in order to correctly define the extent of ester saponification. To properly define the results obtained for hindered esters by the described procedure, an assumption must be made that the actual alkali attack on the glass is uniform for both the sample and the reference reactions. To determine the validity of this assumption, all saponification reactions involving highly hindered esters were conducted using a random distribution of the glassware. The excellent precision of the results obtained by different operators in this laboratory show that within experimental limits the above assumption is correct.
ACKNOWLEDGMENT
The author expresses sincere appreciation to L. D. Fisher, F. C. Veatch, G. R. McNitt, and C. F. Maddox for their assistance in this work, to C. M. Starks for helpful suggestions, and to C. E. Thompson and the Petroleum Products Division for a generous supply of the hindered esters. LITERATURE CITED
J. T., et al., Eds. Interscience, S e w York, 1954. (3) Isagulyantz, V. I., Ti-. X o s k . Inst. Seftekhzm. z. Guz. Prom. 24, 286-97 (1957). (4) Johnson, A. E., Lawrence, R. V., ANAL.CHEW27, 1345-6 (1955). (5) Koroton, I. S., Byrodov, V. A,, Izv. Vysshckh. C‘chebn. Zavedenz, Lesn. Zh.
2 . 150-4 ilOS9).
(6) ’Lucius, ‘BruAng, German patent Y o . 1,011,392 (Julv 1957). (7) Shaefer, W.E., Bslling, W. J., AXAL. CHEM.23, 1126-8 (1951). (8) Tobev, S. W.) MeGregor, S. D., Cottrell, S. L., J . Chem. Educ. 38, 611-13 (1961).
(1) Andreas, F., Chem. Tech. ( B e r l i n ) 1 1 ,
24-8 (1959).
(2) Hall, R. T., Shaefer, W. T., “Organic Analysis,” Vol. 11, pp. 20-70, Mitchell,
RECEIVEDfor review May 4, 1964. hccepted July 16, 1964.
Fluorometric Determination of Uranium with Rhodamine B N. R. ANDERSEN’ and DAVID M. HERCULES Department of Chemistry and laboratory for Nuclear Science, Massachusetts Institute of Technology, Cambridge, Mass. 02 1 39
b A rapid, direct, fluorometric method for uranium has been developed utilizing the reaction between the uranylbenzoic acid complex and Rhodamine 8. The uranyl complex is extracted into a benzene layer containing Rhodamine 8, and the intensity of fluorescence produced is proportional to uranium concentration. The fluorescence intensity is a function of benzoic acid concentration, Rhodamine B concentration, pH, volume of the aqueous phase, and time of ultraviolet irradiation. A variety of cations and anions have been checked for possible interferences. The lower limit of detection is about 5 X 1O-*M uranium (about 12 p.p.b.).
F
LUOROMETRIC
DETERMINATION
Of
uranium in the past has been carried out almost exclusively by the sodium fluoride method ( 7 , 9-11, 19). This method is very sensitive and quite specific, but is inconvenient because of the necessity for firing the sodium fluoride pellet. Other fluorometric methods for the determination of uranium have employed silica gel (IS), uranium-HC1 complexes ( 8 ) changes in fluorescence intensity as a function of calcium (16) and chromium ( 1 7 ) concentration, L7Cls in LaC13 ( 3 ) , and direct measurements of the fluorescence of minerals ( 4 , 6 ) . However, these determinations are no more desirable than the sodium fluoride method because of
experimental complexities or, for silica because it is only semiquantitagel (I8), tive. Feigl (5) has reported that when a neutral aqueous solution of uranyl ions is shaken with a benzene solution of Rhodamine B, the uranyl ions are extracted into the organic phase. The benzene layer turns red and exhibits an intense orange fluorescence when exposed to ultraviolet radiation. The use of Rhodamine R in quantitative fluorescence measurements has been demonstrated recently for the determination of thallium ( f a - l g ) and indium (2). The present communication reports a direct fluorometric method for the determination of uranium using Rhodamine B and benzoic acid as reagents. The procedure developed is as sensitive as methods currently being used, but has the advantage of being simpler and more convenient for general laboratory use. The present method is being incorporated into actinium-227-uranium-235 disequilibrium studies in sea water and marine sediments ( I ) , where numerous analyses must be carried out routinely.
~
1 Present address, Laboratory of AIarine Science, Woods Hole Oceanographic Institution, Woods Hole, M a s s .
2 138
ANALYTICAL CHEMISTRY
EXPERIMENTAL
Reagents. Fisher Scientific Co. Spectrograde benzene was used. Rhodamine was purified according to the procedure of Ramette and Sandell (16). All other chemicals ivere reagent grade and all were used without further purification. STAKDARD t 7 ~ ~ 4 SOLUTIOS. ~ ~ ~ m ~ V02(X03)2.6H20,0.0484 gram (J. T.
