Anal. Chem. 1989, 6 1 , 1787-1789
to Spendley (5) and corrects some mistakes in the literature. It should be noted that ref 3 and 4 use different definitions in Table I. It is the purpose of this comment to clarify several misrepresentations of our article (I),not to dispute the validity of Brumby's approach. The following discussion will use the notation of ref 4. In this notation, P1 is a modified simplex method without quadratic convergence (5), P2 uses P1 plus the method of ref 1-3, and P3 uses Brumby's method added to P1. (1) Reference 4 states that single- and double-precision versions of programs P1-3 did not always give the same results "possibly because of errors in calculating the variance of function values". It should be noted that ref 1 codes the simplex algorithm in single precision except for the function values, yi, and variance calculations, which are done in double precision to avoid round-off errors. The work of Chambers and Ertel (6) was cited in ref 1 to reinforce this point. (2) Reference 4 incorrectly states that Phillips and Eyring use the quadratic approximation only for error estimation, and not in locating the precise minimum of the function. (3)Brumby is also mistaken when he states that P2 requires an extremely small value of REQMIN to locate the precise minimum. For example, the six test functions considered in ref 4 used the same value of REQMIN for P2 and P3 and a value 3 orders of magnitude smaller for PI. (4) It is not our contention that the simplex must be contracted until round-off error strongly influences the result, and the simplex then enlarged. The simplex is enlarged only if required to obtain reliable error estimates.
1787
Finally, a general comment concerning execution times is in order. The last paragraph of ref 4 discusses the routine use of P3 to fit a 13-parameter function. Results in ref 2,4, and 7 suggest such a problem will require several hundred iterations and several hours for minimization and is not a typical example. Simplex routines are fairly competitive with other methods for a small number of parameters, but become much less efficient than Newton-Raphson or quasi-Newton routines for more than five parameters (6-11).
LITERATURE CITED Phillips, G. R.; Eyring, E. M. Anal. Chem. 1988, 6 0 , 738-741. Nekier, J. A.; Mead, R. Comput. J. 1985, 8 , 308-313. Philllps, G. R.; Eyring, E. M. Anal. Chem. 1988, 6 0 , 2656. Brumby, S . Anal. Chem., preceding paper in this issue. Spendley, W. I n Optimization: Symposium of the Institute of Mathematics and its Applications, University of Keele, England, 1968; Fletcher, R., Ed.; Academic Press: New York, 1969. Chambers, J. M.; Ertel, J. E. Appl. Stat. 1974, 2 3 , 250-251. Reddy, B. R. Appl. Spectrosc. 1985, 3 9 , 480-484. Powell, M. J. D. SIAMRev. 1070, 12, 79-97. Chambers, J. M. Biometrika 1973. 6 0 , 1-13. Oisson, D. M.; Nelson, L. S. Technometrics 1975, 17, 45-51. Bard, Y. Nonlinear Parameter Estimation; Academic Press: New York, 1974.
G. R. Phillips E. M. Eyring* Department of Chemistry University of Utah Salt Lake City, Utah 84112 RECEIVED for review February 27, 1989. Accepted April 17, 1989. This research was supported in part by the Office of Naval Research.
TECHNICAL NOTES Construction of an Optically Transparent Thin-Layer-Electrode Cell for Use with Oxygen-Sensitive Species In Aqueous and Nonaqueous Solvents Matthew B. G. Pilkington, Barry A. Coles, and Richard G . Compton* Physical Chemistry Laboratory, Oxford University, South Parks Road, Oxford OX1 3QZ, England We present a design for a reusable optically transparent thin-layer-electrode (OTTLE) cell, which is easy to assemble and does not require technical skills or services. All the complex parts are obtainable as standard items from the manufacturers quoted (vide infra). Designs for 02-free nonaqueous OTTLE cells have appeared in the literature (1-8). However, unlike the cell reported here, these all required instrument-making facilities for construction. The cell can be reused without dismantling for either aqueous or nonaqueous work. Leakage is not a problem over the lifetime of several experiments, and an additional advantage is the ability to thermostat the cell (0-40 "C).
