Anodic Stripping Voltammetry:

This technique is used to determine trace amounts of metal ions that can be preconcentrated at an electrode by reduction to the metallic state. The microelectrode behaves as a cathode during the deposition step and as an anode during the stripping step with the analyte being oxidized to the original form:

This may be explained in detail taking the example of Cu+2, the deposition reaction at HMDE (Hanging Mercury Drop Electrode) held at a cathodic potential sufficient to deposit the metal ion on the electrode

Cu ²⁺ (aq) + 2e^- = Cu (Hg)

where Cu(Hg) is copper amalgamated with mercury. This step is basically a preconcentration step where the metal ion from a large volume of the solution is concentrated on a small volume of the electrode. The solution is stirred during the electrolysis to increase the rate of deposition. Near the end of the deposition time stirring is stopped and usually 1-30 mins are used for deposition. In the second step, the potential is scanned anodically towards more positive potential. When the potential becomes positive, the analyte is stripped from the electrode returning to its oxidized form

Cu (Hg) = Cu ²⁺  (aq) + 2 e ^-                                                                             

And the current during this step is monitored as a function of potential providing increment to the voltammogram, the peak current of that is proportional to the analyte concentration in the solution. This method is sensitive to experimental conditions such as area of mercury film electrode or size of the Hg drop, deposition time, rest time, rate of stirring and scan rate in during the stripping step. If properly controlled that method will be quite sensitive and accurate and is best used for metals which form amalgams.