Anodic Stripping Voltammetry

 

In this example, a sample containing copper and cadmium (the two analytes) are first concentrated in a mercury working electrode and the two metals then sequentially reduced back to their metal ions and the current time curves reported. The results obtained are shown in figure 20. The first stage is to reduce the metal ion to the metal that will form an amalgam with the mercury working-electrode.

 

Figure 20. The Potential Time Trace Used in an Anode Stripping Volatammetry to Determine the Copper and Cadmium Contents

 

This can be depicted by the following equation.

 

 

If a mixture of cations are present the working electrode should be maintained at a potential at least 0.4 V more negative than the standard potential of that metal most difficult to reduce. The deposition of all the metal components of the mixture will be determined by the magnitude of the respective mass transport of each. The speed of deposition can be accelerated by electrode rotation or by vigorously stirring the electrolyte.

 

The concentration of the reduced metal(s) (e.g. metal (A)) in the mercury will be given by,

 

where  () is the limiting current for the deposition of the metal (A),

                () is the deposition time period,

               () is the volume of the mercury electrode.

 

The time of the deposition will vary with a number of factors including the analyte concentration and the electronic parameters, but the conditions should be adjusted so that the deposition takes from one to ten minutes. (the deposition step in figure 20).

 

The pre-concentration step can the be followed by a change in the electrolyte composition to make it more amenable to the stripping procedure and this process takes place in the Ôrest periodÕ in figure 20. Electrode rotation or stirring should be arrested during this procedure. A change in the electrolyte composition can increase the sensitivity of the procedure and can improve resolution if there is more than one analyte being determined.

 

In the third stage the metals are stripped from the mercury by applying an appropriate voltage/time program (that shown in figure 20 being a linear voltage /time relationship that is the most common program form). When the applied working electrode potential reaches the standard metal-metal ion redox potential the metal is stripped from the amalgam according o the following equation.

 

 

In the current time curve in figure 20, peaks are observed for the analytes, cadmium and copper. The peak potential of each metal is characteristic of the analyte and can be used to identify the metal. The height of each metal peak (i_p) is proportional to the concentration of the metal in the test solution. Quantitative analysis is normally achieved using a calibration curve.

 

If a thin-film mercury electrode is employed, under which circumstances the metal is rapidly depleted from the amalgam, then the expression for the peak current () is given by ,

 

 

                                where () is the program rate,

                                           (A) is the film area

                                     and (l) is the film thickness.

 

The other symbols having the meaning previously ascribed to them.