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ANALYTICAL CHEMISTRY, VOL. 50, NO. 3, MARCH 1978
over the wavelength range of observed emission). This result implies the validity of the "average plate luminescence" assumption made earlier. However, the absolute CI, response, which depends on S, is not independent of the aperture and increases as the aperture does, as expected. It would be desirable, then, to increase A as much as possible to achieve better sensitivity and lower detection limits. However, a fourth set of spacers constructed to this end, and having IC = 1.5 cm and h = 2.5 cm, did not rinse efficiently enough for analytical use. Thus, the requirement for efficient rinsing places an upper limit on the cell width. For our flow system, this limit is a t about 1 cm. Presumably, a limit is placed on cell depth in the same way; however, we did not use any cells with depths greater than 1 cm. Obviously, need for a reasonable sample size and reactant volume will also limit the cell dimensions. Use of a CL system which has a large value of tc will also place a definite upper limit on cell depth; in the case of gallic acid, use of cell depths greater than about 0.4 cm is a waste of reacting solution. From Equation 10, if r = 1, and P, = S(2.3 tc)-I which is the response observed for a cell of infinite depth, then PQt= P, (1 10-*"'). For gallic acid cc = 1.6, and a t the practical maximum value of 1 which is 0.4 cm, Ptot= 0.95 P,. By the time tcl has increased to 0.65, about 95% of the maximum response is obtained. As tcl becomes very large, then, the reflectance of the rear wall has no effect on the CL observed. This is seen experimentally and theoretically in Figure 6. For gallic acid, beyond about 0.4-cm cell depth, only light which is front emitted (P,,) is detected and the responses of cells having r = 0 and r =
1 become identical. Thus, the desirability of a CL flow cell to collect light that is not front emitted (Pb,J is dependent on the absorbance of the resident solution. All of the work reported in this paper was done with a stopped flow delivery system ( I ) , but the results and conclusions are applicable to both stopped flow and continuous flow systems. We routinely use cells based on this modular design for measurements in both stopped flow and continuous flow systems. Results dealing with the effect of tc, I , and r on the observed signal are also applicable to CL measurements which are not done in flowing streams.
ACKNOWLEDGMENT We thank Richard Geiger for helpful discussions concerning the mathematical model.
LITERATURE CITED (1) S. Stieg and T. A. Nieman, Anal. Chem., 49, 1322 (1977) (2) E. W. Cottman, R. B. Moffett, and S. M. Moffett, Proc. Indiana Acad. Sci., 47, 124 (1937). (3) U. Isacsson and G. Wettermark, Anal. Chim. Acta, 83, 227 (1976). (4) W. R. Seitz and M. P. Neary in "Methods of Biochemical Analysis", Vol. 23, D. Glick, Ed., John Wiley and Sons, New York, N.Y., 1976, p 169. ( 5 ) Model FSA 980, 2n stearadian flow-throuqh cwette, Schoeffel Instrument Corp., Westwood, N.J. (6) D. Slawinska and J. Siawinski. Anal. Chem., 47, 2101 (1975). (7) J. Lee and H. Seliger, Photochem. Photobiol., 11, 247 (1970). (8) NBS, Opt. Rad. News, Sept. 1975.
~
RECEI~ZD for review September 27,1977. Accepted December 19, 1977. This work was supported, in part, by a grant from Research Corporation.
Flow Photometric Monitor for Uranium in Carbonate Solutions B. B. Jablonski and D. E. Leyden" Department of Chemistry, University of Denver, Denver, Colorado 80208
The reaction between U( V I ) and 2,3-dihydroxynaphlhaIene-6-sulfonic acid is the basis of a continuous flow photometric monitor for uranium in carbonate solution. Linearity in the 0-100 ppm range has been observed with relative standard deviation of 1.1 YO at 60 ppm uranium and a lower detection limit of 3.5 pprn. The applicability of the method has been tested by analyzing process samples from a solution mining operation. Very few interferences have been observed.