EXPERIMENTAL SECTION The main cell body was an Hellma optically transparent flowthrough cell available in UV-visible and IR quartz glass (catalog no. 136K,light path lengths of 100, 200, and 500 jim, Hellma (England),Ltd., Westcliffe-on-Sea,Essex, England). The working chamber was formed by sandwiching a semitransparent gold minigrid (100wires/in., 80% transmittance, Buckbee Mears 0003-2700/69/036 1- 1787$01.50/0
Co., St. Paul, MN) between the two cell plates (Figure 1). The solution under investigation could flow in the direction indicated. This aided cleansing and drying of the cell for experimentation on different solutions. The dimensions of the working chamber were as shown (Figure 1). In order to prevent leaks due to the Au minigrid passing out through the gap between the cell plates, the cell was heated while being held together firmly by a Hellma metal flowthrough cell holder (catalogno. 013.000)and low-melting wax (facial depilatorywax, Vychem, Ltd., Poole, Dorset, England) fed into the gap by capillary action. This wax has excellent adhesion to silica, melts at 50 "C, sets rigidly on cooling, and dissolves (for dismantlinglcleaning purposes) in ethanol. This provided an adequate seal for acetonitrile, dichloromethane,and water. The cell was jacketed with flowing argon to prevent O2 from diffusing through the wax seal. A slot could be ground with simple glassworking facilities to allow the electrode to be introduced without altering the path length of the original cell, but an 02-freeseal would still be required at the point of entry of the minigrid. The use of a thermoplastic material for sealing allows easy assembly and disassembly with little risk of breakage-an important consideration with silica components-and justifies 0 1989 American Chemical Society
1788 * ANALYTICAL CHEMISTRY, VOL. 61. NO. 15, AUGUST 1, 1989
2 c
0 ._ c L a
0 v)
Approximate beom Oren in centre :.no edgeeffectr
s 1
Au minigrid. Lengthof grid wordirtoted by position of beon
.... .. ..
-.
5.
I
I
\ Upstreom Agvire
I
I
300
400
600 Wavelength Inm
quan-referenre electrode
Fipure 1. Diagram of a Hellma Row-through cell converted for spetroelectrochemistry.
I
500
Development of the thin-layer spectrum with time for the reduction of 1.1 X 10.' M BA In acetonitrile with 0.1 M TBAP at 25 OC: (-) BA at i = 0. (---) BA and EA'- at f < 130 s a n d (. ..) EA'at i 2 130 S. The time elapsed to reach exhaustive electrolysis was 130 s. Flgure 2.
0 8r
the acceptance of a finite life for the seal. The wax seal is slowly degraded with a lifetime of at least 1 h of continuous experimentation in dichloromethane, 10 h in acetonitrile, and longer with water. Intermittent use may extend over a long period if the flow system is drained by drawing dry air through after each experimental session. Some solvents (e.g. dimethylformamide) or higher temperature experiments would require a different sealant, hut we would expect to use the same method. We have found no evidence of electroactivity caused by exposure of these solutions to the wax. Optical absorption for dissolved wax in acetonitrile, dichloromethane, and water is too low at the concentrations involved to affect the UV-vis spectrum a t h 2 250 nm. The cell and cell holder assembly fits a standard water thermostated Perkin-Elmer cell holder (Europecatalog no. B008-0819) and was used mounted in a Perkin-ElmerLambda-5 spectrometer. The O T n E cell plus holder have the same externalcross sectional dimensions of 12.5 X 12.5 mm as a standard 10 mm path length UV-vis cell. The cell was fed via a gravity feed reservoir mounted outside the spectrometer where degassing of Orsensitive solutions with argon could take place. Connection from the reservoir to the cell was via 1.5 mm bore Altex poly(tetrafluoroethy1ene) (PTFE) tubing (Anachem,Luton, England). This was jacketed with an argon-purged tube to prevent ingress of 0,.The electrical connection to the Au minigrid was made by silver paint (RS Components, part no. 555-156) to a copper wire. A silver wire quasi-referenceelectrode was placed within the PTFEtubing close to and upstream of the cell but sufficiently close to avoid excessive ohmic drop. This electrode was brought out for connection via an Altex *tee" connector and an Araldite seal. The PTFE flow tubing was connected to the cell by using short lengths of Ar jacketed silicone rubber tubing. The Pt counter electrode was placed in a wider section of tubing downstream from the cell to prevent interference by counter-electrode products in the working electrode chamber. The Pt was in the form of a 60 x 20 mm, 52-mesh gauze of 0.1-mm wire wiled into a spiral. Electrochemical measurements were made with a potenticatat, scan generator, and potential step module (Oxford Electrodes, England, Ltd.). l-Bromo-9,10-anthracenedione(BA) and l-iodo-9,lOanthracenedione (IA) were prepared by D. Bethell a t Liverpool University to 99% purity. Acetonitrile was triply distilled and dried over calcium hydride pellets and stored over 4A grade activated molecular sieves under dry N,. Tetra-n-hutylammonium perchlorate (TBAP) was dried as received (Fluka, purum) under vacuum for 24 h prior to use. The solutions were prepared in a drybox and were degassed by purging with dry 0,-free Ar.
