1. Stripping Methods of Analysis
Lecture 2

2. Adsorptive Voltammetric Stripping Analysis
In trace analysis, mainly of heavy metal ions, anodic stripping voltammetry (ASV) is popular because of the low limit of determination - ranging to sub ppb concentrations, its accuracy and precision, as well as the low cost of instrumentation. ASV is based on a preliminary step of accumulation of the metal ion/compound to be determined on the working electrode. This is followed with voltammetric dissolution (oxidation) of the reduced substance formed. In addition, some anions can be accumulated on a mercury electrode to form an insoluble compound with the mercury ions obtained by dissolution of the mercury electrode at positive potentials. In this type of cathodic stripping voltammetry (CSV), the reduction process of the mercury compound on the electrode surface is studied. The most important step, leading to a substantial increase in the sensitivity is the electrolytic accumulation of the species on the working electrode.

3. Some other principles can be used for accumulation of the substance to be determined. One of these is adsorption. Many organic compounds exhibit surface-active properties that are manifested by their adsorption from solution onto the surface of a solid phase. This phenomenon forms the basis for adsorptive stripping voltammetry (AdSV), where the species to be determined are accumulated on the electrode by adsorption. The first description of this method was published many years ago in connection with the observation that the faradaic response increases after adsorptive accumulation of sulphur, poorly soluble inorganic alkaloids, and some benzophenones on a mercury electrode. Also, the adsorptive accumulation of the reduction products of methylene blue at a hanging mercury drop electrode (HMDE) leading to an increase in the height of the anodic polarographic peak in dependence on the accumulation time was reported. Similar measurements performed with a graphite electrode were also observed.

4. From the definition of AdSV it follows that this method is characterized by nonelectrolytic nature of the accumulation process, where adsorption plays an important role. The adsorption of the analyte itself is, however, not the only way of accumulation in AdSV. Modes of Adsorption 1. Reaction of analyte with oxidized material of the working electrode. 2. The reaction of a metal ion to be determined with a suitable reagent may lead to the formation of a complex which is adsorbed on the surface of the electrode, or 3. The reaction of a metal ion with the reagent adsorbed on the electrode surface, represent another other way of adsorptive accumulation which is utilized for the determination of metals.

5. In spite of the fact that the accumulation process is not fully of electrolytic nature (and in many cases the accumulation mechanism is not known in details) these methods are refereed in the literature as CSV methods. 4. Another type of nonelectrolytic accumulation in stripping voltammetry is the chemical interaction of the analyte with a modified electrode surface. AdSV can be employed in the trace analysis of a wide variety of organic compounds exhibiting surface active properties. When the given compound contains an electrochemically reducible or oxidizable group, the peak current on the voltammetric curve then corresponds practically only to the reduction (or oxidation) of the whole amount of the adsorbed electroactive species. In AdSV determinations of reducible organic compounds, the deposit is stripped off during a cathodic potential scan, and vice versa.

6. The Adsorption Process
If the interfacial forces (at the solid electrode surface/solution interface) lead to an increase in the concentration of some substance at the solid phase- solution interface compared to the concentration in solution, then this substance is said to be adsorbed on the surface of the solid. Adsorption equilibrium is established between the concentration in the solution and that on the surface of the electrode. At a given temperature, the amount of substance adsorbed is dependent on its concentration in solution. The velocity of formation of the adsorbed layer is affected both by the rate of the actual adsorption of the substance from the solution layer, and also by the rate of diffusion of the substance from the bulk of the solution to the electrode surface. The slower of these two processes then becomes the rate-controlling step in the adsorption process.

7. For complete electrode coverage, and for unidirectional diffusion transport to the electrode, the maximal value of the surface excess of the adsorbed substance, ymax , it can be stated that: Ipmax = k A ymax Ymax = 7.36*10-4 C D1/2 tacc1/2 where tacc, is the time required for complete electrode coverage. It has been found experimentally that for diffusion-controlled adsorption, Ip increases linearly with tacc (of course, assuming that surface coverage << 1 and that there is no interaction among the adsorbed molecules and also that the adsorption of the compound is controlled by its diffusion to the electrode).

8. The slope of the dependence of Ip on tacc1/2 is then proportional to the concentration of the studied substance. It thus follows that the AdSV method should be employed under conditions where the peak height increases linearly with the concentration of the studied substance. When this dependence deviates from linearity (especially when Ip approaches a limiting value), the experimental conditions must be modified (dilution of the solution, decrease of the accumulation time, accumulation under non-stirring conditions) It also follows that Ip is dependent on the rate of transport of the substance to the electrode, which can be increased by stirring the solution. In contrast to electrolytic accumulation (e.g. anodic stripping voltammetry, where Ip a n1/2), the Ip value obtained for adsorptive accumulation is directly proportional to the scan rate.