Uranium can be effectively recovered from low grade deposits through the use of a solution mining technique which has only recently been applied to uranium mining. The process relies on the solubilization and complexation of uranium in the ore deposit by ammonium carbonate solutions. Carbonate forms a strong complex with the uranyl cation, facilitating its solubilization. When the uranium is in solution, it can be removed from the deposit. The next step in the process is to recover the uranium as U02(C03)14-or UOn[ (C0J*.2Hz0]*-on an ion-exchange bed. Because the anionic complex is recovered, quaternary ammonium ion exchangers are used. The removal of uranium from solution can be improved by cycling the carbonate host through the bed. Once the uranium concentration in the bed effluent drops below several parts 0003-2700/78~0350-04n4$01.00/0
per million, further cycling no longer improves the recovery of uranium and, for economic reasons, the cycling process is stopped. A t present, it can take considerable time to obtain a sample of the bed effluent, transport the sample to the laboratory, and determine the amount of uranium in the sample. A simple on-line monitor for uranium would improve the efficiency of uranium processing by reducing the time spent waiting for analytical results. Numerous methods for determination of trace amounts of uranium exist. An early review of methods for uranium determination covers a wide range of techniques ( I ) . Colorimetric methods have enjoyed wide popularity. Rodden ( 2 ) lists at least 40 different colorimetric procedures for uranium. The most widely used photometric procedure involves extraction of the sample with trioctylphosphine oxide (TOPO) and the use of either dibenzoyl methane or 4-(2-pyridylazo)resorcinol as the colorimetric reagent ( 3 ) . Other instrumental methods of analysis have been applied to the problem of uranium quantitation. A partial listing of techniques includes neutron activation analysis ( 4 ) , UV fluorescence (2), x-ray fluorescence (5), polarography ( 6 )and spectrophotometry (2). Despite the abundance of methods for uranium analysis, very few applications of these methods to automated analysis have been found. In 1958, Bertram et al. (7) published the design of a flow system for a polarographic monitor for 0 1978 American
Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 50, NO. 3, MARCH 1978
1
SAMPLE
DELAY COIL
2
to
405
flowcell
CyDTA
Figure 1. Flow system design
uranium in process solutions. The system is critically dependent on accurate dilution of the sample with supporting electrolyte and requires close control of temperature. Recently, a continuous flow method designed for a Technicon AutoAnalyzer has been proposed ( 8 ) . The method is a colorimetric procedure employing 2-(2-pyridylazo)-5-diethylaminophenol (PADAP) as the colorimetric reagent. The reported relative standard deviation is 2-3%. The technique which is intended t o be used with acid digested ore samples is not totally automated and involves an extraction of the ore sample or leach solution into TOPO. The extraction is performed manually and the organic phase is manually placed in the Technicon sampler. The complexities of existing instrumental methods and the requirement of an acid p H for most colorimetric procedures make it difficult t o automate uranium analysis. We have developed an unsegmented, continuous flow monitor for uranium based on the colorimetric reaction between uranium and 2,3-dihydroxynaphthalene-6-sulfonic acid a t alkaline pH. The reagent was chosen on the basis of its selectivity for uranium and that color development takes place in alkaline solution (9, 10).
EXPERIMENTAL Reagents. All reagents employed were reagent grade. A stock solution of 1000 ppm uranium (as U)was prepared by dissolving 1.7819 g of uranyl acetate (Baker) in 1 L of deionized water. Solutions for the standard curves were made by dilution of the uranium stock solution with appropriate amounts of either deionized water or ammonium carbonate. Stock solutions of 10 M NaOH, 20% (NHJ2C03and 5% cyclohexanediaminetetraacetic acid (CyDTA, Aldrich) were also prepared. A 64% solution of hydrazine (Eastman) was used as received from the manufacturer. The mixture of reagents indicated in Figure 1 (pump channel No. 3) was prepared by mixing 20 mL of the hydrazine solution with 200 mL each of the sodium hydroxide and ammonium carbonate solutions. A 3% solution of 2,3-dihydroxynaphthalene-6-sulfonic acid, sodium salt (Pfaltz and Bauer) was prepared daily since the reagent oxidizes upon exposure to atmospheric oxygen for long periods of time. Oxidation of the reagent increased the blank by 0.002 absorbance unit over a period of 18 h. This change in absorbance is equivalent to 0.8 ppm uranium. Apparatus. The flow system employed a $-channel peristaltic pump (Polystatic) as shown in Figure 1. Latex tubing inch) was used in all four pump channels. Tygon tubing inch) served as transmission tubing. All connections were made with Technicon AutoAnalyzer glass connectors. The delay coil was made from 40 inches of 3/32 inch Tygon tubing mounted on a glass support. A Perkin-Elmer model 200 double beam spectrophotometer was used for detection. A Savant Precision Cell (Savant In-
struments, Inc.) with 250-pL volume and 10-mm path length was used in the sample beam. A cuvette filled with deionized water was placed in the reference beam. Procedure. In laboratory studies the uranium solution was manually placed in the sample channel of the flow system (channel No. 1. Figure 1). For on-line analysis, this channel can be connected to the process stream. Enriched process samples were analyzed after dilution by an appropriate amount to bring the concentration into the linear range. Once steady state was achieved, a percent transmittance measurement was made from the digital readout of the spectrophotometer. Both the uranium stock solution and concentrated process samples were also analyzed by oxidimetry as described by Main (11). The titration procedure involved pre-reduction of the sample with SnC12and titration against standard 0.02 N K2Cr207to a diphenylaminesulfonate endpoint.