Raker), was dissolved in deionized H20 and diluted to 1.000 liter. The final solution contained 23.0 p.p.m. uranium calculated from the uranium salt weighed out. BEKZOIC ACID SOLUTION (SOLUTION A). Benzoic acid, 3 . 5 weight % (Fisher Reagent), in benzene. RHOD.4WNE B SOLUTION (SOLTJTION B). Purified Rhodamine R,10 mg., was transferred to a 100-ml. volumetric flask and 100 ml. of benzene were added. The solution was allowed to remain in the dark for about 1 hour with intermittent shaking. The solution was then filtered on Khatman No. 41 filter paper to remove the undissolved Rhodamine R. Apparatus. A Turner Model 110 fluorometer was used for all fluorescence measurements. A S o . 47-B filter (Turner S o . 110-813), a narrow pass filter peaking a t 436 mM, was used as a primary filter, and a S o . 26-12 filter (Turner S o . 110-518), a sharp cutoff filter at 510 mk, mas used as a secondary filter. -1 Leeds and Northrup Model 7664 line-operated pH meter was used for all pH measurements. Procedure. d d j u s t the pH of the sample to 5.0 (v.ith 0 . l X S a O H ) using a pH meter. Pipet 1.00 ml. of a uranium solution, containing less than 2 X 10-jJf uranium, into a 60-ml. separatorp funnel. Transfer 5.00 i d . of solution .1 and 5.00 ml. of solution B into the same separatory funnel. Extract the uranium-benzoic acid complex into the benzene phase by h a k i n g for 1 minute. Allow the phases to separate. Remove the aqueous phase and discard it. Transfer the organic phase to a cuvette taking care that no water droplets are transferred because they
may adhere to the cuvette wall and cause an error caused by scatterina A blank of 1.00 nil. of deionized H2O is treated in the same manner a< the sample. The fluorometer is adjusted to read 0.07, fluorescence with the blank. The relative fluorescence intensitv of the samr)lrl is then measured directly. Care must be taken to keen all alassware extremely clean and d r i . K&mg all elassv,are in alcoholic KOH and spectrograde acetone proved to be sufficient to remove interference from diverse ions and moiature on the glass\tar’. Standard solution$ of uranium were used to prepare a standard calibration curve of fluorescence intensity us. uranium concentration. The unknown concentrations of uranium were read off the graph dii ectly. RESULTS AND DISCUSSION
Table I shows ,the variation of relative fluorescence intensity with uranium concentration. 1 calibration curve was established from these data and was checked daily to ensure that the calibration was accurate to within experimental error. The relationship between fluorescence intensity and uranium concentration is linear to about 1.6 x lO-EM; beyond that a slight curvature is observed. The slope of the curve can be varied by changing the slit setting on the Turner fluorometer without affecting the linearity of the curve. Extrapolation of the data from Table I yields a lower limit of detectability of about 5 X 10-8LlIuranium (about 12 1l.p.b. Table I1 is a compilation of measurements performed on :standard uranium solutions over a period of 3 weeks. h solution was prepared on a given day by dilution of the standard uranium stock solution and the analysis performed without adjustment a f the instrument, except for zeroing of the blank. Therefore, these data represjent a measure of the between-days reproducibility of the method, and give a relative standard deviation of L 1 ,Oc/;,. n’it,hin days variation and accuracy of the method were checked using ? , t a n d a d uranium solutions as shown in ‘Table 111. These data indicate clearly that the method is capable of yielding both accurate and precise results. Rhodamine I3 in benzene exists almost entirely in the colorless lactone form (I). K h e n uranium is extracted from aqueous solutions into benzene, in the presence of benzoic acid, it is probably extracted as the complex (11) (f2). In benzene, the complex and the lactone form of Rhodamine B probably react to form the ion pair (111), which contains the conjugated form of Rhodamine B as a cation, responsible for its red color and brilliant fluorescence.
r).