9. In principle, adsorptive stripping voltammetry is even more efficient than anodic stripping voltammetry; the determination limits are in the upper to medium ng/L range. The higher sensitivity of the method is based on the fact that the adsorbed compound remains on the electrode surface, whereas in ASV the deposited metal diffuses into the mercury film or mercury drop. As a result, after adsorptive accumulation the analyte concentration available for the stripping process at the electrode surface, is larger than that after electrolysis and amalgam formation (as in ASV).

10. Substances expected to adsorb on electrode surfaces.
Adsorptive stripping voltammetry has been shown, to be highly suitable for measuring organic compounds that exhibit surface-active properties. Compounds with p bonds ( aromatic rings), sulfur groups, disulfide linkage or long hydrocarbon chains are more strongly adsorbed than those without. In alkaline media, the partial deprotonation of - NH3+ group of amino acids leads to the formation of strongly adsorbing species, at platinum electrodes. In general, using aqueous solutions, the less soluble an organic compound is, the stronger its adsorption.

11. The presence of some metal ions in addition to certain functional groups might cause adsorptive stripping properties. For example histidine can be determined at the nanomolar level at an HMDE as its copper(lI) or Cu(l) complex, and Cu(ll) can be accumulated rapidly and selectively at an HMDE modified by adsorption of a poly (L-histidine) film. Reduction current in such cases is due to reduction of the metal species in the adsorbed complex as well as reduction of the ligand in the adsorbed complex.

12. Electrodes
AdSV can be carried out at practically all types of electrodes employed in voltammetry for which a completely reproducible constant surface area can be ensured over the whole measuring period or during a series of measurements. 1. The HMDE . The above requirement for reproducible surface is best fulfilled by the hanging mercury drop electrode. The best results are obtained with a static mercury drop electrode in which the flow rate of mercury in the capillary and thus formation of a mercury drop with the required, precisely reproducible volume is ensured by a needle valve or other type of valve controlled by an electronic circuit.

13. The hanging mercury drop electrode is used to study substances that are accumulated by adsorption on the electrode and then reduced during a scan to more negative potentials. The selection of a base electrolyte for AdSV can often be based on data published for the polarographic determination of a given substance. These are most often various types of buffers, but hydroxide solutions can also often be used. It has sometimes been observed that the peak height increases on dilution of the base electrolyte up to a concentration of 10-3 M. This can often be useful to limit interference from impurities present in the compound used in preparing the base electrolyte.

14. Carbon paste and platinum electrodes: CPE are made by mixing carbon powder or graphite with a binder (various types of mineral oils, etc.) and pressing the mixture into a glass tube.
Similarly, a platinum disk electrode can also be employed. Both these types of electrodes are especially suitable for studying adsorbable substances that can be oxidized at the working electrode, at positive potentials (e.g V vs. SCE), while mercury electrodes, on the other hand, can be used in a wider negative potential range.
However, it is somewhat more difficult to work with nonmercury electrodes.

15. The accumulation is carried out either at a set Eacc value, or at the open circuit potential. In a number of cases, accumulation is carried out either by simply immersing the electrode in a stirred solution for a given tacc. Then the electrode is rinsed, cleaned and transferred to the pure base electrolyte, in which the actual voltammetric determination is carried out. This procedure is useful because it eliminates the effects of accompanying substances in the sample on the recording of the voltammetric curve. However, the possibility of adsorption of interferents during accumulation cannot be eliminated, which can sometimes be a serious drawback in the use of AdSV.

16. In contrast to the mercury electrode, the use of carbon paste electrodes is more difficult as the determination often depends on the paste composition, on previous electrode treatment, cleaning, etc. In addition, adsorptive accumulation is often accompanied by dissolution of the substance in the binder. Furthermore, the sensitivity attained with a CPE is less than that for a mercury electrode. Most authors work in the concentration range from 10-6 M to 10-8 M; determination of concentrations of 10-9 M are less common. Because of all these possible complications, it is recommended that the original literature be consulted in deciding on the conditions for the determination of a specific substance.

17. The choice of binder is very important for paste electrodes: suitable selection of a binder can sometimes lead to more specific adsorption. It should be noted when using paste electrodes that the substance can also be accumulated as a result of dissolution in the binder during tacc. This is then a combined adsorption-extraction effect , and in some cases, can consist purely of extraction.