RESULTS A N D DISCUSSION The success of the carbonate solution mining process is dependent on the large overall formation constant for uranyl-carbonate complexes (12).
uo,2++
2CO32- 3
[ u 0 2 ( C 0 3 ) , ~ 2 H 2 0 ] 2 p~2 = 4 x UO,” + 3C0,2- 2
ioi4
(11
u0,~c0,~,4-P , = 2 x 1 0 1 8 (2) Unfortunately, the stability of the complexes limits the possibility of a quantitative reaction between uranium and any known colorimetric reagent. T o free uranium from carbonate so that the colorimetric reaction may take place, concentrated NaOH is added to induce the formation of a diuranate anion (13): ZU0,(C0,),4-
- 6 0 E - c’ U , 0 , 2 - + 6C0,2- t
3H,O
(3)
The reaction takes place above pH 11. Because the uranium is liberated from the carbonate complex, it is then able to undergo the reaction with the colorimetric reagent more quantitatively. Compared to acidic leaching, the presence of carbonate in the solution mining process streams is a n asset as far as possible interferants are concerned. In solutions with a moderate to high carbonate content very few ions are soluble, thus reducing the number of interferants in the analysis. Unfortunately, in practice the carbonate concentrations have been observed t o vary as much as a factor of a thousand (0.01% to 10%)creating interference problems in solutions with a low carbonate content. Ca2+and Fez+ may be present in such solutions. T o prevent precipitation of Ca(OH)2 and Fe(OH)2upon addition of the NaOH, CyDTA is employed as a masking agent. CyDTA was chosen rather than EDTA
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ANALYTICAL CHEMISTRY, VOL. 50, NO. 3, MARCH 1978
Table I. Statistics of Resultsa
Trial No. 1 2 3
4 5 Mean Std dev RSD, % Lower limit a
Uranium found, PPm 8.25 9.40 8.25 7.09 8.25
Table 11. Interference Study
Trial No. 1 2
3 4 5
8.25 0.817
Mean Std dev 9.89 RSD, ?k of detection, ppm
Uranium found, PPm 60.6 59.4 59.4 60.6 59.4
Uranium taken, ppm 100
hl00,2-
59.9 0.658
50
Ca
a
d
-
u
w
Fe( 11)
1000 10000 100 1000 1 0 000
2 5 10 1OQ
1.1 3.5
Samples are in 1%carbonate solutions.
BI0"b
Interferant VO;
Concn, PPm 100
L
slant
Figure 2. Response of flow cell
because CyDTA depressed color intensity to a lesser degree than other complexans of this type (14). A second problem associated with varying carbonate concentration is that for a given concentration of uranium, the absorbance of sample solutions will decrease with increasing carbonate. T o avoid the problem of carbonate dependence, a sufficient amount of 20% (NH4)2C03is added to bring the carbonate concentration of all samples above IO%, a t which point the absorbance is no longer dependent on the carbonate concentration. At the high p H involved in the analysis scheme, 2,3-dihydroxynaphthalene-6-sulfonicacid is susceptible to oxidation. In the presence of hydrazine, no time dependence is observed and oxidation is effectively prevented. The colorimetric reagent forms a tris complex with uranium above p H 10. The complex obeys Beer's law up to 100 ppm uranium. Above 100 ppm, a negative deviation from linearity is observed. A least squares fit of the data for the calibration curve evaluated the equation of the line to be Abs = 1.6 X 10-4(ppm U)-1.1 X The correlation coefficient was calculated to be 0.999 with Student's t = 140. Standards for the calibration curve were 1% in (NHJ2C03. The precision of the analysis was evaluated at 8 ppm and 60 ppm uranium. The precision a t low concentration of uranium (8 ppm) was found to be 9.9%, improving to 1.1% a t 60 ppm. Data relating to precision of the analysis are presented in Table I. A lower limit of detection, taken as the concentration of uranium necessary to give an absorbance equal to three times the standard deviation of the blank determination, was calculated to be 3.5 ppm. In designing the flow system, the philosophy was to keep the device as simple as possible. Therefore, no bubble segmentation is used. In order to minimize the number of pump channels required to deliver the reagents, a mixture of reagents is used in the pump channel No. 3 (Figure 1). A pump rate of 18 mL/min was employed throughout this study. As can be seen in Figure 2, washout time for the flow cell is approximately 0.5 min. An analysis time of approximately 1.5
Uranium found, PPm 100
99.8 102.2 100.1
99.6 106 69 95 91
50
31 50.1
100 1000 2 000
49.9 49.8 47.5
Cation exchange column used t o remove Fe(I1).
min per sample was observed at the flow rate employed. A total reagent volume of 20 mL is consumed per analysis of 6 mL of sample. Since the system has been designed for on-line analysis, there is no shortage of sample and sample consumption as high as 6 mL is not prohibitive. Since the reagents are inexpensive and readily available, reagent consumption does not preclude the utility of the system. Reduction of both sample and reagent volumes by 50% can be reasonably expected in an on-line application. In acquiring the data presented here, the system was allowed to achieve steady state before a new sample was introduced to the flow system. No memory effect has been observed in the tubing. Figure 2 is a recorder trace of the Perkin-Elmer instrument showing the response (as %'or) of the system to alternate injections of 4-ppm and 80-ppm samples. If memory were indeed a problem, analysis of the lower concentration sample after a sample of high uranium content should displace the measurement of the 4-ppm sample to a higher absorbance (lower % T ) reading. In fact, no such effect is observed. As mentioned previously, very few interfering metal ions are anticipated because of the presence of carbonate. The major contaminants in solution mining process streams are low levels (
