458 • Chapter 11. Bulk Electrolysis Methods

The peak current is still proportional to v and CQ and is

(11.7.26)

Thin layer methods have been suggested for determination of kinetic parametersof electrode reactions (63 65), but they have not been widely used for this purpose. Adifficulty in these methods, especially when nonaqueous solutions or very low sup porting electrolyte concentrations are employed, is the high resistance of the thin layerof solution. Since the reference and auxiliary electrodes are placed outside the thin layer chamber, one can have seriously nonuniform current distributions and high un compensated iR drops (producing for example, nonlinear potential sweeps) (66, 67).Although cell designs that minimize this problem have been devised (63, 64), carefulcontrol of the experimental conditions is required in kinetic measurements. Thin layercells have been applied in a number of electrochemical studies, including investiga tions of adsorption, electrodeposition, complex reaction mechanisms, and value de terminations. They have also become very popular in spectroelectrochemical studies(see Chapter 17).

The theory and mathematical treatments used for thin layer cells find application inother electrochemical problems. For example, the deposition of metals (as amalgams) intothin films of mercury and their subsequent stripping (Section 11.8) is fundamentally athin layer problem. Similarly the electrochemical oxidation or reduction of thin films(e.g., oxides, adsorbed layers, and precipitates) follows an analogous treatment (see Sec tion 14.3). Thin layer concepts are also directly applicable to synthetically modified elec trodes featuring electroactive species bound to the surface (Chapter 14). In manyproblems involving surface films, mass transfer truly is negligible over wide time do mains and problems with uncompensated resistance are minimal; thus relatively fast ex periments can be performed. Finally, the observed behavior with a scanningelectrochemical microscope (Section 16.4), where electrochemistry is examined in thegap between an electrode (tip) and a conducting or insulating substrate can be thought ofas that of a leaky thin layer cell.

11.8 STRIPPING ANALYSIS11.8.1 Introduction

Stripping analysis is an analytical method that utilizes a bulk electrolysis step (preelec trolysis) to preconcentrate a substance from solution into the small volume of a mercuryelectrode (a hanging mercury drop or a thin film) or onto the surface of an electrode.After this electrodeposition step, the material is redissolved ("stripped") from the elec trode using some voltammetric technique (most frequently LSV or DPV). If the condi tions during the preelectrolysis step are maintained constant, exhaustive electrolysis ofthe solution is not necessary and, by proper calibration and with fixed electrolysis times,the measured voltammetric response (e.g., peak current) can be employed to find the so lution concentration. This process is represented schematically in Figure 11.8.1. Themajor advantage of the method, as compared to direct voltammetric analysis of the origi nal solution, is the preconcentration of the material to be analyzed on or within the elec trode (by factors of 100 to >1000), so that the voltammetric (stripping) current is lessperturbed by charging or residual impurity currents. The technique is especially usefulfor the analysis of very dilute solutions (down to 10~ 10 to 10~ n M). Stripping analysisis most frequently used for the determination of metal ions by cathodic deposition, fol

Stripping Analysis 459

Deposition(preelectrolysis)

Stripping

E -0.5

Figure 11.8.1 Principle of anodic stripping. Values shown are typical ones used; potentials andEp are typical of Cu2+ analysis, (a) Preelectrolysis at Ed; stirred solution, (b) Rest period; stirreroff. (c) Anodic scan (v = 10-100 mV/s). [Adapted from E. Barendrecht, Electroanal Chem., 2, 53(1967), by courtesy of Marcel Dekker, Inc.]

lowed by anodic stripping with a linear potential scan and, therefore, is sometimes calledanodic stripping voltammetry (ASV) or, less frequently, inverse voltammetry. The basictheoretical principles and some typical applications will be described here. Several com-plete reviews describing the history, theory, and experimental methodology of this tech-nique have appeared (68-74).