18. 3. Chemically modified electrodes: AdSV can also be carried out using electrodes modified by bonding a substance with a known structure to the electrode surface. However, electrodes with chemically modified surfaces are still not often used in analytical chemistry because of the complicated and difficult reproduction of the electrode surface. In addition, the accumulation process at these electrodes is far more complex than for reversible adsorption at a mercury electrode and they are thus difficult to employ for routine analyses. The detection limit is also usually much higher.

19. Increases in selectivity and sensitivity has been achieved by attaching molecules with suitable functional groups to the surface of the working electrode and preconcentration of electroactive species by interaction with these groups. To trap analytes on the electrode surface, complexation, electrostatic attraction or covalent linkages can be used. Carbon-paste electrodes modified with an ion exchanger (tricaprylmethyl-ammonium chloride) have been used to determine nitrite. The accumulation process at these electrodes is far more complex than for reversible adsorption at an HMDE and the detection limit is usually higher.

20. Practical Aspects
It is relatively simple to decide whether a substance can be determined using AdSV at a mercury electrode.
Base Electrolyte: First, the voltammetric behaviour of the compound (at a concentration of 10-6 M) is examined at a hanging mercury drop electrode (HMDE) in different supporting electrolytes.
Accummulation Potential: In the optimum supporting electrolyte, the initial potential is then set (zero Volt or -0.1V vs. SCE), a new mercury drop is formed and the voltage scan towards more negative potentials (at a rate of 20 mV s-1) is immediately begun. After the voltammetric curve has been recorded, a new mercury drop is again formed and the same initial potential is applied for a period of 60 s to the working electrode in stirred solution. After this accumulation period (tacc), stirring is stopped and the voltage scan is run as previously after a quiescent period of 10 s.
If the surface activity of the compound leads to its accumulation, a substantial increase in the peak current is obtained.

21. For oxidizable organic compounds, a solid type of working electrode is used in a similar way: the accumulation is studied at 0 V or with an "open circuit" and then the stripping voltammetric curve is recorded towards more positive potentials. The stripping process can be evaluated by the DPV or DCV mode (in the latter case at a scan rate of 100 mV s-1); the DCV method yields higher limit of determination but yields improved signal to background characteristics and thus peak measurement is usually easier (and more precise), compared to the DPV method (especially at positive potentials). After these preliminary investigations, the most suitable accumulation potential Eacc is found by examining the dependence of the peak current Ip on Eacc. The optimal accumulation time tacc must also be found. The dependence of Ip on the analyte concentration should be linear over a reasonably wide range.

22. The method of standard additions can be used for quantitative measurements. Three additions of a standard solution are recommended to ensure that the measured Ip values correspond to the linear part of the calibration curve. The other parameters in AdSV have similar significance as in anodic stripping voltammetry: for example, the dependence on the magnitude of the electrode surface area, the stirring rate, the rate of increase of the potential, the amplitude and duration of the potential pulse in the DPV method. As the AdSV peak height often increases by up to 7% on an increase in the solution temperature by one degree C (depending on the substance studied), the measurement should be carried out in vessels thermostatted with a precision of at least oC.

23. Determination of organic compounds
AdSV can be used without serious complications for studying organic compounds (charcterized by surface activity) in the concentration range from 1*10-6 – 1*10-8 M. So far, the lowest detection limit has been attained for riboflavin (using a mercury electrode) with a value of 2.2*10-11 M (tacc = 30 min) using DPV. These values for organic substances are similar to these obtained for anodic stripping voltammetry method and therefore the application of adsorbance considerably broadens the region in which voltammetry can be employed in the trace analysis of organic compounds. AdSV can be used in a wide variety of cases to determine organic compounds. It follows from work published so far that AdSV has been used to determine a wide range of biologically active substances, such as various pharmaceuticals, growth stimulants, pesticides and industrially important substances.

