Anal. Chem. I 9 8 4 56, 2387-2392
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Automated Electrochemical Stripping of Copper, Lead, and Cadmium in Seawater David R. Turner,* S. G. Robinson, and Michael Whitfield
Marine Biological Association of the United Kingdom, Citadel Hill, Plymouth PLl 2PB, England
Fast staircase stripping at rotating-disk, ring-disk, and splitdtsk electrodes has been compared with potentiometric,linear scan, and differential pulse stripping for the analysis of Cd, Pb, and Cu in seawater at the mercury film electrode using a purpose-bulit hard-wired instrument designed for field work. The dependence of rapid staircase stripping peak currents on the staircase parameters at short step widths (>I ms) was in agreement with theory for Cd and Pb but not for Cu, suggesting interferencefrom following chemical reactions in the latter case. The technique Is much faster than differential pulse stripping and can be carried out in a stirred solution making it better suited for field work. Discrimination of faradaic from capacitance currents Is comparable with differential pulse in the case of Cu but is a little poorer in the case of Cd. The performance of the rapid staircase can be improved by the use of a spilt-disk electrode. The rapid Staircase technique is not, however, suitable for use in low ionic strength waters. The performance of the potentlometric Stripping technique Is critically dependent on the nature of the background subtraction procedure employed.
Over the last decade, stripping techniques involving electrochemical preconcentration have become widely used for studying the behavior of trace metals in natural waters (see reviews ( 1 , 2 ) ) .Although much of this effort has been devoted to the measurement of totalmetal concentrationsafter suitable chemical pretreatment, electrochemical techniques are uniquely suited to the direct study of trace metal speciation in natural waters. The low levels of electroactive trace metals in most natural waters dictate the use of highly sensitive stripping techniques ( l ) ,and the chemical mobility of biologically active systems suggests that the techniques should be adaptable for work on site, if not in situ. For field use the techniques selected should preferably retain high sensitivity on stripping into a stirred solution so that they are less affected by vibration and movement of the cell and should be rapid and simple to use. Furthermore, the results should be capable of a clear interpretation in terms of the chemical speciation of the elements studied. Rotating electrodes are well suited to this purpose since the solution is efficiently stirred throughout the analysis and the well-defined diffusion layer thickness at the electrode surface simplifies the interpretation of the measurements. The number of Coulombs qm equivalent to the trace metal deposited per unit area of a rotating electrode during a deposition time td is given by q m = 1.61ndFD2/3v1/6~1/2Ctd
where nd = number of electrons involved in the deposition process, F = the Faraday constant, D = diffusion coefficient, v = kinematic viscosity, w = electrode rotation speed, and C = trace metal concentration in the solution. At a given value of C the overall sensitivity of a stripping technique can be enhanced either by increasing qm(via w or td) or by improving 0003-2700/84/0356-2387$01.50/0
the sensitivity of the measurement of the resulting qm. The extent to which w can be increased is limited by turbulence and the physical stability of the mercury film, while large increases in td result in unacceptably long analysis times. The approaches adopted to improve the measurement of q m include modulated stripping waveforms ( 1 , 2 ) ,the use of a second working electrode (2),and digital treatment of the signal (3). We report here a comparison of a number of stripping techniques at rotating electrodes which exemplify the application of these procedures. The techniques have been selected on the basis of suitability for field measurements, i.e., they are all rapid stripping techniques carried out in stirred solution, and include both electrically induced stripping (anodic stripping voltammetry, ASV) and chemically induced stripping (potentiometric stripping analysis, PSA). The four techniques, rapid staircase stripping (4) and potentiometric stripping (5) at a rotating-disk electrode and stripping at ring-disk (6, 7) and split-disk (8) electrodes, have been incorporated into a purpose-built digital field instrument and are used as previously described except for the rapid staircase stripping which was extended to cover step widths down to 1 ms. The PSA technique provides a sharp contrast to the ASV techniques in that the trace metal in the amalgam is chemically oxidized at a constant rate, and the resulting change in electrode potential is followed. The length of time spent by the