Comparison of Pulse Techniques

Goals

The goals of this experiment are to:

• Understand how variations in the applied potential waveform affect the appearance
  of the current data
• Determine the E_1/2 for copper using three different techniques
• Determine the amount of copper in an unknown sample using standard addition.

Experimental Apparatus

• Gamry Instruments Framework^™ software package installed on a host computer
• Seven 10 mL volumetric flasks
• Volumetric pipets (1, 2, 3, 4, 5 mL)
• Screen-printed electrode cell stand (Gamry part number 990-00420)
• Carbon working screen-printed electrode (Gamry part number 935-00120)

Reagents and Chemicals

• 1.0 M sulfuric acid, pre-purged to remove dissolved O₂
• Unknown sample
• 100 ppm Cu in 0.1 M sulfuric acid, pre-purged to remove dissolved O₂

Background

Copper-poisoning occurs when copper builds up in the body over the course of months or years. The copper contamination can originate from many sources, including eating acidic foods that have been prepared in copper cookware, or excess copper in drinking water. Poisoning is more serious for children under the age of six because copper can delay physical and mental development. In adults, poisoning can cause symptoms such as memory loss and mood disorders.

The United States has occasional problems with the discovery of large amounts of copper in the drinking water. The Environmental Protection Agency’s action-level limit for copper in drinking water is 1.3 parts per million, meaning that cities and home owners should take steps—such as filtration—to reduce the copper levels if the level exceeds this value (reference 1). In the city of Flint, Michigan (for example), while the focus has been on high levels of lead in the city’s drinking water, many samples have been found to contain high levels of copper as well. (reference 2).

Traditional sweep techniques, such as cyclic voltammetry, have limits of detection in the parts per thousand range. To lower the limit of detection, a more sensitive technique is needed. For this reason pulse voltammetry was developed. The sensitivity is increased for pulse experiments over sweep experiments because the current is measured towards the end of the voltage pulse. This allows a decay in the capacitive current—as in a chronoamperometric experiment—before the measurement of the faradaic current, where as there is no decay of the capacitive current before the measurement in sweep voltammetry. A higher faradaic response is also observed because of the time between pulses, which allows the surface conditions to refresh.

In all forms of pulse voltammetery, the current-response is proportional to the amount of analyte that is electrolyzed at the surface. For this reason, the peak current is dependent upon:

• Electrode surface area,
• Diffusion coefficient of the analyte,
• Concentration of the analyte.

Because the peak current only accounts for analyte that is being electrolyzed, it is also dependent upon the step height of the pulse. That is to say, the greater the pulse, the more conversion of analyte occurs.