24. LIMITATIONS OF THE ADSORPTIVE STRIPPING METHOD
A serious drawback is interference from other surface-active substances that may be present in the sample solution. In this case, competitive adsorption usually occurs and leads to a decrease in the measured current or, at very high surface-active substances concentrations, to significant suppression of the signal. Interfering effects depend on the nature of both the analyzed and interfering substances and on their concentration ratio: in the determination of trichlorobiphenyl (tens of pg L-1), a thousand-fold excess of Triton X-100 produced a 90% decrease in the signal; on the other hand, when the Triton X-100 concentration was comparable to that of trichlorobiphenyl, practically no change occurred in the signal. Evidently, the interfering effect of surface active species can be minimized by employing short accumulation times; however, this approach is not suitable in the determination of trace amounts of analyte. It is then necessary to employ a suitable separation scheme of interfering compounds

25. Solutions to interference problems in AdSV
First type of interferences (surface-active species). Many approaches can be used to minimize this type of interference. Using shorter accumulation times or the proper choice of accumulation potential may decrease the affinity of surfactants toward the electrode surface.
For example, nonionic surfactants exhibit more pronounced depression following accumulation around the potential of zero charge. By a proper choice of the accumulation potential, interferences of urine constituents on the measurement of thiourea were eliminated. More severe peak depressions usually require a separatory technique to isolate the analyte from coadsorbing interferents.

26. (b) The second type of interference (nonadsorbable constituents) may be eliminated by a medium exchange procedure. After adsorption has proceeded under controlled conditions, the electrode is transferred into a suitable clean electrolyte solution, after brief rinsing with water, where the voltammetric measurement is carried out. Because of the mechanical instability of the hanging mercury drop electrode, most applications of the medium exchange approach have involved measurement of oxidizable species at solid electrodes.

27. (c) The third type of interference (metal ions & complexing ligands)
(c) The third type of interference (metal ions & complexing ligands). It is often sufficient for the elimination of this interference to add a suitable masking agent or to adjust the accumulation potential or the pH. In general, the high sensitivity of AdSV permits significant dilution of various samples, thus potentially minimizing possible interferences. In measurements of metal ions via their complexation, the interferent surfactants can be destroyed by UV irradiation prior to adding the complexing ligand of interest. Sample acidification is recommended prior to this treatment to minimize losses by adsorption on the walls of the quartz tube used.

29. Differential-pulse adsorptive stripping voltammogram for 5x 10-7 M Reactive Blue 19 in pH 4.5 Britton-Robinson buffer. Accumulation time: 2 min, accumulation potential: 0.0 V and scan rate: 5 mV S-I.

30. Square wave-AdSV voltammogram for danazol drug in B–R buffer (pH 2.6),
Tacc: 60s, Eacc: 0.0V, scan rate: 300mV s−1, SW frequency: 50 Hz and
pulse amplitude: 80mV. danazol concentration: (A) 5.0×10−7 and (B) 2.5×
10−7 mol l−1.

31. The language and grammar are that of the authors
Adsorptive Stripping Voltammetry for Determination of Cadmium in the presence of Cupferron on a Nafion-coated Bismuth Film Electrode
Nipaporn Meepun, Suphawuth Siriket, Saravut Dejmanee
Int. J. Electrochem. Sci., 7 (2012) – 10591
The language and grammar are that of the authors

32. Summary
The method for reliable determination of Cd(II) by square wave adsorptive cathodic stripping voltammetry was done. The technique involves an interfacial adsorption of Cd(II) as Cd-cupferron complex on a Nafion-coated bismuth film electrode and subsequent stripping from the electrode by square wave cathodic voltammetric and the reduction of Cd-cupferron complex. The optimum conditions of direct reduction of Cd(II) ions was observed by 0.1 M acetate buffer electrolyte (pH 4.0) containing with 0.04 mM cupferron and 0.4 mg L-1 Bi(III) of an accumulation potential at -300 mV versus Ag/AgCl and accumulation time 60 seconds.

33. .The advantages of mercury based electrodes are numerous; easily prepared, sensitive and reproducible with fast and simple electrode kinetics. However, because of the toxicity of mercury, it is important to develop environmental friendly electrodes for stripping voltammetric determination of cadmium. Bismuth film electrodes (BiFE) have been successfully used in anodic stripping voltammetric analysis (ASV) and adsorptive cathodic stripping voltammetry (AdCSV). Bismuth films can be deposited on different substrates such as, glassy carbon, carbon paste, carbon fiber, boron-doped diamond and pencil-lead , showing excellent advantages with respect to mercury films. However, various surfactants in real samples can be adsorbed on the surface of BiFEs, which will result in serious interference and bad analytical performance. The problem can usually be solved by using a protective Nafion layer.

34. Instruments
Square wave voltammetric measurements were performed with PalmSens instrument connected to PC (PalmSens BV, The Netherlands), using a miniaturized cell with three electrodes. A three electrodes system, including a glassy carbon electrode (2 millimeter diameter) served as a working electrode for study electrochemical behavior (Metrohm, Switzerland), a double junction Ag/AgCl (3 M KCl) (Metrohm, Switzerland) and a platinum wire (Metrohm, Switzerland) were employed as reference and auxiliary electrodes, respectively.

