15 Anodic and Cathodic stripping voltammetry

Dr. Heena Rekhi Rekhi and Neha Sharma

  1. Description

Stripping voltammetry, also called stripping Chronoamperometry, has the lowest detection limit of any commonly used electroanalytical technique. Basically, electrochemical stripping analysis is a two-step operation. During the first step, analyte is electrolytically deposited onto or into the surface of electrode typically consisting of a thin film or a drop of mercury or in some cases a solid electrode by controlled potential electrolysis. This is followed by a reverse electrolysis, or stripping step, in which the deposited analyte is removed from the electrode. Each electrochemical species strips at a characteristic potential.

 

The preconcentration or electrodeposition step provides the means for substantially improving the detection limit for the analytical (stripping) step. Since the volume of the mercury electrode or the solid electrode surface is considerably less than the volume of the sample solution in the electrochemical cell, the resulting amalgam or deposit of metal atoms into the electrode may be more concentrated than the original test solution by a factor up to 1 million.

 

The procedure for shipping out stripping analysis involves initially preconcentrating the metal on electrode surface or into Hg (liquid) at negative potentials followed by selective oxidation of each metal phase species under the influence of anodic potential sweep. This analytical technique is sensitive and serves as a reproducible method for analysis of trace metal ion in aqueous media, 2) concentration limits of detection for many metals are in ppb range and approximately 12-15 metal ions can be analyzed by this method. The size coverage and distribution of metal ions on the surface of electrodes influence stripping peak currents and peak widths. Stripping techniques differ in their method of accumulation (e.g., electrolytic vs adsorptive) and method of detection (e.g., voltammetry vs potentiometry).

 

Stripping voltammetry is a sensitive electroanalytical technique for the determination of trace amounts of metals in solution. The technique consists of three steps. Firstly there occurs deposition of metal ions onto an electrode held at a suitable potential. The solution is stirred during this step to maximize the amount of metal deposited. Second, stirring is stopped so that the solution will become quiet. Third, the metal deposits are stripped from the electrode by scanning the potential. The value of current in the stripping step gives the idea of amount of metal in the solution.

 

The stripping step may consist of a positive or a negative potential scan, creating either an anodic or cathodic current respectively. Cathodic Stripping Voltammetry (CSV) and Anodic Stripping Voltammetry (ASV) are two specific stripping techniques.

 

Anodic Stripping Voltammetry

 

Anodic stripping voltammetry (ASV) is used primarily to determine the concentration of trace metals that can be preconcentrated at an electrode by reduction. The method is especially effective for metals that dissolve in mercury by forming amalgams. Either a hanging mercury drop electrode or a thin-film mercury electrode can be used. Very electropositive metal ions, such as mercury(II), gold(III), silver and platinum(IV), are deposited on solid electrodes such as glassy carbon. The geometry of diffusion is critical within or on the electrode surface, such as a mercury pool, is not useful.

 

In the first step, a deposition potential is chosen that is more negative than the half-wave potential of the metal or metals to be determined. Regions in which particular metal ions are reduced at mercury electrode are determined from the current-potential curves discussed in previous sections. A suitable potential would lie on the diffusion current plateu of a dc polarogram or a normal pulse voltammogram. For example, copper (II) can be selectively reduced in the potential range 0 to -0.25V (vs. SCE), whereas copper (II), bismuth, lead, cadmium and zinc could be reduced simultaneously at a potential of -1.3 V. Similarly copper and bismuth could be selectively deposited at -0.4 V. The solution is generally stirred during deposition to maximize analyte-electrode contact.

 

If more sensitivity is required, the deposition time is simply increased. This increases the degree of preconcentration, making more deposited analyte available at the electrode during the stripping step. However, the deposition step is seldom carried to completion. Usually, only a fraction of the metal ions needs to be deposited, just a sufficient amount to produce a measurable current during the stripping step. However, it is important that the same fraction of metal ion be removed during each experiment. Thus the temperature and stirring of the sample solutions must be kept as constant and reproducible as possible, and the deposition time must be carefully controlled for samples and standards alike.