the mechanism for the reaction suggested above. The large concentration of benzoic acid used in this investigation ensures quantitative extraction of the uranium with the benzene phase. Fluorescence Intensity as a Func-t H~U02(~C00)3~ tion of pH. T h e variation of fluores(11) cence intensity n i t h pH of the aqueous phase is shown in Figure 2 . There is a slight variation in fluorescence intensity ( 2 . 5 7 0 ) between pH 4 and [UOn(+COO) 31L
---?-
@=O (1)
Table I. Relative Fluorescence Intensity as a Function of Uranium Concentration
Fluorometer slit KO. 3 Relative fluorescence intensity, 70 Uranium concentration
Fluorescence Intensity as a Function of Reagent Concentration. Fig-
ure 1 shows t’he variation of fluorescence int,ensity with Rhodamine B concent,ration. The number of millilit’ers of solution B indicated were added t o t h e separat,ory funnel, along with sufficient benzene to make the volume of the organic phase 5.00 ml. It’ is evident t h a t , a t concentrations corresponding to 3 ml. of Rhodamine B added, fluorescence intensity became independent of Rhodamine 13 concentration. This indicates that negligible error will arise from small variation in Rhodamine €3 concentration introduced by preparing the solution or by pipetting error. Plotting relative fluorescence intensity as a function of benzoic acid concentration gave a curve similar to that shown in Figure 1 with the plateau occurring a t 2.6y0 benzoic acid. To ensure a margin of safety, all benzenebenzoic acid solutions were prepared to contain 3.57( benzoic acid. The observation that fluorescence intensity becomes independent of the concentration of Rhodamine B, or benzoic acid, a t high reagent concentrations is entirely consistent Kith
Table 111.
Taken,
Found,
p.p.m.
p.p.m.
2.300
1.150
0.392
0.130
2.30 2.31 2.31 2.28 1.17 1.14 1.16 1.14 0,402 0,388 0,394 0.392 0,135 0.126 0.131 0.133
9 63
2 4 4 4 11 5 16 0 22 4 38 0 42 0
1 4 6 9 1 1
x
i o - 7 ~
89 x lO-G*lf 82 x 10-fiM 75 X 10-”M 70 x l0-8M 64 x 10-5.w 93 x 1 0 - 5 ~
Table II. Measurements Performed on Standard Uranium Solutions Randomly during a 3-Week Period Fluorometer slit S o . 10: concentration of solutions, 9.66 X 10-fiM (2.30 p.p.m.)
Relative fluorescence intensity,
7c
42.2 42.2 41.6 41.2 42.8 41.2 41.8 41.8 41.8
Analyzed uranium concentration 9 9 9 9
70 x 10-6M 70 X 10-6M 60 X 10-6.1f 49 x 10-6M
9 64 X 10-6M 9 64 X 1W.V 9 64 f 0 10 X 10-6.1f
Analysis of Synthetic Uranium Samples
Av. f std. dev.
Relative std. dev.
Rel. error
2 . 3 0 f 0.014
0.61
0.00
1.15 f 0 . 0 1 5
1.31
0.00
0.394
0.006
1.53
0.51
0 . 1 3 1 Z+Z 0.005
3.64
0.77
z!=
VOL. 36,
NO. 11, OCTOBER 1964
2139
701
60
7 0
L
6o
50L
t
I
I
I
I
I
I
I
IO
I
I
2.0
30
I 4.0
I
5.0
I
6.0
I
I
I
20
80
9.0
rnl Solution B
Figure 1 , Variation of relative fluorescence intensity with concentration of Rhodamine B Uranium concentration = 9.66 X lO%i; Fluorometer slit No. 10; total volume of organic phase w a s 5.00 ml. for each run
Figure 2. Variation of relative fluorescence intensity with pH of the aqueous phase Uranium concentration =
9.70 X 1 O - W ;
fluorometer slit No. 10
diverse ion-uranium
Table IV. Relative Fluorescence Intensity as a Function of Volume of Aqueous Phase
Fluorometer slit No. 3 Relative fluorescence intensity, v01. of % aqueous phase, ml. 1.00 2.00 3.00 4.00
36.6 23.3 18.4 13.4
Table V. Effects of Diverse Ions 2.30 p,p.m. uranium used in all cases; all solutions 100.00 ml.
u
Additive La+3 c u +2 Fe+3 Fe’3
Amt., mg.
10.0 10.0 10.2 9.P Fe-3 11 Fe+2 10 0 Fe+2 9 2a Fe+2 1 1 Ph+2 10.8 T h C 4 10.2 T h + 4 10 2 Th+4 1 4 ZrO+2 11 8 Cr’3 10 0 SiOnc2 30 9
Xi64-216
po4-3
5
io o
X/V ratio 43: 1 43: 1 44: 1 43: 1 5 :1 43: 1 40: 1 5 :1 47: 1 44: 1 44: 1 6: 1 51: 1 43: 1 134: 1 9: 1 84: 1 10: 1 72: 1 43: 1 3: 1 100: 1 43: 1 42: 1
Rerecovered, covery p.p.m. 2.38 2.29 tinan spectral grade tlimethylformamide (D 11F)-redistilled 95% e thnnol. The substituted 2,2'-dihydrosyazomethines were prepared according to the directions below. The aldehyde.: which were not comnicrcially available were prepared by the I h f f reactiun (4). The corresponding amine (0.01 mole) '