35. Preparation of the Nafion-coated electrode
Prior to Nafion coating, the glassy carbon electrode was polished with a 0.05 mm alumina in aqueous slurry, rinsed with ethanol and deionized water. The Nafion coating was made by applying 5 mL of a solution of 1.5% Nafion in ethanol on the surface of the glassy carbon electrode with a micropipette and solvents was then allowed to evaporate at room temperature for 10 min. Then the Nafion membrane was treated with a jet of warm air for about 1 min and left to cool at room temperature. This heat treatment has been shown to improve the stability of the Nafion membrane. The bismuth film was then deposited on the Nafion-coated glassy carbon electrode according to an in situ procedure.

36. Analytical procedure
The three electrodes were immersed in an electrochemical cell containing the 0.1 M acetate buffer (pH 4.0), 0.04 mM cupferron and 0.4 mg L-1 Bi(III). An initial accumulation potential (-0.3 V vs. Ag/AgCl) was applied to the Nafion-coated glassy carbon electrode for 60 s and then the cathodic stripping voltammogram were recorded from -0.6 V to -1.1 V in square wave mode, for which the scan rate is 5 mV s-1, the pulse amplitude is 100 mV and modulation frequency is 25 Hz each scan was repeated five times. Prior to the next cycle, a 30 s conditioning step at +0.3 V was used to remove the target metals and bismuth. The cadmium concentrations in the samples were determined by the standard addition method.

37. RESULTS AND DISCUSSION
Preliminary experiments were carried out to identify the general features which characterize the behavior of Cd(II)-cupferron systems on a Nafion-coated bismuth film electrode. Figure 1 illustrates the influence of cupferron on the cathodic stripping behavior of cadmium on the Nafion-coated bismuth film electrode (NCBiFE) in acetate buffer, following an initial accumulation for 60 s. No peak response was obtained for the blank solution (Fig. 1(a)), as well as when cupferron alone was added to the blank solution (Fig. 1(b)).

38. Figure 1: Cathodic stripping voltammograms demonstrating the influence of cupferron on the reduction of Cd(II) on the NCBiFE. (a) blank solution consisting of 0.1 mol L-1 acetate buffer (pH 4.0), 0.4 mg L-1 BI(III); (b) solution (a) with 0.04 mM cupferron; (c) solution (a) with 25 mg L-1 Cd(II); (d) solution (a) with 25 mg L-1 Cd(II) and 0.04 mM cupferron. Conditions: Eacc -0.3 V; tacc 60 s; Epulse 100 mV; Estep 5 mV; frequency 25 Hz.

39. However, when cadmium alone was added to the blank (Fig
However, when cadmium alone was added to the blank (Fig. 1(c)), a small peak due to the reduction of Cd(II) at V was obtained. The addition of small amount of cupferron to the cadmium solution resulted in a considerable enhancement of the cathodic peak (Fig. 1(d)), a high well-defined peak. The enhancement of the cathodic peak by cupferron clearly indicates that the accumulation of cadmium as the Cd-cupferron complex onto the electrode surface is involved. Compared with NCBiFE, the electrochemical signal on NCBiFE in the presence of cupferron was improved about two times and the Cd(II) peak was shift to more positive value about 30 mV. This phenomenon can be concluded that the Cd-cupferron complex was adsorbed on the surface of electrode.

40. Effect of supporting electrolyte and pH
Different types of buffer were investigated as supporting electrolyte such as acetate, phosphate and borate. It was found that the acetate buffer gave the sharper peak and much enhanced peak currents. The influence of pH value of the used buffer solution on the cathodic stripping peak current of Cd(II) was also investigated in the range of 3.6 to 4.4. It was found that at pH 4.0 the peak current of cadmium was at maximum value, the reduction of peak current were decreasing both at lower and higher pH values due to the protonation of coordination sited of the ligand and the hydrolysis of the Cd(II), respectively. Thus, pH 4.0 of acetate buffer solution was used for further experiments.

41. Figure 2. Effect of pH on the peak current of 50 mg L-1 Cd(II)
Figure 2. Effect of pH on the peak current of 50 mg L-1 Cd(II). Conditions: 0.5 mg L-1 Bi(III); 0.02 mM cupferron; Eacc -0.4 V; tacc 60 s; Epulse 80 mV; Estep 5 mV s-1; frequency 25 Hz.

