Читать книгу Electroanalytical Chemistry - Gary A. Mabbott - Страница 20

Voltammetry experiments, where currents are measured, require ions in the solution to carry charge between electrodes. Even though water ionizes to a small degree (for pure water [H⁺] = [OH⁻] = 10⁻⁷ M), the conductance of water is usually too small for the purposes of most voltammetry experiments. Instead, one usually adds a pure salt to the sample solution. The salt is often referred to as the supporting electrolyte. Because the pH of a solution influences the electrode reaction in many cases, often an acid/base buffer is also included in the supporting electrolyte. The supporting electrolyte keeps the electrical resistance down. Lower resistance helps minimize voltage errors due to “ohmic losses” in voltammetry experiments. In a voltammetry experiment, current passes through the cell. The solution that carries that current has a finite electrical resistance. Energy, in the form of voltage, is lost overcoming the resistance according to Ohm's law. This loss represents an error in the measurement of the true voltage. The energy lost in volts, V, is given by

(1.15)

where i is the current driven through the solution resistance, R. The actual voltage that reaches the electrode, V_actual is

(1.16)

In typical voltammetry experiments, the resistance is on the order of 100 Ω. Consequently, errors on the order of 1 mV or bigger occur when the current reaches 10⁻⁵ A (=10⁻³ V/100 Ω) or more. The energy lost in overcoming the solution resistance is energy that is not applied to the working electrode. Whenever the product, iR, is greater than a few millivolts, the assumption that all of the energy applied to the cell is focused onto the working electrode/solution interface no longer holds and the data are suspect.