2161
Anal. Chem. 1988, 60, 2161-2162
200000~
I
R a 150000
50000--
equivalent to 2.0 V/s. It can be seen from the figure that some voltammetric discrimination is provided by the detector. The peak height ratio between the two compounds is approximately 3.6 for the upper trace, 3.0 for the middle trace, and only 2.6 for the lower trace. While this would certainly not be a sufficient amount to provide complete resolution without any chromatography, the figure illustrates that voltammetric detection augments the resolution provided by the column. Registry No. TEAPTS, 733-44-8;fenoldopam, 67227-56-9; deschlorofenoldopam, 75510-66-6; N-methylfenoldopam, 115534-32-2;SK&F 102698,95333-81-6;6-mercaptopurine,50-442; methimazole, 60-56-0; DL-lo-camphorsulfonic acid, 5872-08-2.
j/ llil
1
+1.8
"
LITERATURE C I T E D +1.6 V I
Samuelson. R. E.; Odea, J.; Ostewouna. . - J. Anal. Chem. 1980, 5 2 , 22 15-22 16. Kafil, J. B.;Last, T. A. J. Chromatogr. 1985, 348, 397-405. Last, T. A. Anal. Chim. Acta 1883, 155, 287-291. Barnes, A. C.; Nieman, T. A. Anal. Chem. 1983, 55, 2309-2312. White, J. G.; St. Claire, R. L., 111; Jorgenson, J. W. Anal. Chem. 1986. 58. 293-298. Gunasingham, H.;Tay, B. T.; Ang, K. P. Anal. Chem. 1984, 56, 2422-2426. Jane, I.; McKinnon, A.; Flanagan, R. J. J. Chromatogr. 1985, 3 2 3 , 191-225. Gunasinaham, H.; Tay. B. T.; Ana, K. P. Anal. Chem. 1987, 5 9 , 262-266. Last, T. A. Anal. Chem. 1983, 5 5 , 1509. Rabenstein, D. L.; Saetre, R. Anal. Chem. 1877, 49, 1038-1039. Allison, L. A.; Shoup, R. E. Anal. Cbem. 1983, 5 5 , 8-12. Abounassif, M. A.; Jefferies, T. M. J . Pharm. Biomed. Anal. 1983, 1 , 65-72.
RECEIVED for review December 23, 1987. Accepted May 6, 1988.
Wall-Tube Electrode Cell for Computerized Potentiometric Stripping Analysis Amando F. K a p a u a n
Department of Chemistry, Ateneo de Manila University, P.O. Box 154, Manila, Philippines Computerized potentiometric stripping analysis (PSA) as developed by Jagner et al. provides a rapid determination of electroadive metals in solution at very low concentration levels (1-3). In this technique, the metal is first plated at a constant potential onto a suitable electrode surface, usually mercury film on glassy carbon, and chemically stripped off while the voltage of the electrode is followed as a function of time. The length of time that the potential of the electrode stays in a range characteristic of the metal is a function of the amount plated on and therefore of the original concentration in the solution. Reproducibility of the method is strongly influenced by the hydrodynamic regime at the working electrode and only the rotating disk electrode (RDE) system and Kissinger laminar-flow thin film liquid cells have been found useful for analyses at very low concentrations where stripping plateaus are on the order of a few milliseconds ( 4 , 5 ) . However, the RDE system is difficult to construct and use properly while Kissinger type flow cells, especially with electrodes that use a mercury film, are hard to maintain since they have to be taken apart and reassembled to clean and polish the working electrode. Recently, Albery and Bruckenstein (6) pointed out the complete hydrodynamic equivalence of the wall-tube electrode and the RDE. We have exploited this equivalence with a PSA cell that uses a wall-tube electrode configuration and has a built-in centrifugal pump.
EXPERIMENTAL SECTION Apparatus. The important parts of the wall-tube cell are shown in Figure 1. The motor M drives the 35-mm smooth disk impeller D at 3600 rpm and pumps solution through the tube T at 10.2 mL/s against the glassy carbon disk electrode E. In the wall-tube configuration, the tube's internal bore should be larger than the working electrode's active diameter. Thus the tube in this cell has a 6 mm diameter bore and the working electrode is a 3-mm glassy-carbon rod (Tokai 20) cast in epoxy (AralditeCY230 resin and HY951 hardener, 1O:l). Distance between the tube's end and the surface of the working electrode is fixed at 4 mm. A platinum coil counter electrode and a Ag/AgCl reference electrode are mounted as close as possible to the working electrode. Acrylic plastic is used for all components in contact with the solution. All of the dimensions and operating parameters given above follow closely the limits given by Albery and Bruckenstein as those required for "safe" operation of this electrode system. Solution in a beaker (85 mL minimum) can be introduced and withdrawn from the bottom of the setup. The working electrode's mount allows it to be removed and replaced for cleaning and polishing without difficulty. Data Acquisition System. Since a fairly rapid data acquisition rate is required in millisecond stripping PSA, a computerized system is a practical necessity for the technique. The lab-built data acquisition system (DAS) used here has two 12-bit 25-ps analog-to-digital converters equipped with a teraohm input resistance instrumentation and sample and hold amplifiers, two 12-bit digital-to-analog converters, a 24-bit clock with a 1-MHz crystal time-base, and four independent DPDT reed switches. A
0003-2700/88/0360-2161$01.50/00 1988 American Chemical Society
2162
ANALYTICAL CHEMISTRY, VOL.