11.8.2 Principles and TheoryThe mercury electrode used in stripping analysis is either a conventional HMDE or a mer-cury film electrode (MFE). In current practice, the MFE is typically deposited onto a ro-tating glassy carbon or wax-impregnated graphite disk. One usually adds mercuric ion(10 -10 M) directly to the analyte solution, so that during the preelectrolysis, the mer-cury codeposits with the species to be determined. The resulting mercury films are oftenless than 100 A thick. Since the MFE has a much smaller volume than the HMDE, theMFE shows a higher sensitivity. There is evidence that mercury electrodes with platinumcontacts dissolve some platinum on prolonged contact, with possible deleterious effects;hence platinum is usually avoided. Solid electrodes (e.g., Pt, Ag, C) are used (less fre-quently) without mercury for ions that cannot be determined at mercury (e.g., Ag, Au,Hg).

The electrodeposition step is carried out in a stirred solution at a potential Ed, whichis several tenths of a volt more negative than E° for the least easily reduced metal ion tobe determined. The relevant equations generally follow those for a bulk electrolysis (see

460 Chapter 11. Bulk Electrolysis Methods

Section 11.3.1). However, since the electrode area is so small, and td is much smaller thanthe time needed for exhaustive electrolysis, the current remains essentially constant (at / d)during this step, and the number of moles of metal deposited is then idtdlnF. Because theelectrolysis is not exhaustive, the deposition conditions (stirring rate, td, temperature)must be the same for the sample and standards to achieve high accuracy and precision.

With an HMDE, one observes a rest period, when the stirrer is turned off, the solu tion is allowed to become quiescent, and the concentration of metal in the amalgam be comes more uniform. The stripping step is then carried out by scanning the potentiallinearly toward more positive values.

When an MFE is used, the stirring during deposition is controlled by rotation of thesubstrate disk. A rest period usually is not observed, and rotation continues during thestripping step.

The behavior governing the i E curve during the anodic scan depends on the type ofelectrode employed. For an HMDE of radius r0, the concentration of reduced form, M, atthe start of the scan is uniform throughout the drop and is given by

(11.0.1) nF(4/ 3)7rr3

0

When the sweep rate v is sufficiently high that the concentration in the middle of thedrop (r = 0) remains at C ^ at the completion of the scan, then the behavior is essen tially that of semi infinite diffusion and the basic treatment of Section 6.2 applies (75).Correction must be made for the sphericity of the drop [see (6.2.23)]. In this case thespherical correction term must be subtracted from the planar term, since the concentra tion gradient builds up inside the drop and the area of the extended diffusion field de creases with time. Thus, the equations that apply for a reversible stripping reaction atthe HMDE are (75)

ip AD^C

nFi

M

\ ( ) (( )

(2.69 X 105)n3/ 2vm

D M < Jr

(0.725

JX 10 5

J

(11.8.2)

(11.8.3)

where / p is in A, A in cm2, DM in cm2/ s, C ^ in mol/ cm3, v in V/s, and TQ in cm; the func tions xi&t) a n d <t>(crt), where a = nFv/ RT, are tabulated in Table 6.2.1. These equationshold for the usual HMDE for v > 20 mV/s, and clearly, under these conditions, a largefraction of the deposited metal remains in the drop. A comparison of the i E curve pre dicted by (11.8.2) and a typical experimental stripping voltammogram at an HMDE isshown in Figure 11.8.2 (76). At very large scan rates the spherical term becomes negligi ble and linear diffusion scan behavior, with / p proportional to i>1/ 2, results. Practical strip ping measurements are usually carried out in this regime. At smaller rates, when thediffusion layer thickness exceeds TQ, the finite electrode volume and depletion of M atr = 0 must be considered. At the limit of very small v, when the drop is completely de pleted of M during the scan, the behavior approaches that of a thin layer cell or MFE (seebelow) with / p proportional to v.