 

In the final stripping step, there is no stirring of the solution. The potential is scanned in the positive direction (typically linear potential sweep voltammetry, differential pulse voltammetry, or square-wave voltammetry). At characteristic potentials the deposited metal atoms are stripped from the electrode back into the solution by oxidation to the ionic form. The potentials of the stripping peaks identify the respective metals, since, ideally, the different metals strip back into solution in reverse sequence to their reduction potentials. The area under the resulting current peaks is proportional to the concentrations of the respective analyte species.

 

Prior to each anodic stripping experiment, the supporting electrolyte must be conditioned or purified. In the conditioning step, a potential of 0.0V versus SCE (usually just negative of oxidation potential of mercury) is applied to the electrode for a controlled time (60-120 sec) to clean the electrode by removing contaminants from the mercury drop or material not removed during the prior stripping step. In order to prevent the formation of thin film electrode in situ, the conditioning potential may be set positive of the oxidation potential of mercury to provide a clean electrode surface for the deposition step. The solution is stirred during the conditioning step. Purging the solution with purified nitrogen gas for 2-10 min eliminates interference from oxygen. Standard samples and blank are carried through identical electrodeposition and stripping steps. Often the method of standard addition is used for evaluation. The limit of detection is nearly always governed by the magnitude of the blank value and not by instrumental sensitivity.

 

Differential pulse anodic stripping has two advantages. First, it discriminates against the capacitive component of the stripping signal. Only a portion of the metal atoms oxidized as a result of potential step that ends an anodic pulse has a chance to diffuse away from the electrode surface before the potential step that begins the next pulse returns the potential to a value at which the metal is redeposited. Consequently, metal atoms make multiple contributions to the analytical signal and render the detection limit of differential pulse anodic stripping voltammetry lower than that of linear potential sweep anodic stripping voltammetry. The second advantage is that linear sweep methods have continuous interference from charging as long as the potential is scanned.

 

Anodic stripping voltammetry can be complicated by intermetallic formation, particularly with a thin-film mercury electrode. This occurs when metals such as zinc and copper are in high concentrations. When such intermetallics are present, the strippng peaks for the constituent metals may be shifted, severely depressed, or absent completely. The formation of intermetallics is less likely to be a problem with a hanging mercury drop electrode because the large electrode volume diminishes the maximum achievable preconcentration. Intermetallic compound formation can render the results of standard addition evaluation incorrect.

 

Principle

 

The most sensitive of commonly used electroanalytical techniques is anodic-stripping analysis. Trace level analysis can be performed for solutions containing metal ions in range of 10-6-10-12 M. Additional applications of stripping voltammetry include simultaneous multielement determination and it serve as cost effective instrumentation technique as compared to other spectroscopic techniques. Three types of working electrodes can be used:

 

(a) The classical hanging mercury-drop electrode (HMDE),

(b) The thin-film mercury electrode (TFME),

(c) Solid electrodes (gold, silver, surface-modified carbon).

 

The sequence of steps of the anodic stripping analysis is described below.

 

Step 1. The electrodeposition step

 

(a) Deposition of metal ions onto the surface of hanging mercury-drop electrode (HMDE), in the form of amalgam, M(Hg):

(b) Mn+ + ne- → M(Hg)

(c) The reduction of metal ions of interest (E-E1/2 about -200 mV) during electrolysis carried for (30sec to 5 mins) in a stirred solution is achieved under proper conditions. This step is called the electrodeposition step. Factor affecting the concentration of metal ion in the film are time of electrolysis, concentration of metal ion in solution and rate of stirring. Since the electrodeposition is carried out on small electrodes, the amount of material deposited into it usually does not change significantly the concentration of the metal ions Mn+ in the solution. Their occurs a preconcentration in this step1. However, it can be estimated.

 

Amalgated metal mercury with respect to the initial concentration of Mn+, using Nernst simplified model, concentration is estimated in stirred solutions as

i =   +

However the value of thickness of Nernstian layer is 20  m.

 

Cathodic Stripping Voltammetry

 

The procedure for cathodic stripping voltammetry (CSV) follows the same steps as outlined for anodic stripping voltammetry. It involves preconcentration by oxidation with subsequent stripping via a negative potential scan. Cathodic stripping voltammetry is used to determine those materials that form insoluble mercury salts on the electrode surface. At a relatively positive potential, mercury(I) ions are produced at the mercury electrode surface during an anodic pre electrolysis. Materials that precipitate with mercury (I) ions form an insoluble film on the surface of the mercury electrode. After a rest period, a cathodic scan causes the reduction of the salt to mercury and the original anion, giving a cathodic current peak.

