Open Circuit Potential (OCP)

Posted 1/4/2016 in Uncategorized

This article is part of the AfterMath Data Organizer Electrochemistry Guide

Open Circuit Potential is a technique that measures the potential difference between the working and reference electrodes.

Detailed Description

Like most other electrochemical techniques, this experiment begins with an induction period. During the induction period, a set of initial conditions which you specify is applied to the electrochemical cell and the cell is allowed to equilibrate to these conditions. Data are not collected during the induction period.

After the induction period, the potential difference between the working and counter electrodes is monitored for a specified period of time.

The experiment concludes with a relaxation period. During the relaxation period, a set of final conditions which you specify is applied to the electrochemical cell and the cell is allowed to equilibrate to these conditions. Data are not collected during the relaxation period.

At the end of the relaxation period, the post-experiment conditions are applied to the cell, and the instrument returns to the idle state.

Potential is plotted as a function of time.

Parameter Setup

The parameters for this method are arranged on two tabs on the setup panel. The Basic tab contains the parameters relating to the electrolysis. An additional tab for Post Experiment Conditions is common to all of the electrochemical techniques supported by the AfterMath software.

Basic Parameters Tab

You can click on the “I Feel Lucky” button (located at the top of the setup) to fill in all the parameters with typical default values (see Figure 1). You may want to change the Duration in the Electrolysis period box to a value which is appropriate for the electrochemical system being studied. You may also want to change the Number of intervals in the Sampling Control box.

Figure 1: Basic Setup for OCP.

Post Experiment Conditions Tab

After the Relaxation Period, the Post Experiment Conditions are applied to the cell. Typically, the cell is disconnected but you may also specify the conditions applied to the cell. Please see the separate discussion on post experiment conditions for more information.

Typical Results

Figure 2 shows the typical results for a solution of $2 mM \;K_3Fe(CN)_6 \;in \;0.1 M \;KCl, \;2 mm$ Pt WE, Pt mesh CE.

Figure 2: Open Circuit Potentiogram of a Potassium Ferricyanide Solution

Theory

Consider the reaction $O + n e^{-} \rightarrow R$ where $O$ is reduced to $R$ in an n electron reaction with formal potential $E^0$ . By measuring the OCP you could determine the ratio of $O$ to $R$ through the use of the Nernst Equation.

$E = E^0- {\frac{RT}{nF}} \; ln \left({\frac{[R]}{[O]}}\right)$

where $R$ is the universal gas constant ( $8.314 \;J/mol K$ ), $T$ is the absolute temperature ( $K$ ), $n$ is the number of electrons, and $F$ is Faraday's Constant ( $96485 \; C/mol$ ). Knowing the OCP, you could calculate the ratio of products to reactants. Please see the BE-RDE webpage for an example of using OCP to calculate the ratio of products to reactants.