potential in the region of the electrode potential of a metal therefore gives a measure of the amount of metal in the amalgam; a more detailed treatment can be found in ref 5. The measurement of cadmium, lead, and copper in seawater at natural pH was used as the test system. Natural organic matter was destroyed by irradiation so that the methods could be compared without the uncertainties introduced by organic interference. EXPERIMENTAL SECTION Instrumentation. Conventional linear scan and differential pulse ASV measurements were carried out with a PAR 174A polarographic analyzer modified to give a 0.1-sclock time. The majority of the measurements reported here were, however, carried out with an automated bipotentiostat purpose-builtfor field work which incorporated a staircase waveform for the ASV stripping process (Figure 1A). Four different preprogrammed experimental sequences were used for ASV and PSA at a single working electrode and ASV at ring-disk and split-disk electrodes, respectively. An additional sequence was used for preplating mercury film electrodes. The experimentalsequences are controlled by hard-wired timing sequences rather than by software control. This greatly simplifies the use of the instrument in the field and results in a compact, robust experimental system. The parameters defining the experiment are all individually preset (Table I) before the automated sequence is initiated. The potential-time sequence used for the split-disk experiments is reproduced in Figure 1B (8). For the laboratory studies described here all current-voltage curves were recorded in a 1K word memory and then replayed to a pen recorder. The current was sampled once on each staircase step by a 14-bit ADC (Figure 1A). On slower scans the signal was passed through a low-passfilter (Table I) before conversion. PSA data were collected directly from a monitor point on the field instrument by a CAMAC-based interface system controlled by 0 1884 American Chemlcal Society
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ANALYTICAL CHEMISTRY, VOL. 56, NO. 13, NOVEMBER 1984 A
.
measure C"ll*"I
Y
calculated according to the equation given by Albery and Hitchman (IO). The cell used was a covered glass beaker (volume ca. 250 mL) with a Pt counterelectrodeand an inlet for oxygen-free nitrogen. The reference was an Ag/AgC1/0.1 M NaCl electrode in a glass tube terminating in a porous Vycor plug. The reference electrode contacted the solution via a Luggin capillary placed 1 mm below the rotating electrode (11). Procedures. Saline water measurements were carried out on irradiated seawater, using a preplated mercury film electrode. Preplating was carried out at -1.0 V in 0.1 M NaC104/0.01 M Hg(N03)zin a separate cell. The mercury film thickness was calculated from the charge passed during the preplating. The preplated electrodewas rinsed with distilled water and transferred to the analysis cell without allowing the film to dry. Freshwater measurements were carried on water collected from the surface of Lake Windermere in an acid-washedpoly(ethy1ene) bottle and the mercury film was plated in situ. One deposition/stripping cycle was carried out before any data were recorded from an in situ plated film. The sample (150 mL) was deoxygenated for 20 min before analysis, resulting in a pH close to 8.2 for the saline samples. Standard trace metal solutions were prepared by dilution from atomic absorption standards (Fisons). Standard experimental parameters are listed in Table I1 and apply to all the results given unless otherwise stated.
Flgure 1. PotentiaVtime sequences: A, staircase stripping waveform: B, subtractive ASV sequence (8). The dashed llne In B represents the potential pulse applled to the "background" halfdlsk before Stripping. The parameters A€,7,f,, t t2, f d and ramp limit are all Independently variable (Table I).
,,
Table I. Parameters Defining Stripping Experiments parameter deposition potential deposition time td' staircase step height AE staircase step width 7 sampling delay t, ramp limit (range of stripping scan) SASV timings tl, tz (Figure
range
resolution
12000 mV 30-2970 s 1-10 mV 1-99 ms 0.5-100 ms 12000 mV
1 mV 30 s 1 mV 1 ms continuous
1-99 ms
1 ms
1 mV
RESULTS AND DISCUSSION Although stripping peaks were recorded for cadmium, lead, and copper in all experiments, the responses of lead and cadmium to changes in the stripping parameters were very similar, so that certain results are shown for cadmium and copper only. Rapid Staircase Stripping (RSASV). A theoretical treatment of rapid staircase stripping when the sampling delay ( t d )equals the step width (7)(cf. Table 11) results in the expression (12)
1)
curient sensitivity time constant of low-pass filter
0.02 @A-10m-- 1, 2, 5 step/-xaL0, 10-3000 ms 1, 3 step/decade
'For the ASV techniques tl, t 2 (minimum values 1 included in the total deposition time.
