Potentiometric Stripping Analyses Using a Flexible, Low-Cost Computerized Instrument C. W. K. Chow, D. E. Davey, M. R. Haskard, D. E. Mulcahy, and T. C. W. Yeow Sensor Science Engineering Group, University of South Australia. P.O. Box 1, lngle Farm, SA5095 Australia Potentiometric stripping analysis (PSA) has gained a place in trace metal analysis, with many applications of this technique reported in the past decade. The advantages reported in the literature, such a s lack of organic interference and use of low ionic streneth solutions in analv~6.3, have made this technique an ahernatwe method to the well-established anodir striminr! voltilmmrtnc rASV! procedure (1-2,8). Owing to theincreasing popularity of this techniaue. including it in the underaaduate chemistrv course is worth consideration. unfortunately, most chemiitrv schools are unlikelv to be able to purchase the commercial PSA equipment fo; a classroom kxperiment. However, the equipment required for PSA is, in principle, rather
-
Potentiostat
:ntiostat
om
)n I
Stripping Step
I Plating Step
: O M
'
:
O
'
2+
M
O M
: M(Hg) M
2+
+ O jM + R
2-,
:
~ . ~ . . . . . . . . . . . . . .. .
M fl 2++ 2;
jM ( H ~ )
:
Smpping .. .Time (Proportional to
Time
Potentiostat - - - - Switch I
---
I I I I
Measuring unit
Cell
LQ
resistance ~mia~e
I D.C. source I
.
I
U
Reference electrode Glassy carbon workmg electrode
Figure 1. Principles of the PSA system. (a) Illustration of the plating and stripping steps in PSA (0 is a chemical oxidant and R'- is its reduced form);and (b) basic equipment.
simple and can he constructed using a potentiostat and a pH meter with chart recorder (Fig. l(b)) ( I ) , and such a n arrangement is suitable to illustrate the theoretical basis of the technique. Up-to-date practical training for the student also suggests the help of a computer. In this paper, a flexible, low-cost PSA system incorporating a n IBM compatible PC is described. Potentiometric stripping analysis (PSA) is a two-step analytical procedure involving plating and stripping steps. During the plating step, metals deposit and concentrate onto the working electrode a t the applied potential. In general, a mercury film is coated onto a glassy carbon electrode and the plated metals dissolve in it, forming a n amalgam. To this point, PSA and ASV are identical. I n ASV, however, the stripping method utilizes a potential scan, each metal being oxidized a t its characteristic potential and the current generated being. ~ . r o.~ o r t i o ntoa l concentration. In the PSA stripping step, thcs amalgamated metals are brought back from the surface of th(: workinrr electrode into soiution by an oxidant (e.g., H e ) present in the sample solution. This chemical stripping step means that apparatus can he simplified. The resulting potential (El versus time ( t )curve is then used to obtain both qualitative and also quantitative information ( I ) . Such a n E-t curve, illustrating the plating and stripping steps (including the stripping time determination), is shown in Figure l(a), and a n illustration of a basic PSA system is shown in Figure l(b). Although PSA has several aspects in common with ASV, the major difference is the measurement of the physical parameters. ASV involves the measurement of current against potential while PSA involves the measurement of time against potential. Thus, PSA has several advantages over ASV First, time measurement is much easier and more accurate than current measurement. Moreover, because current is not involved in the PSA measurement, a sample of low ionic strength can be measured without the addition of supporting electrolyte. Another important advantage occurs when samples containing surface-active substances (e.g., dissolved organics) are examined. After successive depositions, the working electrode may be covered partially by such substances. This will affect the deposition step because the transport of metal ions to the electrode surface is restricted. The analytical signal obtained in ASV is directly dependent on the number of ions deposited in a given time and is diminished as a consequence. However, in PSA such surface processes not only affect the transport of metal ions to the electrode surface during the deposition step, but also slow the transport of oxidant ions to the electrode surface during the stripping step. I t turns out that these two effects are partially self-compensatory in PSA, stripping time thus being in practice less influenced by surface-active organic species than stripping current (1-2). In general, PSArequires a fast-data acquisition rate for high sensitivity, as will be discussed in detail later in this paper. High speed is a distinct advantage in PSA. With a sufficiently large number of data points per second, excelVolume 71
Number 11 November 1994
997
