some undesirable results. Iron gives a weak line a t 3170.3 A. which interferes seriously with traces of molybdenum. The high iron content suppresses palladium line intensities and probably suppresses other lines disproportionately. When alumina ash FTeights are high, results are apt to be poor because of poor spectroscopic buffering and line interferences, and possibly incom-
plete recovery due to deficiency of precipitants. Too rapid raising of the pH to 5.2 with ammonia can cause precipitation of much calcium and magnesium from solutions rich in phosphate, such as bone ash. Dilute ammonium hydroxide buffered t o pH 8.0 with ammonium acetate should then be used. The method has been used chiefly for determination of copper, cobalt, zinc, and molybdenum in plants, but also for mixed feeds, fertilizers, soil extracts (acetic acid and ammonium acetate extractants), bone ash, feces, urine, animal soft tissue, and blood. No difficulties other than those just mentioned have been encountered. Recoveries of other elements such as nickel, chromium, vanadium, tin, and lead probably are as complete as copper, as claimed by Mitchell. Spectrographic checking of recovery is reliable enough for most purposes and the high precision of the radioisotope results is needed only for special purposes. The precision of the spectrographic measurement may be estimated from a series of 17 exposures of the ash obtained from a composite of alfalfa samples. The standard deviations from the mean were 6.4% for copper, 7.6% for cobalt, 8.4% for molybdenum, and 13.1% for zinc. Mitchell and Scott recently reported considerably better precision (8),but their handling of the spectrographic work, mentioned above, is less convenient,
LITERATURE CITED
Chichilo, P., S echt, A. W., Whittaker, C. J. Assoc. OBc. Agr. Chemists 38, 903 (1955). Cholak J., Hubbard, D. M., Mcdary, R. R., Story, R. V., IND.ENQ. CHEM.,ANAL. ED. 9, 488 (1937). (3) Friedlander, G., Kennedy, J. W;: “Nuclear and Radiochemistry pp. 261-3, Wiley, New York,
2,
1955. (4) Heggen, G., Strock, L. W., ANAL. CHEM.25, 859 (1953). (5) Kolthoff I. M., Sandell, E. B.,
“Textbook of Quantitative Inorganic Analysis,” 3rd ed., p . 122-37, Macmillan, New Yo$, 1952.
Kuenetsov, V. I., Zhur. Anal. Khirn. 9, 199 (1954).
hlitchell, R. L., Analyst 71, 361 (1946).
Mitchell, R. L.. Scott, R. O., Appl. Spectroscopy 11, 6 (1957). Mitchell, R. L., Scott, R. O., J . SOC. Chem. Ind. 66, 330 (1947). Mitteldorf, A. J., Appl. Spectroscopy 6, 21 (1951).
Piper, C. S., “Soil and Plant Analysis,” pp. 272-4, Interscience, New York, 1950. Pohl, F. A., 2. anal. Chem. 142, 19 (1954).
Smit, J., Smit, J. A., Anal. Chim. Acta 8 , 274 (1953). Stetter, A,, Exler, E., Natunuissaschaften 42, 45 (1955). Waldbauer, L., Rolf, F., Frediani, H., IND. ENG. CHEM., ANAL. ED.13,888 (1941). (16) , . Wark. W. J.. ANAL. CHEM.26. 203 (1954). ‘ (17) West, P., Conrad, L. J., Anal. Chim. Acta 4,561 (1950).
RECEIVEDfor review May 31, 1957. Accepted July 31, 1957.
Rapid Spectrophotometric Determination of Submilligram Quantities of Uranium CARL A. FRANCOIS Western Division, The Dow Chemical Co., Pittsburg, Calif.
b Uranyl ion can be separated from contaminants by solvent extraction from an aluminum nitrate solution using tributyl phosphate in iso-octane. Color is developed by introducing an aliquot of the extract into an acetone-water solution of dibenzoylmethane and pyridine. The absorbance of the resulting uranyl-dibenzoylmethane complex is measured spectrophotometrically at 41 0 mp. Common cations, except thorium in a concentration exceeding 10 times that of the uranyl ion, do not interfere. Precision generally within 2% was observed when 0.05 to 0.50 mg. of uranium was determined in a 50
*
ANALYTICAL CHEMISTRY
variety of samples. The speed of analysis was comparable to fluorometric procedures.
