What medium is required for electrochemical experiment?
What solvents and salts are appropriate for an electrolyte solution?
What are some typical electrolyte solutions?
How does solution resistance affect my experiments?
What is a Luggin capillary?
What is positive feedback iR compensation?
How does uncompensated resistance affect a cyclic voltammogram?
The medium must be conducting. This can be achieved by using either a molten salt or an electrolyte solution. An electrolyte solution is made by adding an ionic salt to an appropriate solvent.
The salt must become fully dissociated in the solvent in order to generate a conducting (i.e., ionic) solution. The electrolyte solution must also be able to dissolve the analyte, must be electrochemically inert over a wide potential range (i.e., no current due to electrolyte solution oxidation/reduction), and must be pure (e.g., the presence of water decreases the size of the potential range). It must also be chemically inert, so that it will not react with any reactive species generated in the experiment (e.g., acetonitrile is nucleophilic, so can react with electrogenerated cations). If the temperature is to be varied, the electrolyte solution must have an appropriate liquid range.
Electrolyte solutions can be aqueous or non-aqueous. A wide range of salts can be used for aqueous electrolyte solutions. Since the redox potentials of some compounds are pH sensitive, buffered solutions should be used for these compounds. Suitable non-aqueous solvents include acetonitrile, DMF, DMSO, THF, methylene chloride, and propylene carbonate. Salts for non-aqueous electrolyte solutions typically consist of a large cation (e.g., tetraalkylammonium cations), and large anions (e.g., hexafluorophosphate, tetrafluoroborate, and perchlorate) to ensure full dissociation. N.B. Perchlorate salts must be handled with care, since they are potentially explosive.
Although the addition of fully dissociated salts improves the conductivity of the electrolyte solution, many electrolyte solutions (particularly those based on non-aqueous solvents) have a significant resistance (hundreds of ohms). This leads to a potential drop between the electrodes (termed iR drop – potential = current (i) x solution resistance (R)). Some of this iR drop can be compensated for by using a potentiostat and a three electrode system. However, some resistance (between the working and reference electrodes) remains uncompensated. This uncompensated resistance can be decreased or eliminated by careful cell design (including use of a Luggin capillary), positive feedback iR compensation, or post-run data correction.
Uncompensated resistance can be decreased by placing the reference electrode close to the surface of the working electrode. This can be achieved using a Luggin capillary, which is a hooked capillary that is attached to the end of the reference electrode (i.e., it is an extension to the reference electrode). The tip end of the capillary is placed close to the surface of the working electrode. However, it must not be placed too close, otherwise part of the surface may be blocked. In addition, exact placement of the capillary tip is required to obtain reproducible results.
Positive feedback iR compensation is available on the epsilon. This method feeds back a voltage into the cell electronics to compensate for the iR drop due to the solution resistance. However, care must be taken when selecting the applied feedback voltage, since too high a voltage can drive the electronics into oscillation, which can adversely affect the surface of the working electrode (since extreme potentials are applied). This is prevented in BASi instruments by first measuring the uncompensated solution resistance, and then increasing the magnitude of the feedback incrementally, testing the system for stability after each increment.
If the uncompensated resistance is significant (hundreds of ohms), then the peak potential separation increases and the peak current decreases. These effects become more pronounced with increasing scan rate. Unfortunately, these effects are also characteristic of slow electron transfer kinetics. Since slow electron transfer kinetics are not dependent on analyte concentration, and the effects of uncompensated resistance are (E = iR), the two can be differentiated by running the experiments at different analyte concentrations.