lent resolution of elements with similar reduction potentials is achievable. In contrast, as ASV potential scan rates are increased, resolution is lost due to peak widths increasing with scan rate (9,121. It has been suggested that deoxygenation is not necessary in PSA(1-2). In principle, this is true, with oxygen, i n fact, assisting the stripping step required. However, for better control with equipment such a s described here (the data acquisition rate being relatively slow) deoxygenation is recommended. Although t h e equipment t o be discussed was constructed using a n existing commercial polarographic analyzer a s potentiostat, a cheaper potentiostat could be used easily in its place. The hardware built for this application includes: a eomouterized hieh t meter: .. i n.~ uim~edance nn o p i ~ o n n on l n f f u n ~ used t ~ucuntrolthe plntingtimcdthe pot~ntinsrnr.and: an mtcrfwr card rhnt firs inw the expamion slot ofrhe computer. The operation of the equipment is controlled fully under software written in Quick-BASIC (an upgraded version of the BASIC language). The optional odoff unit described is used only for our selected equipment, and i t should not be required if the polarographic analyzer (or potentiostat) has its own built-in timing control. Instrumentation A 174A polarographic analyzer (Princeton Applied Research Corporation, New Jersey, USA) was employed as the potenti&at. The data acqu&tion system included an 1BM compatible 3%-SX personal computer with a 80387 maths c o - n n ~ w i mOlicrobits. r South Australia. Australia~ running a't 20 MHz, a n interfacing card, a measuring unit and a n odoff control unit. The cell assembly contained a magnetic stirrer, a 3-mm dia. glassy carbon electrode, (Chemtronics, Western Australia, Australia) employed as working electrode, a platinum wire, used a s auxiliary electrode, and a AgiAgCli 3M KC1 double-junction electrode a s reference (HNU ISE 40-02-00, HNU Systems Inc., USA.). A diagram illustrating the experimental arrangement is given in Figure 2. Hardware The data acquisition system is a n in-house designed system employing the Inter Integrated Circuit (IIC) bus system (Philips Components P t y Ltd., Artamon, NSW, Australia) (7). The IIC PC interface card (A i n Fig. 2) installed in the expansion slot of the IBM personal computer (B), provides a bi-directional interface between the serial IIC bus and the parallel data bus of the computer. In other
words, the function of this card is to serve as a communication unit between the computer and other external IIC serial devices. The measuring unit (C) serves severalfunctions. I t comprises a n amplifier, with selectable gain stages, and a Philips-Signetics 80C552 microprocessor. This microprocessor provides both the IIC and the RS232 communication links. While the normal RS232 serial port available with most IBM-com~atiblePC's as a standard accessorv vrovides a cheaper and simpler arrangement, i t allows only one serial device to be connected a t a time. The IIC bus provides a n easy way to connect several compatible devices to the serial bus a t the same time. I t also has a n in-built 10-bit resolution AiD converter, although only the lower eight bits are used in this application to obtain a higher sampling rate (the IIC bus is eight bits wide, hence two eight-bit data transfers are required to obtain the 10-bit data from the measuring unit that results i n a lower overall sampling rate). The signal input range for eight-bit data transfers is limited between 0 to 1000 mV with a 4 mV resolution. Resolution is determined by dividing the range by the number of steps. For a n eight-bit unit there are 256 (2') steps and the resolution can be improved to 1 mV for 10-bit data transfer. if this o ~ t i o nis utilized. The measurement accuracy (resolution) can be improved further through four gain ranges, selected manually via dip switches on the measuring unit. Amaximum resolution of 0.1 mV is achievable by selecting the highest input gain. Maximum input impedance is 1x 1 0 ' ~ohm. Thus, this unit can be considered a s a highimpedance voltmeter similar to a commercial pHlion-selective electrode meter. Thus, the PC may be viewed as an alternative to a chart recorder. The optional odoff controller (Dl is a software-driven two channel odoff switch that controls the power leads to solenoid levers that turn the potentiostat on and off. This arrangement removed the need to alter the circuitry of the polarographic analyzer. A complete circuit design of the measuring system is available from us, on request. Software The equipment i s operated under software control, a flow diagram of the operational sequence being shown in Figure 3. The software developed for this application has been written i n Quick-BASIC (Version 4.5, Microsoft Corporation, Redmond, USA). The full program consists of three
f
Equipment Control Start Plating
~ e c b r dPotential vs Time and Transfer Data to RAM (until full potentiogram recorded)
I
Store Recorded Data on Hard Disk
I
1 Data Processing
Figure 2. A schematic diagram of the computer-controlledapparatus used in this study.