0
60
120
180
300
2LO
Timelr
Flgure 3. Parallel current and absorptiin transients in a stationary soIUtimn: (a)absorption against time at 565 nm: (b)current against time. At (i) the potential was stepped from 0 to -0.9 V vs an Ag wire psevdo
reference electrode, and at (ii) the potential was stepped back to 0 zero current to what would be a limiting current potential under flowing conditions. Q. This corresponded to stepping from a
RESULTS AND DISCUSSION The performance of the 5 W p m cell was evaluated by using the BA/B.4- redox couple. BA undergoes a one-electron reversible reduction to BA'. on gold in 0.1 M TBAP in dry acetonitrile (9). The electrochemistry was carried out while cms d. Potential solution was flowing through the cell at was slowly swept in a cathodic direction until a limiting current was reached. The solution and potential were then held stationary, and the spectrum was observed to change. The current decayed to zero as exhaustive electrolysis occurred over a time scale of about 130 s. The spectra displayed (Figure 2) describe the change in absorption as the parent molecule BA underwent 1-e- reduction to B.4.. Absorption/current transients were produced by potential steps (Figure 3). The cell thickness was confirmed from the absorption of BA at 335 nm (< = 4.01 X M-' u d (9))using the BeerLambert law. The optical path length was 560 Sm, longer than the original cell path length due to the Au minigrid. The 100-pm cell was similarly found to have a 160-fim optical path length with the minigrid in place. The 100-Sm cell was used to observe the spectrum of the unstable 1.4- radical. IA undergoes a 1-e-
Anal. Chem. 1989, 61, 1789-1791
reduction to I&- on gold in 0.1 M TBAP in dry acetonitrile. The UV-visible absorption spectrum of IA'-was not observed with the 560-pm cell, but was observed with the 160-pm cell. When stationary IA solution was reduced in the 100-Mm cell, the 1A'- spectrum grew t o a maximum over 10 s and disappeared over a totalperiod of 35 s. Decreasing the cell thickness ( I ) from 560 to 160 pm produced a 12-fold decrease in the time taken ( t ) to achieve exhaustive electrolysis for electroactive species with similar diffusion coefficients (D). This agrees with the approximate relation
t
a
12/D
The compound under observation must have a molar extinction coefficient above a certain limit, with the minimum requirements being c > 200 M-' cm-' for the 560-pm cell and e > 600 M-' cm-' for the 160-rm cell. The flow operation of this system permits cleaning and O2 removal so that the cell need not be removed from the spectrometer between experiments on different solutions. Thus the cell has an application for photoelectrochemical detection in high-performance liquid chromatography and for flow injection analysis (10, 11). T o conclude, we have shown that a well controlled spectroelectrochemical system may be constructed without in-
1789
strument workshop facilities, thus making the technique much more widely available.
ACKNOWLEDGMENT We thank D. Bethel1 for providing samples of l-bromo9,lO-anthracenedione and l-iodo-9,lO-anthracenedione. Registry No. BA, 632-83-7; Baa-, 121176-25-8;IA, 3485-80-1; IA*-, 121176-26-9;Au, 7440-57-5.
LITERATURE CITED (1) Murray, R. W.; Heineman, W. R.; O'Dom, G. W. Anal. Chem. 1967, 3 9 , 1666. (2) Yildiz, A.; Kissinger, P. T.; Reiiley, C. N. Anal. Chem. 1968, 4 0 , 1018. (3) Heineman, W. R.;Burnett, J. N.; Murray, W. M. Anal. Chem. 1968, 40, 1974. (4) Muth, E. P.; Fuller, J. E.; Doane, L. M.; Blubaugh, E. A. Anal. Chem. 1982, 5 4 , 604. (5) Porter, M. D.; Dong, S.; Gui, Y.-P.; Kuwana, T. Anal. Chem. 1984, 56, 2263. (6) Sanderson, D. G.; Anderson, L. 6 . Anal. Chem. 1985, 5 7 , 2388. (7) Nevin, W. A.; Lever, A. B. P. Anal. Chem. 1988, 6 0 , 727. (8) Zhang, C.; Park, S A . Anal. Chem. 1988, 6 0 , 1639-1642. (9) Compton, R. G.; Pilkington, M. B. G.; Bethell, D., unpublished work, Physical Chemistry Laboratory, Oxford University, November 1988. (IO) Dewald, H. D.; Wang, J. Anal. Chem. 1984, 766, 163. (11) Lacourse, W. R.; Krull, I. S. Anal. Chern. 1985, 5 7 , 1810.