BO. NO.
19, OCTOBER 1, 1988
M
- 1.2
Zn
%
5 Q
-o'8
Vi>
>
-0.4
Flgue 1. Cross section of the walUutm PSA cell. See text fw details.
Princeton Applied Research Model 174A polarographic analyzer is used as a t b e l e c t r o d e potenticetat while a Yokegawa Electric Works Model 3022 X-Y recorder is used to produce hard copy of the resulting chronopotentiograms. The microcomputer is an IBM-PC XT compatible equipped with an 8087 math copmeessor and the PSA programs are written in compiled Basic, with machine-language calls for speed in the data acquisition. Reagents. Reagent grade zinc, cadmium, and lead nitrates and mercuric chloride were used to make stock solutions 1000 pg/mL in the metal, using deionized distilled water. The acid used was hydrochloric acid kept in 1 M stock concentration. Potentiometric Stripping Analysis Pmcedure. Before each set of runs,the working electrode is preplated with mercury film at -1.0 V vs Ag/AgCI for 10 successive 1-min intervals in a solution 40 %/mL in mercuric chloride and 0.01 M in HCI. Polishing with an aqueous slurry of 0.05-pm alumina as needed keeps the swface of the glassy-carbon working electrode mirror smooth. Solutions to he anal& are made 0.01 M in HCI and 10 pg/mL in mercuric ion. Plating is carried out at a voltage at least 500 mV more negative than the substance's stripping potential for a time interval that varies from a few seconds to several minutes, depending on the concentration. At the instant that the electrodes are disconnected from the potential source, data acquisition is started and voltage between the working electrode and the reference is digitized at the rate of 12000 samples/s and stored in the computer's memory together with the time readings. A total of 5000 readings is normally taken (25000 bytes of data) and processed immediately for output to the X-Y recorder. When needed, the data are stored in a disk file for later recall. The stripping plateau lengths are obtained graphically from the chronopotentiogramsby using the method descrihed by Jagner (I). Data smoothing, derivative plotting, and background suhtraction are also available in the PSA program. Quantitation is done with the use of standards in a calibration run or hy the method of standard additions.
RESULTS AND DISCUSSION Six successive runs on the same solution are plotted on a single graph in Figure 2 to show the shape of typical chronopotentiograms and at the same time the reproducibility of the system. These were run on a solution 0.4 pg/mL in zinc, cadmium, and lead with a plating voltage of -1.37 V vs &/@I applied for 10 s. Relative standard deviations of the measured plateau lengths of each of these elements for all six runsare 1.1%, 0.6%. and 1.8%, respectively for zinc, cadmium, and lead, which stripped off in that order. The total elapsed time for each run, including the time required for plotting the result, is less than 1min. More extended runs with up to 35 trials give slightly bigger values of the relative standard deviation, on the order of 2-3%, most probably as a consequence of the thickening of the mercnry film, since during each plating period a certain amount is deposited with the analyte. Table I shows the resnlts of a typical standardization run for the determination of lead a t the nanogram-per-milliliter level. Plating time for these runs was 180 s at -1.37 V vs
0
ea
0
240
160
msec. Flgure 2. SIX consecutive PSA runs on a solution 0.4 pglmL each in Zn, Cd, and Pb. 0.01 M in HCI. and 10 pglmL in Hg. Plating was 10 s at -1.37 V vs AgIAgCI. Table I. Standardization Data for Pb(I1) Analysis' Pb(I1) concn, ng/mL 10.0 20.0
30.0
stripping plateau, ms
Ph(I1) concn, ng/mL
22.5
40.0
39.5 60.2
50.0 60.0
stripping plateau, ms 77.2 101.7 123.5
Plating was at -1.37 V v8 Ag/AgCI for 180 s and solutions were made 0.1 M in HCI and 10 pg/mL in Hg(I1). Ag/AgCl. Coefficient of correlation for the straight line plot of concentration vs stripping plateau is 0.998. With the sensitivity at this level, 80 ng of lead can he dertermined to about 5% accuracy if a plating time of 15 min is used (stripping plateau defined by 100 data points). The PSA cell has been used to determine low levels of the heavy metals in drinking water and extractable lead and cadmium in PVC pipes used for drinking water supplies, where there is a relatively large amount of low concentration sample available. Reproducibility of the analyses done with it suggests that among other things, the mean boundary layer thickness is constant during the plating and the stripping period. Since these have very different time scales, the design chosen for the pump seems to be a correct one for this application. We have designed and are testing out a wall-tube cell with a different configuration than the one described here in which the wall-tube parameters like the volume flow rate and the tube radius to tubeeelectrode distance can he varied so that the equations given by Albery and Bruckenstein can he validated experimentally. This will he reported in a subsequent paper.
LITERATURE CITED (1) Jagner. D. Anal. Chem. 1978. 50. 1924-1929. (2) Jagner. D. Anal. &m. 1979, 57. 342-345. (3) Oranell. A,; Jagner, D.; JoYlhon. M. Anal. Chem. 1980. 52. 2220-2223. (4) Jagner, D. Analyst ( 1 0 " ) 1982. 107, 593-599. (5) Anderson. L.; Jagner, D.; Josefwn. M. Anal. Chem. 1982, 54, 1371-1376. (6) Albery. W. J.: Bruckensteln. S. J . E!e~fmanaI.Chem. W8S. 144, 105-112.
RECEIM) for review January 12,1988. Accepted May 9,1988.