998
Journal of Chemical Education
Figure 3. A block diagram of the operational sequence of the PSA equipment.
Cd potential range .......
400
-. . . . . . . . . . .
Pb porcnlial range
Figure 4. A Potentiogram for 300 ppb of Cd(ll). Pb(ll). and Cu(ll) in 0.1 M HCI, using a 1-minuteplating time, and 10 ppm ~g'' as oxidant. major modules; equipment control, which includes the control of all external devices via the IIC bus, potentiogram display, and data processing. In this paper, only the module for equipment control will be discussed. The other two modules are straightforward and user-dependent. Once again, details may be obtained from us. uuon reauest. The eauioment control module initialize; [he 116bus by ad'dressing t h e IIC-PC interface card. After these initialization steus. measured data is read and stored in the PC's RAM in the form of a n array. The recorded data can then be processed to produce a graphical output, the potentiogram (Fig. 4). Testing Procedure
A 50-mL aliquot of 10-ppm mercurous nitrate in 0.1 M hydrochloric acid (AR) was transferred to the analytical cell. Initial deaeration was carried out by bubbling high purity nitrogen gas into the mixture for 15 min (nitrogen purging is required for good data reproducibility). The H e ion is present to act as oxidant and also for the formation of a mercury lilm, the latter being formed in sit" by repeating the plating and stripping cycle 10 times prior to the addition of standards 121.A standard addition calibration in the range of 0-900 ppb was then performed with nine 50-pL additions of a 100-ppm cadmium, lead and copper nitrate standard solution. The solution was added to the analytical cell by means of a 50-200 FL adjustable micropipet (Socorex, Switzerland). The analysis follows the analytical sequence shown in Figure 3. A 1-min plating time a t -900 mV was selected for the experiment. Astream of nitrogen gas was maintained over the surface of the solution during the analysis. Calibration data was obtained by determining the time that e l a ~ s e sbetween two consecutive elemental eauivalence p&ts (Fig. 4). The time is proportional to the concentration of the uarticular metal in the solution (1).However, a simplified method is suited to a computerized technique. The number of data points within a potential range for a n element is counted, and divided by the sampling rate. This method gives essentially the same result and is less demanding on programming. The selected potential ranges against the reference electrode were -700 to -550, -500 to -350, and -200 to -50 mV for Cd, Pb, and Cu, respectively. With experience, the elemental potential ranges can be defined precisely, with the midpoints of the plateaux in theory correlating with the E" 's (2).
F gJre 5 CallDratlon graphs for the lnree elements Co Pb, and CL n 0 1 M hC , -5 ng a 1 -mln-te p at ng llme and 10 ppm hg2' as oxloant (the mean ol If pr cate reaoings oemg plonea) (a, agatnsl ppo and. (b)against molarity.
Results and Discussion
The data is presented in the form of a potential (El versus time (t) curve (potentiogram)(I).However, other forms of graphical presentation such a s a derivative potentiogram (61,or a plot of sampling count versus potential (51,can be obtained easily by altering the software. The potentiogram obtained from a 300 ppb Cd(II), PMII), and Cu(I1) standard is shown in Figire 4. The stripping time ( t , ~ , ) ,for the plateau region, measured a s indicated in Figure 4, is proportional to concentration. In Figure 5, the calibration curve for the three elements, Cd, Pb, and Cu, over the range of 0-900 ppb, shows excellent linearitv in each case. In Fieure 5(a) different sensitivities were-observed for the three elements. This is due to the molar mass differences for the three elements. In accordance with Faraday's law (91,for cations of the same charze, identical sensitivities would be e x ~ e c t e d when the calibration curves are plotted as tstnpvers;s molar concentrations. Figure 5(b) shows the expected correlation between the data sets for cadmium and copper. However, greater sensitivity than expected is observed for lead. This is due to lead being deposited both in the mercury film and also on the exposed glassy carbon surface. Accumulation of the other two elements depends solely on their solubility in the mercury film (10). This difference in behavior stems from the fact that mercury deposits on the glassy carbon electrode in the form of mercury droplets rather than a s a continuous film (11). The limit of detection of the PSA system depends upon the precision of the timing device used, for the signal is derived from the duration of the stripping time, tatn,. When the sampling rate of the data acquisition system is increased,~the~resolutiou of the sampling time also is increased, improving the precision of the stripping time measurement (5).The sampling rate used in our experiments was 500 data points per second (as determined by software) and this is more than adequate for most cases. The speed of the system is restricted both by the limitations of the computer's processor clock speed and also by the IIC bus system (7). Higher sampling rates could be achieved by using a faster computer or a n alternative communication method such a s an RS232 link. The sampling rate of the version illustrated could be imuroved further by incorporating a bank of RAM chips to store the sampled data until the full potentiogram had been registered, with Volume 71
Number 11 November 1994
999
data from the RAM chips then being transferred to the PC. The s a m.~ l i n rate e that could be obtained bv this improvespeed or ment would not be limited by either the the IIC bus svstem: thus, the full 10-bit resolution of the A/D converter could be used. However, sampling rates a s low a s 100 data points per second (using a relatively slow IBM compatible PC-XT) still can provide data acceptable for educational purposes. Further details can be provided if desired.
