Anodic Stripping Voltammetry: Hints and Tips
It is not uncommon for newcomers to analytical electrochemistry to have teething difficulties with anodic stripping voltammetry (ASV) experiments.
The most common problems are:
- The working electrode should be scrupulously clean before the experiment. Especially no oily deposits (eg fingerprints) should be left on the tip of the electrode as these stop the formation of an even mercury film.
- The mercury film itself is actually a collection of small mercury droplets with an overall gray appearance. Inexperienced users often want to deposit more mercury to get a mirror like finish but this would require the deposition of more mercury than is required. Also excess mercury can fall from the electrode mid-experiment.
- Control of solution pH is critical for most ASV experiments. Most transition metal ions are hydrolysed to an appreciable extent at pH 7. That is hexa-aquametal ions, [M(H2O)6]2+, will convert to hydroxyl species eg [M(H2O)5(OH)]+ as the pH is raised, eventually resulting in the formation of insoluble hydroxides and oxides. These processes interfere with the simple chemistry of the conversion of a metal ion in aqueous solution to a metal atom in the mercury phase in the ASV experiment. One of the first signs of trouble is a shift in the ASV peak position (and sometimes an unsymmetrical peak). In general ASV of simple metal dications should be done at around pH 4 or less. Even pH 5 would be too high for copper. Use of a narrow range indicator paper can be useful to determine if the pH has drifted out of range far. However the pH should not be so low as to cause dissolution of the substrate metal out of the mercury amalgam. For example while pH 2 would probably be OK for copper determination it would be too low for ASV of zinc. A pH of about 4 is usually a good compromise.
- Another problem can be the temptation to use too high concentration of substrate ions. ASV is normally used to detect metal ion in the ppb ranges (or even ppt). If using ppm solutions, you normally would have use very short deposition times to avoid putting too much substrate into the mercury film - which leads to a mercury amalgam that no longer behaves ideally (eg changes in viscosity can occur). Again the first sign of trouble can be changes in peak position, misshapen and peaks even double-humped peaks.
- Changes in peak position (especially drift in the one direction over several days) can indicate that the reference electrode is in need of regeneration or replacement. Reference electrodes do age! If you are using a reference electrode without a knowledge of its history or age then do not be surprised by strange peak positions.
For more information, download the teaching note (550 KB PDF) covering the quantitative determination of copper, lead and cadmium in tap water at a thin mercury film electrode.