RECEIVED for review December 19, 1988. Accepted March 27, 1989. M.B.G.P. thanks the SERC for a studentship.
Application of a Nested-Loop System for the Simultaneous Determination of Thorium and Uranium by Flow Injection Analysis Jose Luis PBrez Pavbn, Bernard0 Moreno Cordero,* Jesiis Herniindez MBndez, and Rosa Maria Isidro Agudo Department of Analytical Chemistry, Bromatology and Food Sciences, University Numerous methods have been described for the determination of uranium (1-6) and thorium (7-11), most of them colorimetric. However, owing t o the low concentrations in samples of interest and the presence of interferents, direct determinations are difficult, and separation or preconcentration techniques such as liquid-liquid extraction, ion-exchange chromatography, and extraction columns are always employed prior t o analytical measurement. Continuous automated or semiautomated analytical techniques are to be preferred over wet methods when one is dealing with hazardous materials or when large numbers of samples have to be analzyed. Several flow injection analysis (FIA) procedures have been proposed for the determination of uranium (12-15) and thorium (16);in this work we propose for the first time a sensitive and selective FIA method for the simultaneous determination of thorium and uranium without previous separation from the matrix; Arsenazo 111 is used as the reagent, with monitoring of the systems a t X = 665 nm. The proposed FIA system is a two-channel manifold with a two-valve nested-loop injection system, the loop of one valve being a lead powder reducing column (Figure 1). The injected sample is split into two sections, one of them passing through the reducing column.
EXPERIMENTAL SECTION Reagents. Stock solutions of uranium and thorium at a concentration of 2.0 X M were prepared by dissolving appropriate amounts of uranyl nitrate hexahydrate (Merck) and thorium nitrate pentahydrate (Merck) in water. Stock solutions of Arsenazo I11 were prepared by dissolving 0.4100 g of the solid product (Fluka) in 250 mL of water. Aqueous 10% (w/v) solutions were of Triton X-100 (Analema). The reagent solution was prepared by mixing 25.0 mL of the stock Arsenazo I11 solution, 25.0 mL of the 10% (w/v) solution, and 75 mL of concentrated HC1 and then diluting with water up to 250 mL. All chemicals
Qf
Salamanca, Salamanca, Spain
reagents were of analytical grade. Apparatus. The flow system comprised a peristaltic pump (Gilson Minipuls 2 HP-4) and a Perkin-Elmer Coleman 55 with a l-cm flowthrough cell (18 pL, Hellma 178 12-QS). All connections were 0.5 mm i.d. Teflon tubing. Injection System (Figure I ) . Dasgupta and Hwang (17)report a configuration of a six-port injection valve installed within the loop of another six-port injection valve for the determination of aqueous peroxides; the loop of the nested (inner) valve contains an immobilized packed reactor with differentiating action on the analyte components. We have adapted this system for the simultaneous determination of thorium and uranium; the loop of the inner valve contains a lead reductor minicolumn. The minicolumn was a 5-cm length of 2 mm i.d. glass tubing; it was packed with lead powder (0.1-0.3 mm) with small glass wool beds at each end to prevent the escape of the material. How the nested-loop device should be handled is described in the original paper (17)and summarized in our procedure. The resulting detector output is two separate sequential signals. When a uranium solution is injected into the system, the first peak (section L1, nonreduced sample) is much lower than the second peak due to the low molar absorptivity of the U(V1)-Arsenazo I11 complex; the second peak (section L2, reduced sample) corresponds to the U(1V)-Arsenazo I11 complex and is similar to the peaks obtained when a thorium solution is injected into the system. Procedure. The system is started with both valves in the inject mode; this allows the filling of the column and R1with carrier solution. Later, both valves are switched to the loading mode and the sample (thorium and/or uranium, 3.6 M HC1) is pumped to fill L1 and L2. Then, V2 is switched first, followed by VI, and the carrier solution (3.6 M HCl) directs the sample toward the reagent stream (2.0 X 10"' M Arsenazo 111, 3.6 M HCl, 1% (w/v) Triton X-100); the signal is recorded at X = 665 nm. The conditions under which these determinations were carried M out were as follows: carrier, 3.6 M HCl; reagent, 2.0 X Arsenazo I11 (3.6 M HC1, TX-100 1%); L , = 143 pL; L2 = 254 pL;
0003-2700/89/0361-1789$01.50/0 0 1989 American Chemical Society