s)
are also
a DEC LSI 11/2 processor with 56K RAM. The data were input via a 14-bit ADC with a 16K word memory buffer operating at sampling rates of up to 10 kHz. The raw data in the 16K word buffer thus consisted of potential measurements equally spaced in time. This information was converted before storage on disk to a counts vs. potential curve (9),where the counts are proportional to the length of time spent by the potential in a given voltage range or channel width (typically several millivolts). The time of interest can then be measured as the area of the resulting peak. Before being replotted as potential/time or frequency curves the data were filtered by a simple moving average technique in which each data point is replaced with the mean of the (2N + 1) points of which it is the center; the parameter (2N + 1) is termed the filter width. Peak areas were measured by using an interactive graphics terminal. The data acquisition routines were written in MACRO and CATEX (a language specific to the CAMAC system) and the analysis routines in FORTRAN. Further details of the hardware and software used are available from the authors. Cell and Electrodes. All measurements were carried out by using a rotating electrode system (Mee Instruments, Oxford) capable of rotation speeds in the range 0-3000 rpm. Disk, split-disk, and ring-disk electrodes were of glassy carbon (Ringsdorff Carbon Co., Croydon) set in epoxy resin and were polished with fine wet and dry paper, alumina, and finally diamond paste. The disk and split-disk electrodes were 6 mm in diameter, and the radii of the ring-disk electrode assembly were 2.9, 3.0, and 3.3 mm for the disk, and inner and outer ring edges, respectively, giving a theoretical collection efficiency of 0.245
where ip is the stripping peak current and n, is the stoichiometric number of electrons involved in the stripping process. The parameter gp is a function of the staircase step height (AE),but for thin films g, is independent of film thickness. The limit of this thin film behavior is reached as the film where DR is the diffusion thickness approaches (DRT)~/~ coefficient of the metal in the amalgam. The shortest step width (7)used here is 1ms (Table I), which gives a thin film limit of loo00 A taking DR as lo4 cm2s-l. Peak currents were found to be independent of film thickness in the range 1000-8000 A but were reduced as the film thickness approached 500 A probably due to incomplete coverage of the glassy carbon surface (12). Stripping peak currents were confirmed to vary linearly with deposition time (30-300 s), square root of rotation speed (200-1600 rpm), and concentration (up to 300 nM) for all three metals (cf. eq 1). Since the present work extends the RSASV technique to the short step width of 1 ms, we have compared our results with theory for d three metals for r in the range 1-10 ms and AE in the range 1-10 mV. Theoretical peak currents were calculated from eq 1and 2 with the parameters listed in Table 111. Theoretical and experimental peak currents are compared
Table 11. Standard Parameters Used for Stripping Measurements' stripping technique
deposition w/rpm
td/S
rapid staircase (RSASV) 32 65 subtractive ASV at split-disk (SASV) ASV with collection at ring-disk (ASVWC) 362 potentiometric stripping (PSA) 60 linear scan (LSASV) 60b differential pulse (DPASV) 60*
w/rpm
1000
1000
1000 1000
1000 1000 1000
1000 1000 1000
0 0
stripping other parameters t, = T - 1 ms (0.5 ms when 7 = 1 me) AE = 10 mV, T = 2 ms, t, = 1ms, tl = tz = 5 8 AE = 5 mV, 7 = 10 ms, t, = 9 ms, collection potential -0.9 V channel width 4 mV, filter width 6, [Hg] 68 pM scan rate 50 mV s-l scan rate 10 mV s-l, clock time 0.1 s, pulse amplitude 50 mV
aFilm thickness = 2000 A and deposition potential = -0.9 V throughout. *Plus 20-s quiescent.