Because the volume and thickness of the mercury film on an MFE are small, thestripping behavior with this electrode follows thin layer behavior more closely (seeSection 11.7), and depletion effects predominate. The theoretical treatment for theMFE has appeared (77, 78); a diagram of the model employed is shown in Figure

Stripping Analysis 461

0 . 6

Figure 11.8.2 Experimental anodic stripping i E curvefor thallium. Experimental conditions: 1.0 X 10~5 MTl+ , 0.1 M KC1 solution, Ed = 0.7 V vs. SCE, td = 5min, v — 33.3 mV/s. Circles are theoretical pointscalculated from (11.8.2). [Reprinted with permissionfrom I. Shain and J. Lewinson, Anal. Chem., 33, 187(1961). Copyright 1961, American Chemical Society.]

11.8.3. If the stripping reaction is assumed to be reversible, the Nernst equation holdsat the surface:

CM+ n(O, t) = , vt)\ (11.8.4)

The solution of the diffusion equations with this condition and the initial and boundaryconditions shown in Figure 11.8.3 leads to an integral equation that must be solved nu merically. Typical results for / p for films of different thicknesses, /, as a function of vare shown in Figure 11.8.4a. At small v and /, thin layer behavior predominates and/ p « u. For high v and large /, semi infinite linear diffusion behavior predominates and/p ^ vm. The limits of these zones are shown in Figure 11.8.4b. MFEs used in currentpractice fall within the region where thin layer behavior can be expected for virtuallyall usual sweep rates (< 500 mV/ s). An approximate equation for the peak current in thethin layer region based on a diffusion layer approximation in solution has also beenproposed (79):

l«pl = 2.1RT(11.8.5)

Notice the similarity between this expression and the corresponding limiting thin layerequation (11.7.17) (where Al = V).

SolutionCM + n(y, t)

M(Hg) » M + n + Hg + ne

Figure 11.8.3 Notation,initial conditions, and boundaryconditions for theoretical treatmentofMF E.

462 • Chapter 11. Bulk Electrolysis Methods

25

12.5

6.251/8 1/4 1/2 1 2 4

(V/min)

(b)

16 32

Figure 11.8.4 (a) Calculatedvariation of peak current withscan rate for different thickness ofMFE. (b) Zones where semi infinite diffusion and thin layerequations apply at MFE. [FromW. T. de Vries, J. ElectroanalChem., 9, 448 (1965), withpermission.]

11.8.3 Applications and VariationsThe technique of controlled potential cathodic deposition followed by anodic strippingwith a linear potential sweep has been applied to the determinations of a number of metals(e.g., Bi, Cd, Cu, In, Pb, and Zn) either alone or in mixtures (Figure 11.8.5). An increasein sensitivity can be obtained by using pulse polarographic, square wave, or coulostaticstripping techniques. Other variants, such as stripping by a potential step, current step, ormore elaborate programs (e.g., an anodic potential step for a short time followed by a ca thodic sweep) have also been proposed (68 74).

Important interferences that sometimes occur with mercury electrodes involve (a)reactions of the metals with the substrate material (e.g., Pt or Au) or with the mercury(e.g., N i Hg), or (b) formation of an intermetallic compound between two metals de posited into the mercury at the same time (e.g., Cu Cd or Cu N i). These effects are much

Stripping Analysis 463

01

/

//

///f

-0.21 1

Cu0.1 ppb

£(V vs.-0.4 -0.6

I ^ — \ '

\ /\JPb

0.4 ppb

SCE)-0.8

I I

\

\

\]VCd

0.2 ppb

-1.0I I

A50 nA

T

-1.2 - 1 .I I I

Zn0.1 ppb

Figure 11.8.5 Anodic stripping analysis of a solution containing 2 X 1 0 9 M Zn, Cd, Pb, and Cuat an MFE (mercury-plated, wax-impregnated graphite electrode). Stripping carried outby differential pulse voltammetry.

more serious with mercury films than with hanging drops, because MFEs feature fairlyconcentrated amalgams and a high ratio of substrate area to film volume. They can beoverriding concerns in the choice between an MFE and an HMDE.

