Quantitative Determination of Ions by Stripping Voltammetry

Goals

• To understand the effect of deposition time on the current response
• To use standard addition to analyze for lead
• To compare the amount of lead from different locations

Experimental Apparatus

• Gamry Instruments Interface 1000T potentiostat
• Gamry Instruments Framework^™ software package installed on a host computer
• Standard UV/Vis disposable cuvette
• SPE cuvette adaptor (990-00421)
• SPE cuvette top (972-00065)
• Carbon working screen-printed electrode (935-00120)
• Micro stirbar (935-00065)
• Seven 10 mL volumetric flasks
• Volumetric pipets (1, 2, 3, 4, 5 mL)

Reagents and Chemicals

• Three to five water samples, 10–15 mL each, pre-purged to remove dissolved O₂
• 100 ppm aqueous Pb, pre-purged to remove dissolved O₂
• 1.0 M sulfuric acid, pre-purged to remove dissolved O₂

Background

The ability to perform trace analysis of heavy metals is important to studying the
effects of metals on everything from ecology to their effects in cells. Traditionally
atomic absorption spectroscopy has been used to determine the concentrations of
these analytes. But one of the main disadvantages of this technique is that only one
analyte may be analyzed for at a time, because the instrument can be set for only
one wavelength.

In voltammetry determination the concentrations of several analytes simultaneously
is possible via the use of stripping voltammetry. In stripping voltammetry, a potential
is applied to the working electrode so that the analytes of interest are reduced
to a metallic state on the surface of the electrode. The solution is stirred during this
process to allow more solution to contact the surface of the working electrode, thus
pre-concentrating more of the analyte to the surface. The goal of this pre-concentration
step is to have as much of the analytes as possible adsorb onto the surface of
the working electrode (to increase the technique’s sensitivity), but not more than a
single monolayer (single layer of atoms or molecules). More than one monolayer
distorts the shape of the peaks, because there are different interactions from the analyte-
electrode interface versus the analyte-analyte layers’ interface, and these affect
the stripping potentials of the different molecular layers.

The potential is then scanned using one of several waveforms (linear sweep, staircase,
pulse, square wave). When the potential reaches the point at which an analyte
on the surface is oxidized back to its ionic form, the analyte is said to strip from the
electrode because it desorbed rapidly. The advantage of this technique is because
different metals have different redox potentials, many analytes can be evaluated at
the same time. The limiting factor in the number of analytes that can be determined
at the same time is the ability to achieve sufficient separation between the peaks of
the different analytes of the voltammogram for qualitative analysis.