ANALYTICAL CHEMISTRY, VOL. 56, NO. 13, NOVEMBER 1984
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Table 111. Parameters Used in the Calculation of RSASV Peak Currents (Equations 1 and 2) parameter
source
value
2 1O8D/cm2s-l 7.17 (Cd),9.45 (Pb),7.33
nd
ref 13
(CU)
0.0093 ref 13 9.6,8.5,7.4,6.7, estimated from Figure 6 of ref 12 6.2,5.6 for AE = 1, 2,4, 6,8,10 mV, respectively 2
v/cm2 s-l g,/v
ns
800
400
-
icalc
1200
-
icalc
600
in Figure 2, and regression analyses are shown in Table IV for the equation i,(exptl) = bi,(calcd)
(3)
Equation 2 does not provide an exact description of these experiments since it was derived for t, = T (12),while here t, < T (Table 11). It was observed experimentally from repeated measurements at constant step width ( 7 ) with varying t, that the peak current decayed according to
i, = ip*(l - at,-1/2)
/
(4)
where ip* is the current at the start of the step (t, = 0) and a is a constant for each metal. The regression analyses of eq 3 were repeated by using experimentalpeak currents corrected according to eq 4 but with only minor effect on the results (Table IV). Two features emerge clearly from an examination of Figure 2 and Table IV. Firstly, both cadmium and lead show the predicted dependence of i, on AE and 7 (eq 2), although the observed currents are a little less than half the predicted values. The reason for this discrepancy is not clear, although the similarity between the values for Cd and P b suggests a systematic error in the calculation of qm (eq 1). Secondly,the stripping of copper does not follow the theoretical predictions but gives peak currents much smaller than those predicted and results in curved relationships between ip(exptl) and ip(calcd) (dashed lines in Figure 2). This is undoubtedly related to the stability of Cu(1) in the seawater medium which has been clearly demonstrated in recent papers (14,15), so that the reoxidation process of Cu can be expected to be a complex process involving both Cu(1) and Cu(I1). At very short step widths where the time for associated chemical reactions is minimized, the peak currents are closer to the theoretical values and the sensitivity of the measurement for Cu improves significantly. Thus, at AI3 = 10 mV and T = 1ms, the highest current shown in Figure 2, the observed current is 32% of the theoretical value if copper is assumed to strip as Cu(I), cf. 42% and 47% for P b and Cd, respectively (Table IV). At long deposition times a double Cu peak was observed in RSASV, following which normal Cu stripping behavior could not be obtained until the electrode was repolished, suggesting that the double peak had been caused by deposition of a separate Cu phase in addition to the amalgam. This was
I
I
200
400
I
Figure 2. Plots of staircase stripping peak currents vs. theoretical predictions (eq 1 and 2) for Cu (299 nM) and Cd (151 nM). (0)A€ = 1 mV, (A)A€ = 2 mV, (0)A€ = 4 mV, (0)A€ = 6 mV, (A) A€ = 8 mV, (4) A€ = 10 mV, (-) calculated linear regresslon without correction (Table IV), (---) arbitrary lines joining points wkh the same A€.
tested by a calculation of the relevant Cu amalgam concentration. The critical conditions were Cu concentration 205 nM, deposition time 240 8, rotation speed 1000 rpm, film thickness 2000 A, and electrode area 0.183 cm2. Calculating the concentration of Cu in the amalgam from eq 1,we obtain a concentration of 0.02 mol %, in reasonable agreement with the Cu amalgam solubility of 0.01 mol 70quoted by Whitfield (16). Other Anodic Stripping Techniques. For the purpose of comparison, measurements were also carried out by using conventional stripping techniques (linear scan and differential pulse stripping) and twin-electrodetechniques (split-disk and ring-disk electrodes). The conventional techniques are compared with RSASV in Figure 3, where the higher currents measured in RSASV are particularly noticeable. Subtractive ASV (SASV) at the split-disk electrode was carried out according to the method of Sipos et al. (Figure 1B) which is designed to ensure that different potentials are applied to the two half-dish for only a short time (tl,here 5 8). The split-disk
Table IV. Regression Analyses of Equation 3 metal
Cd
Pb Cu (no= 2) c u (no= 1) LI Number of data points. according to eq 4.
nu 36 36 33 33
bb,c
0.465 (0.005) 0.416 (0.008) 0.131 (0.007) 0.261 (0.013)
std deviationlpb
bbd
5.1 4.4 14.4 14.4
0.437 (0.004) 0.381 (0.007) 0.123 (0.006) 0.246 (0.013)
Standard error quoted in parentheses. No corrections applied.
std deviation/pA
4.9 4.2 14.5 14.5
Experimental peak currents corrected
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ANALYTICAL CHEMISTRY, VOL. 56, NO. 13, NOVEMBER 1984
Table V. Comparison of Stripping Techniques in Seawater stripping time/sb
stripping technique"
Cd
iF/MAC Pb
RC Cu
Cd
Pb
Cu
0.27
0.29 0.20
0.055 0.059
0.80
0.18 e
signa1:noise ratiod Cd Pb Cu
veak half-&dth/mV Cd Pb Cu
RSASV: bE = 10 mV, 7 = 2 ms hE = 6 mv, T = 1 ms
0.15 0.125 0.15 1.5
SASV ASVWC PSA LSASV DPASV RSASV in freshwater: AE=BmV,.r= 10ms hE = 10 mV, 7 = 10 ms