A
LIMITATION to
the more widespread use of sensitive colorimetric methods for uranium determination has been the lack of specificity of the available colorimetric reagents. To utilize such reagents frequently necessitates isolation of the uranium in a relatively pure state, a step which may become somewhat exacting and lengthy particularly when submilligram amounts must be separated from a preponderance of other material. Of the available methods for uranium separation, solvent extraction with
tributyl phosphate appears to be a rapid and simple means of isolating the metal in the required state of purity. Tributyl phosphate, a colorless liquid only slightly miscible with water, has been used by many investigators for extracting the nitrates of the actinide elements. McKay (9) has summarized this work. Moore (10) shows the mechanism for formation of the uranyl nitrate-tributyl phosphate complex to be:
+
+
UOz++(aqueous) 2 NOa- (aqueous) 2 TBP (organic) -c U02(NOs)2(TBP)~ (organic)
A summary of the various aspects of tributyl phosphate as a solvent extract-
ant for uranium has been provided by M7right ( I S ) . LeStrange, Lerner, and Petretic (8) used tributyl phosphate for extracting nearly macro quantities of uranium, followed by direct spectrophotometric measurement of the yellow uranyl nitrate-tributyl phosphate complex. They applied a polarographic procedure to smaller uranium concentrations in the extract. Paige, Elliot, and Rein (11) developed a procedure in which the uranium extracted into tributyl phosphate was measured spectrophotometrically in the ultraviolet region, where they reported high sensitivity and little interference. Sodium peroxide (6) and 8-quinolinol ( 3 ) have been used as colorimetric reagents for uranium after its separation by solvent extraction 1% ith tributyl phosphate, but the former reagent is lacking in sensitivity while the latter necessitates additional purification steps before the determination can be made with confidence. Recently Kimball and Rein ( 7 ) presented a rapid and sensitive spectrophotometric procedure in which the color is developed by introducing an aliquot of the tributyl phosphate extract into an ethyl alcohol solution of thiocSanate. Dibenzoylmethane was proposed as a colorimetric reagent for uranium by Toe, Kill, and Black (15) and by P'ibil and Jelinek (12). The methods reported for separating uranium from interferences before using the reagent include ether extraction (15), ethyl acetate extraction ( I ) , mixed ethyl acetate-tributyl phosphate extraction id), extraction with an ethyl acetate solution of dibenzoylmethane ( I d ) , and precipitation of interfering metals with sodium carbonate (8). I n the procedure described here, preliminary separation of uranium is accomplished by extraction with tributyl phosphate in iso-octane. By using an extractant with a lower ratio of tributyl phosphate to diluent than used by others, the levels of extracted contaminants are reduced to noninterfering values. ilddition of aluminum nitrate to the aqueous phase maintains a distribution coefficient high enough to enable virtually complete removal of uranyl ion with a single extraction. The yellow uranyl-dibenzoylmethane complex formed by direct addition of an aliquot of the extractant to an acetone-water solution of dibenzoylmethane is measured spectrophotometrically. The method has been used , for uranium in a variety of ores as well as in aqueous and organic leach liquors. APPARATUS AND REAGENTS
ilbsorbance measurements were made u-ith the Beckman Model DU spectrophotometer and 1-cm. Corex cells. A Leeds & Sorthrup line-operated pH
meter was used for pH measurements. Extractions were carried out in 60-ml., Squibb-type separatory funnels. The aluminum nitrate salting agent was prepared by dissolving 900 grams of aluminum nitrate nonahydrate (J. T. Baker Chemical Co.) in enough distilled water to make 1 liter of solution. The chromogenic reagent was prepared by combining 17.5 ml. of a 1% (w./v.) acetone solution of dibenzoylmethane (Eastman Kodak Co.), 809 ml. of reagent grade acetone, 43.5 ml. of pyridine, and enough distilled water to make 1 liter of solution. Commercial grade 2,2,4-trimethylpentane (iso-octane)from the Phillips Petroleum Corp. and commercial grade trin-butyl phosphate from Commercial Solvents Corp. were used without further purification. The extractant consisted of 1 volume of tributyl phosphate combined with 10 volumes of iso-octane. Standard uranium solutions were prepared by dissolving U. S. Atomic Energy Commission MS-ST grade uraniumoxide (U30s,Mallinckrodt Chemical Co.) in nitric acid and diluting with distilled water to an appropriate volume. RECOMMENDED PROCEDURE
