Galvanostat-Potentiostat

Introduction Potentiometer Galvanometer Design of electrochemical cell Types of electrodes Components of potentiostat Uses Evaluation of electrochemical data Summary

Contents

Potentiostat

An electronic instrument that measures and controls the voltage difference between a Working Electrode and a Reference Electrode.

It measures the current flow between the Working and Counter Electrodes.

It is used to keep the potential (voltage) between a working electrode and a reference electrode at a constant value.

Introduction

Galvanostat

It is used to maintain a constant flow of current through an electrolytic cell. 

Its main feature is its nearly infinite (i.e. extremely high respect to common loads) internal resistance.

The designation galvanostat is mainly used in electrochemistry

This device differs from common constant current sources by its ability to supply and measure a wide range of currents (from pico-amperes to amperes) of both polarities.

Simple Potentiostat

Simple Galvanostat

Model: 663 VA autolab

Galvanostat-Potentiostat

Electrodes

a) working electrodeb) counter electrodec) reference electrode

Electrolyte

Cell container

The design of electrochemical cell

Most common is a small sphere, small disc or a short wire, but it can also be metal foil, a single crystal of metal or semiconductor or evaporated thin film

Has to have useful working potential range

Can be large or small – usually < 0.25 cm2

Smooth with well defined geometry for even current and potential distribution

Working electrode

Mercury and Amalgam electrodes.

Inert solid electrodes like gold, platinum, glassy carbon.

Working electrode - examples

To supply the current required by the W.E. without limiting the measured response.

Current should flow readily without the need for a large potential.

Products of the C.E. reaction should not interfere with the reaction being studied.

It should have a large area compared to the W.E. and should ensure equipotentiality of the W.E.

Counter electrodes

The role of the R.E. is to provide a fixed potential which does not vary during the experiment.

A good R.E. should be able to maintain a constant potential even if a few micro-amps are passed through its surface.

Reference electrode

It consists of solvent and a high concentration of an ionised salt and electroactive species

To increase the conductivity of the solution, we have to reduce the resistance between

a) W.E. and C.E. (to help maintain a uniform current and potential distribution)

b) Also between W.E. and R.E. to minimize the potential error due to the uncompensated solution resistance.

The electrolyte solution

Control Amplifier: Supplies the power to maintain the controlled potential between the Working and Reference Electrodes.

Electrometer: Measures the potential difference between the Reference and Working Electrodes.

Current-to-Voltage Converter: Measures the current between the Working and Counter Electrodes.

Three Primary Components of a Potentiostat

This equipment uses three electrode system for investigations of reaction mechanisms related to redox chemistry and other chemical phenomena.

Potentiostats and galvanostats are electrochemical instruments used in electrochemistry, battery and fuel cell testing, corrosion control, voltammetry, biomedical research, surface imaging, and other related applications

Primary Use

The dimensions of the resulting data depend on the experiment.

In voltammetry, electric current in amps is plotted against electric potential in voltage.

In a bulk electrolysis total coulombs passed (total

electric charge) is plotted against time in seconds

The data is always a collection of individual data point, one followed smoothly by another. Types of data that shouldn’t be recorded:

Noisy data Flat-lined data Overloads

Evaluation of Electrochemical Data

Example of curves

The potentiostat does not control the potential of the working electrode

The potentiostat controls the potential of the counter electrode only (relative to the working electrode)

The counter electrode is the most important electrode (followed by the reference electrode – the working electrode is never a problem)

Compliance voltage limits are very important in the choice of the potentiostat.

Summary