42. Effect of Nafion Concentration
The effect of Nafion concentration on the sensitivity of cadmium analysis procedure was investigated in a range from 0.5 to 2% (Fig. 3). The experimental results were shown that the cathodic stripping peak current of Cd-cupferron increasing Nafion concentration increased from 0.5 to 1.5%(w/v). Indeed, at very thin or very thick polymer films, the stripped species were able to diffuse away from electrode due to incomplete coating or cracks, respectively. Thus, a Nafion concentration of 1.5% was selected as the optimum condition in the further experiments.

43. Effect of Nafion concentration on the peak current of 50mg L-1 Cd(II)
Effect of Nafion concentration on the peak current of 50mg L-1 Cd(II). Conditions: 50 mg L-1 Cd(II); 0.1 M acetate buffer pH 4.0; other conditions similar as Fig. 2.

44. Effect of cupferron concentration
The influence of increasing concentration of cupferron on the cathodic stripping peak heights of cadmium at pH 4.0 in 0.1 M acetate buffer at a accumulation potential -0.4 V for 60 s on a NCBiFE was investigated in the range of 0.0 to 0.3 mM and is shown in Figure. 4(A). At lower cupferron concentrations, the cadmium peak height increased with increasing ligand concentration and reached the maximum value for cupferron concentrations in the range 0.0 to 0.1 mM. The presence of higher cupferron concentration level over 0.1mM (0.1 mM to 0.3 mM) caused leveling off of the cathodic stripping peak height. At cupferron concentrations higher than 0.04 mM, the peak current of cadmium increased more considerably, but the peak current of acetate buffer decreased and peak position of acetate at V became shift to more cathodic potential (Fig 4(B)).

45. Figure 4. Effect of cupferron concentration on the peak current of 50 mg L-1 Cd(II) in 0.1 M acetate buffer pH 4.0, (A) mM cupferron, (B) various concentration cupferron; (a)-(h) 0, 0.04, 0.06, 0.08, 0.1, 0.15, 0.2 and 0.3 mM, respectively. Other conditions similar as Fig. 2.

46. Effect of accumulation potential
The effect of varying accumulation potential on the cathodic stripping peak current of cadmium was studied in the range from -0.1 to -0.8 V at a accumulation time of 60 s, and the results were shown in Figure 5. It was found that the peak current was rather high with changing potentials from -0.1 to -0.3 V, probably due to the increased accumulation of the complex on the electrode surface. The peak current slightly decreased at potentials more negative than -0.3 V. An accumulation potential of -0.3 V gave the base sensitivity and was used for further determinations.

47. Figure 5. Effect of accumulation potential on the peak current of 50 μgL-1 Cd(II). Conditions: 0.1 M acetate buffer (pH 4.0); 0.4 mg L-1 BI(III); 0.04 mM cupferron; tacc 60 s; Epulse 80 mV; Estep 5 mV; frequency 25 Hz.

48. Effect of accumulation time
The effect of the accumulation time on the stripping peak currents of Cd(II) was studied in the range from 15 to 300 s as illustrated in Figure 6. The peak current increased with increasing accumulation time until the saturation surface concentration is gradually reached. As is expected for adsorption processes, the dependence of the peak current on the accumulation time is limited by the saturation of the electrode. Thus, accumulation time of 60 s was used throughout the remaining experiments, as it gave good sensitivity and relatively short analysis time.

49. Effect of accumulation time on the peak current of 50 μgL-1 Cd(II)

50. Effect of square wave parameters
In order to obtain well-defined square wave adsorptive cathodic stripping voltammetry (SWAdCSV) response to the signal of Cd(II) in the presence of 0.04 mM cupferron, 0.1 M acetate buffer pH 4.0, following accumulation on the Nafion-caoted bismuth film electrode (NCBiFE) at -0.3 V for 60 s, the square wave parameters investigated were the step potential, the square wave amplitude and the frequency. When the frequency was higher than 30 Hz, the peak shape of Cd-cupferron complex was unsatisfactory. Increasing the potential step did not enhance significantly the peak height of Cd(II) despite of the dramatic increase in the scan rate, in accordance with earlier reports. The best conditions for quantitative measurement of cadmium concentration were then selected at 25 Hz frequency, 5 mV s-1 potential step with 100 mV square wave amplitude.

51. Adsorptive stripping voltamograms and calibration curve for increasing concentration of Cd(II) with NCBiFE.