On the other hand, MFEs offer much better sensitivity in linear sweep stripping andbetter control of mass transfer during the deposition step. If an HMDE is chosen (e.g., toreduce interferences), one can use differential pulse stripping to obtain sensitivities com-parable to those attained by LS V at an MFE.

Since stripping at an MFE gives total exhaustion of the thin film, the voltammetricpeaks are narrow and can allow baseline resolution of multicomponent systems. Thin-layer properties and the sharpness of the peaks permit relatively fast stripping sweeps,which in turn shorten analysis times. In contrast, the falloff in current past the peak in astripping voltammogram obtained at an HMDE comes from diffusive depletion, ratherthan exhaustion, and it continues for quite some time. Thus the peaks are broader, andoverlap of adjacent peaks is more serious. (Compare, for example, Figures 11.8.6 and11.8.6d.) This problem is usually minimized for the HMDE by using slow sweep rates, ata cost of lengthened analysis time.

Cathodic stripping analysis can also be carried out for species (usually anions) thatdeposit in an anodic preelectrolysis. For example, the halides (X~) can be determined atmercury by deposition as Hg2X2 Deposition on solid electrodes is also possible. In thiscase, surface problems (e.g., oxide films) and underpotential deposition effects often ap pear. On the other hand, the sensitivity for stripping from a solid electrode is very high,since the deposit can be removed completely, even at high scan rates. Stripping of filmshas often been used to determine the thickness of coatings (e.g., Sn on Cu) and oxide lay ers (e.g., CuO on Cu).6

6In fact, one of the earliest electroanalytical (coulometric) methods was the determination of the thickness of tincoatings on copper wires (80).

464 • Chapter 11. Bulk Electrolysis Methods

+0.2 +0.4 +0.6 +0.8 0-EN vs. SCE

+0.2 +0.6 +0.8

Figure 11.8.6 Stripping curves for 2 X 10~7 M Cd2+, In3+, Pb2+, and Cu2+ in 0.1 M KNO3.\v\ = 5 mV/s. {a) HMDE, td = 30 min. (b) Pyrolytic graphite, td = 5 min. (c) Unpolished glassycarbon, td = 5 min. (d) Polished glassy carbon, td = 5 min. For (£) to (d), &>/2 = 2000 rpm andH g2 + was added at 2 X 10~5 M. [From T. M. Florence, J. Electroanal. Chem., 27, 273 (1970),with permission.]

Another variation involves the stripping or electrolysis of species that have sponta neously adsorbed on the surface of an electrode without the preelectrolysis step. Thistechnique, called adsorptive stripping voltammetry, can be applied, for example, to sul fur containing species, organic compounds, and certain metal chelates that adsorb onHg and Au (74, 81). Examples include cysteine (and proteins that contain this aminoacid), dissolved titanium in the presence of the chelator solochrome violet RS, and thedrug diazepam. The amounts found by this method would necessarily be limited tomonolayer levels. However, similar approaches can be employed with thicker polymerlayers that can interact with solution species. Related experiments are described inChapter 14.

11.9 REFERENCES

1. J. J. Lingane, "Electroanalytical Chemistry,"2nd. ed., Wiley Interscience, New York, 1958,Chaps. 13 21.

2. P. Delahay, "New Instrumental Methods inElectrochemistry," Wiley Interscience, NewYork, 1954, Chaps. 11 14.

3. N. Tanaka, in "Treatise on Analytical Chem istry," Part I, Vol. 4,1. M. Kolthoff and P. J. Elv

ing, Eds., Wiley Interscience, New York, 1963,Chap. 48.

4. E. Leiva, Electrochim. Acta, 41, 2185 (1996).5. (a) L. B. Rogers and A. F. Stehney, J. Elec

trochem. Soc, 95, 25 (1949); (b) J. T. Byrne andL. B. Rogers, ibid., 98, 452 (1951).

6. D. M. Kolb, Adv. Electrochem. Electrochem.Engr., 11, 125 (1978).