Pipet a volume of aqueous sample not exceeding 5.0 ml. and containing between 0.05 and 0.50 mg. of uranyl ion into a 60-ml. separatory funnel. Adjust the volume to 5.0 ml. with distilled water. Reduce cerium(1V) by adding 1 to 3 drops of 5% (w./v.) sodium sulfite solution. Add 1 drop of Ecresol purple indicator and, by dropwise addition of ammonium hydroxide or nitric acid, adjust the pH to the yellow form of the indicator or to the point where precipitation of metal ions just begins. If titanium or zirconium precipitates during pH adjustment, add approximately 50 mg. of sodium fluoride to complex these cations. If thorium is present, introduce 1 drop of glacial acetic acid a t this point. ildd 8.0 ml. of aluminum nitrate salting solution, mix, then accurately pipet 3.0 ml. of tributyl phosphate extractant into the separatory funnel. Stopper the funnel and shake for 30 seconds. Allow the phases t o separate and discard the aqueous phase. With a dry pipet, introduce 2.0 ml. of the organic phase into a 25-ml. volumetric flask, exercising care to exclude any droplets of aqueous phase which may be clinging to the walls of the separatory funnel. Fill the volumetric flask to the mark with the chromogenic reagent except for samples containing thorium. To the extractions made from the latter. samples, add 1.0 ml. of pyridine and swirl before diluting to the mark with chromogenic reagent. After 1 hour measure the absorbance with a spectrophotometer a t 410 mp. Carry a blank through the procedure. EXPERIMENTAL
Extraction Conditions. To avoid the use of uranium-bearing solvent phase heavier than water, and be-
cause only a single extraction per sample mas contemplated, a tributyl phosphate diluent with specific gravity less than t h a t of water was desired. Ethyl acetate, butyl acetate. xylene, and iso-octane diluents were tested. Ethyl and butyl acetate extracted troublesome amounts of ferric ion from the aqueous phase and were not considered further. Xylene proved sufficiently inert as diluent, but its viscosity was such that minute droplets of the aqueous phase remained suspended in the organic phase after extraction and caused high and variable blanks. Iso-octane was selected because it separated cleanly and rapidly after contact with the aqueous phase and gave no evidence of extracting the common cations. 9 further desirable property of this solvent ivas its relatively lo^ vapor pressure, which enabled a reasonable 11-orking time before evaporation errors became evident. Extraction of uranyl ion into 1 volume of tributyl phosphate 111 3 volumes of diluent, a mixture comnionly employed by others, did not provide adequate separation of uranyl ion from bismuth(III), iron(III), and the cations n ith plus four valences: cerium, tin, thorium. and zirconium. Further dllution of the tributyl phosphate extractant led to reduced extraction of these iorls, but a high concentration of nitrate ion nas necessary In the aqueous phase in order t o maintain a suitably high distribution coefficient for uranj 1 ion. An extraction mixture consisting of 1 volume of tributyl phosphate with 10 volumes of iso-octane was a satisfactory compromise betxeen uranyl ion selectivity and concentration of nitrate ion required. When this mixture was used, only bismuth(III), cerium(IV), and thorium(1V) were extracted in sufficient quantity to interfere in the colorimetric determination. Extraction efficiency as a function of aluminum nitrate concentration is shown in Figure 1. A l o a organic-toaqueous volume ratio was employed to maintain high sensitivity; an aliquot of the aqueous phase was filtered to remove suspended droplets of organic solvent, then analyzed fluorometrically for uranium. Tests proved that equilibrium distribution of uranyl ion between the aqueous and organic phase was attained after 10 seconds' shaking in a separatory funnel. A shaking time of 30 seconds was adopted to ensure this condition. The extraction of free nitric acid from the aqueous phase by tributyl phosphate can result in incomplete extraction of uranyl ion (10). Fluorometric analysis of the aqueous phase after extraction revealed that 9970 of the uranium n-as transferred to the extractant when the pH of the aqueous phase was in the VOL 30, NO. 1, JANUARY 1958
51
range from 3 to 6, whereas extraction was only 86% complete from an aqueous phase 331 in nitric acid. Yeutralizing excess acidity in the presence of mcresol purple. as described in the procedure, is a convenient means of attaining the proper pH; the amount of indicator extracted n ith uranium does not measurably influence the ultimate absorbance of the uranyl-dibenzoylmethane complex. Common cations such as ferric ion may precipitate and mask the indicator color during pH adjustment; in this case the pH is brought to the level where such a precipitate just fails to redissolve. The precipitate redissolves upon addition of aluniinum nitrate salting solution.
