Amalgam voltammetric approach to heavy metal speciation in natural waters: Part 3. Distortion resulting from the electrode sphericity and pulse application

Abstract

The concentration of heavy metals in natural waters is normally too low for direct speciation studies. A new method of studying speciation and complexation was proposed in a previous paper and the theory was given. This method utilizes hanging drop amalgam voltammetry in conventional or differential pulse modes. The amalgam is prepared in situ prior to a medium exchange to the solution to be studied. The method presented has been verified experimentally with acceptable results by studying the complexation of lead using different experimental parameters. However, the method was based on a simplified model using the Nernst diffusion layer concept with a linear diffusion system, ignoring the effect of electrode sphericity and the loss of mass due to the pulse application. These two effects are taken into consideration in this paper. A shift in potential due to the sphericity is found, but this can be ignored in speciation studies because an identical shift appears for the standard. The additional loss of mass due to pulse application results in a rather distorted current-potential curve with the maximum given by a fourth-order equation. The limitations imposed by the use of the simplified diffusion model are discussed, and it is found that the model presented in the previous two papers can be used as an experimental correction method, with sphericity and pulse effects included.