-
Conclusion This paper describes a simple and economic way (approximate cost, $300) to construct a computerized potentiometric stripping analysis system for teaching and research purposes. The IIC bus offers the advantages of flexibility for later expansion and modification of the system. Additional units can he simply plugged into the bus line, allowing a single computer to control two or three external devices. Thus, class experiments could be carried out without the need to provide a computer for each analvtical erouo. considerablv reducine costs. ~heYhar&are designei has pro;ided both the interface for the PC reauired for undereraduate use. and the sampling rate esslntial for high p&formance PSA. The measuring unit (C in Fig. 2 ) has the added advantage that it can be configured also a s a normal pHlion-selective electrode meter. Thus, ordinary pH measurements can be carried out with it. We emphasize that a commercial ISE meter can be employed in this PSA experiment, hut the lower sampling rates obtainable reduce the sensitivity of the analysis. Finally, the experimental setup described included a commercial potentiostat capable of performing ASV. Thus, both PSA and ASV techniques could be performed on the same sample. This is useful in an undergraduate analytical chemistry course where PSA (a relatively new tech-
nique) can be compared in parallel with the well-documented ASV. The correlation of results is then an especially attractive statistical option (13). Literature Cited 1. 2. 3. 4. 5. 6.
7. 8. 9. lo. 11. 12. 13.
Jsgler, D.; Gmneli, A.Ana!. Chim, Aefo 1976,83,19-26 Jaener " . DAnolvst 1982.107.595599. . Jagler,D.Anol. Ckem. 1978.60, 192G1929. Hoyer B.: Skov. H J : Kryger, L.Anol. Chim. Ado 1 9 8 6 , 1 8 8 , 2 0 ~ 2 1 7 . Mortensen, J.; Ouziel. E.; Skov, H. J.; Kryger. L.Anol. Chim. Ado 1919, 112,297312. Jagner,D.;Aren, KAnol. Chim.Acfa 1978, IW,37&388. Data Handbwk. IC20-8051 based eekf-bit micmontrolkrs: Philips Components: The Netherlands, 1991 Florence,T M. Anol~sf1 9 8 8 , 1 1 1 , 4 8 ~ 5 0 5 . Bard, A. J.; F a d h e r . L. R. Eleclrochemiml Methods: Fundomnfols ond Applimfmns: John Wilev & Sons: New York. 1980. Xi., H: Y.: Chou, C.An.1. Chim. A& 1989,222,26%268. Wang, J.;Tian, BAnol. Chsm. 1892. €4, 17061709. in Shuman. M. S.; Martin-Goldberg. M.El~clmhemicolMelhals:AnodicSL~ipping, water ~ n n l y s ~horsnnic s: speier iPan 2). ~ d i t e dby ine ear. K, ~eademicpress: New York, 1984. pp 345-388. Miller. J.C.: Miller. J. N. SlolislicsForAnolytiml Chaminry;Ellis Horwood:Chichester 1984
.
6
.
1000
Journal of Chemical Education
Erratum The figure at the right was inadvertently omitted from the article "ASimpleAlternative to Separatary Funnels for chemical Extractions" by Ragnar Bye that appeared an page 806 of the September 1994 i s s u e .
44