"
I
i
W A V E LENGTH
(mu)
Figure 2. Absorption spectra of uranyl- and thorium-dibenzoylmethane complexes in water-pyridine-acetone medium
'
A . 4.37 X 10-4M uranyl ion B . 2.16 X lO-3-W thorium ion C. 2.15 X l O - a M thorium ion with complexing action of acetate during extraction and additional pyridine during spec-
trophotometric measurement
GRAMS
Al(N03),-9H20
PER I3 N L
AQUEOUS
PHASE
Figure 1. Effect of aluminum nitrate concentration on efficiency of uranyl on extraction
3 mi. of extractant; 13 ml. of aqueous solution coiitaining 0.482 mg. of uranyl ion; 30-second shaking time Conditions for Color Formation. Dibenzoylmethane TT as selected as colorimetric reagent because the uranyl complex formed with this compound is stable, exhibits high absorptivity, and adheres t o Beer's 1aJv (15). The relatively narrow pH range in which the uranyl complex forms reproducibly is of no consequence because control of acidity during extraction enables the proper pH range t o be maintained by a buffer. To simplify the procedure, uranyl ion extracted into the tributyl pliosphate-iso-octane phase was not back stripped into an aqueous solution before colorinietric analysis. An aliquot of the organic phase was pipetted directly into an acetone-water solution of dibenzoylniethane containing pyridine as buffer. The colored complex formed with dibenzoylmethane in this system (Figure 2. curve A ) produced an absorption spectrum similar to that obtained in ethanolic medium (15). The low solubility of iso-octane prevented addition of the organic phase to an ethyl alcohol-iyater medium; acetone proved 52
ANALYTICAL CHEMISTRY
a better solvent for the present procedure. Acetone, nater, pyridine, and dibenzoylmethane can be combined and added to the tributyl phosphate aliquot as a single reagent. The proportions recommended were found to be optimum. With this so-called chromogenic reagent, the intensity or rate of color formation did not differ from that attained by separate addition of reagents. The chromogenic reagent exhibited no evidence of deterioration after standing 1 month at room teinperature.
an initial 15-minute standing period in the acid range before adjusting to final pH resulted in l o r and erratic absorbance readings, particularly not,iceable when the final pH was 6.0 or above. Apparently, immediate addition of base results in partial hydrolysis of uraniuin and prevents complete reaction of the uranyl ion nith dibenaoylmet,hane. The p r w n c e of pyridine in t,he acetone solution buffers the syst,rni within an appareiit p H range of 5.0 to 5.5 and enables formation of the colored complex ivithout further manipulation. Prcsence of excess pyridine has no detrimental effect' on the complex. as indicated by the fact that pyridine concentrations in the syst,em varying from 1 to 40% by volume exhibited no noticeable absorbance changes. -14% pyridine concentration provides adequate buffer action. Color is usually developed completely n-ithin 1 hour and remains stable thcreafter. TThen traces of sulfate or phosphate ions are present in the estractant. ho\rever, the rate of color forniatioii is slon-ed considerably. Sucli a condition occurs when extraction is n l d e froin a solution containing 100 nig. or more of these anions. -1bsorhance rnrasureiiients should be made a t 410 nip rather t'lian a t 395 or 400 mpu, the wave length range of maxinium absorbance, because thoriunl and traces of other tetravalent cations coextracted nith uranium form dibenzoylmethane complexes which absorb strongly bel017 410 mp. Although t'liis nicans a small sacrifice in sensitivity. freedom from interference at this w i v e length leads to greater reliability and increased precision. A large excess of dibenzoylmetharle over that required for reaction with uraniuni is undesirable, as the reagent itself absorbs slightly under the conditions of analysis. The chroriiogenic rragent provides enough dibenzoylmethane to result in a twofold excess over that found necessary for full color developnwnt, with the maximum perniissible quantity of uranyl ion (0.5 nip.). INTERFERENCES
Solutions containing 5 mg. or larger quantities of the folloning ions xere carried through the desired proccdure:
/I
0 5
2
3
4
e
3
7
e
9
IO
Figure 3. Absorbance o f uranyl-dibenzoylmethane complex in acetone medium as function o f pH
BO,---, Bay+, Ca+'+, Cd++ Ce+4, Ce+++ Cr04--, Cr++, Cu++, be+T+, Fe++, GL", Hg", K+, Mg++, Mn04-, %In++, Moo4--, ,?ja+, Xi++ Pb-+, Si04-', Sn-4 Sn+', &++, Th+4,' Ti+4, \TO4---, V-4, \TOA--, Zn++, ZrT4
Figure 3 presents the relationship betneen absorbance of the uranyldibenzoylmethane complex and apparent pH in the water-acetone medium. both in the presence and in the abscncc of pyridine buffer. Failure to allon-
Of these ions only bismuth[IIIj, ceriuni(IT'), and thorium(1V) were extracted in interfering quantity. Bismuth may be prevented from extracting by coinplexing n ith disodium Versenate. Cerium(IT-j is rendered nonextractable
PH
Ag+ As04---> Au+'+,
\\-hen it is reduced to the trivalent state with sodium sulfit'e. Thorium interferes seriously by forming a complex with dibenzoylmethane which absorbs in the n - a w length region of the uranyl complex (Figure 2 , curve B ) . Also, the consumption of dibenzoj-linethane by t'his cation prevents coniplete color deyelopment of the uranyl complex. Introducing acetate ion into the aqueous phase before estractiiig uranium exerts some complexing actio11 on thorium and reduces its estrac*tion. Thorium which is estractecl in spit