 

Silver can be used as the electrode for the determination of anions that form insoluble silver salts. Materials that can be determined by cathodic stripping voltammetry include arsenate, chloride, bromide, iodide, chromate, tungstate, molybdate, vanadate, sulfate, oxalate, succinate, selenide, sulfide, mercaptans, thiocyanate and thio compounds. Lead has been determined by cathodically stripping a film of PbO2 deposited on a SnO2 electrode.

 

CSV is the method used to determine organic and inorganic compounds that form insoluble salts with an electrode. An insoluble film is formed on the surface of the electrode by the application of an anodic (positive) potential to the working electrode. The electrodeposition reaction involved during the preconcentration step may be represented by:

mechanism for this is given below:2RSH + Hg → Hg(RS)2 + 2H+ + 2e-.Thus both inorganic and organic compounds can be determined by CSV.

 

As a result of the elctrodeposition step the decrease of concentration in solution is calculated for a mercury film electrode for the following conditions: the area of the film, A = 0.2 cm2; the initial concentration of the metal ions in the solution, CMn+ = 10-8 M; the volume of the tested solution is 20 ml; the time of deposition is 100 s; the Nernstian layer thickness during the deposition step is 20 m. The number of moles of the metal electrolyzed in the film, at the end of the electrolysis step CM(Hg) = 10-12 mole. Thus, the decrease of concentration of Mn+ in solution after the electrolysis is negligible (0.5 %).

 

Step 2. Rest period

 

After a predetermined time, the stirring of the solution is turned off. The solution is left undisturb to achieve uniformity in the concentration of metal in the amalgam. The extension of rest period for about 30 sec, ensures that no reoxidation of the metal by traces of oxygen takes place and applied potential remains unchanged during this tenure. During the rest period the electrodeposition current decreases.

 

Step 3. Stripping

 

After the oxidation of the depositing metal M i.e. stripping the ion from the mercury electrode back into solution after preconcentration step by oxidation to the ionic form under diffusion controlled conditions, using one of the voltammetric methods:

 

M(Hg) → Mn+ + ne-

 

The anodic diffusion current is used to determine the concentration of the metal in the amalgam, which is proportional to time of electrolysis, stirring rate and concentration of Mn+ in solution.

 

Types of working electrode:

 

Mercury electrodes:

 

For stripping analysis, the working electrode must be stationary, and have a favorable redox behavior of the analyte, reproducible area and low background current over wide range of potential. The most used electrode, which fulfills these requirements, is hanging dropping mercury electrode (HDME) and mercury film electrode (MFE).

 

Solid electrodes:

 

Oxidizable compounds can be monitored pertaining to the limited anodic potential of mercury electrodes. Accordingly, solid electrodes with extended anodic potential windows have attracted considerable analytical interest. There are many different types of solid electrodes used as working electrodes such as gold, platinum, glassy carbon electrode, carbon paste electrode, carbon fiber electrode, and epoxy-bonded graphite electrode. Unlike mercury electrodes, solid electrodes present a heterogeneous surface with respect to the electrode chemical activity. Such surface heterogeneity leads to deviations from the behavior expected from homogenous surfaces. The dependence of response on the surface state of an electrode is an important factor pertaining to utility of solid electrodes. Accordingly, the use of such electrodes requires precise electrode pretreatment and polishing to obtain reproducible results. The nature of these pretreatment steps depends on the materials involved

 

Chemically Modified electrodes (CMEs):

 

Used working electrode may be insensitive to be applying in a certain field. Modification will be used to improve the properties of the selected working electrode. The main idea of the modification depends on incorporating of a reagent on the electrode surface or into the matrix of the selected electrode. The most famous method for the incorporation of a modifier to the electrode surface is covering the electrode surface with suitable polymer film.

 

Electrode surface with the solution of the selected polymer and allowing the solvent to evaporate. Also, electroploymerization may be used to make the polymer film on the electrode surface. As a new type of CMEs, pre concentrated CMEs were described. Such modified electrodes have surface characterized by ability for reacting and binding the target analyte. Preconcentrating agent used in such modifications is usually incorporated in the electrode matrix (as done with carbon paste electrode or may be binding with functionalized polymeric film on the electrode surface.

 

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