ELECTROCHEMICAL

METHODS OF ANALYSIS

Siham AbdounMsc., PhD.

Introduction

Electrochemistry is branch of chemistry

concern with the interaction of electrical and

chemical effects

A large part of this field deals with the study of

chemical changes caused by the passage of an

electrical current and the production of

electrical energy by chemical reaction.

It is named electrochemistry because its

originated from the study of the movement of

electrons in an oxidation–reduction reaction.

Electrochemical methods: are analytical

techniques that use a measurement of

potential, charge, or current to determine an

analyte’s concentration or to characterize an

analyte’s chemical reactivity.

It is a qualitative and quantitative methods of

analysis based on electrochemical phenomena

occurring within a medium or at the phase

boundary and related to changes in the structure,

chemical composition, or concentration of the

compound being analyzed.

These methods are divided into five major

groups: potentiometry, voltammetry, coulometry,

conductometry, and dielectrometry.

Applications

1. Obtaining thermodynamic data about a

reaction.

2. To generate an unstable intermediate such as

radical ion and study its rate of decay or it is

spectroscopic properties.

3. They use to analyze a solution for trace

amount of metal ions or organic species.

4. The electrochemical properties of the

system themselves are of primary interest,

for example, in the design of a new power

source or for the electrochemical methods

have been developed.

Type of electrochemical techniques

1. Bulk techniques, in which we measure a

property of the solution in the

electrochemical cell. An example is the

measurement of a solution’s conductivity,

which is proportional to the total

concentration of dissolved ions,

2. Interfacial techniques, in which the

potential, charge, or current depends on the

species present at the interface between an

electrode and the solution in which it sits.

An example is the determination of pH

using a pH electrode.

Despite the difference in instrumentation, all

electrochemical techniques share several

common features.

(1) The electrode’s potential determines the

analyte’s form at the electrode’s surface

(2) The concentration of analyte at the

electrode’s surface may not be the same as its

concentration in bulk solution;

(3) Current is a measure of the rate of the

analyte’s oxidation or reduction; and

(4) We cannot simultaneously control

current and potential.

Interfacial Electrochemical Techniques

The interfacial electrochemical techniques is

divided into Static techniques and dynamic

techniques

Static technique the current is not pass through

the analyte’s solution. Potentiometry, in which

we measure the potential of an electrochemical

cell under static conditions, is one of the most

important quantitative electrochemical methods

Dynamic techniques, in which we allow current

to flow through the analyte’s solution, it

comprise the largest group of interfacial

electrochemical techniques e.g. Coulometry, in

which we measure current as a function of time,

Amperometry and voltammetry, in which we

measure current as a function of a fixed or

variable potential

Controlling and Measuring Current and

Potential

we cannot simultaneously control both

current and potential

if we choose to control the potential, then we

must accept the resulting current, and we

must accept the resulting potential if we

choose to control the current.

The second electrode, which we call the

counter electrode, completes the electrical

circuit and provides a reference potential

against which we measure the working

electrodes potential. Ideally the counter

electrode’s potential remains constant so that

we can assign to the working electrode any

change in the overall cell potential.

Electrochemical measurements are made in an

electrochemical cell consisting of two or more

electrodes and the electronic circuitry for controlling

and measuring the current and the potential.

The simplest electrochemical cell uses two

electrodes. The potential of one electrode is

sensitive to the analyst’s concentration, and is

called the working electrode or the indicator

electrode.

If the counter electrode’s potential is not

constant, we replace it with two electrodes: a

reference electrode whose potential remains

constant and an auxiliary electrode that

completes the electrical circuit.

Because we cannot simultaneously control the

current and the potential, there are only three

basic experimental designs

(1) Measure the potential when the current is zero,

(2) Measure the potential while controlling the current,

(3) Measure the current while controlling the potential

Each of these experimental designs relies on Ohm’s

law, which states that a current, i, passing through an

electrical circuit of resistance, R, generates a

potential, E. (E =iR)

Each of these experimental designs uses a different

type of instrument

Type of Electrochemical Methods

1. Potentiometry methods: it measures the

potential of a solution between two electrodes.

The potential is then related to the concentration

of one or more analytes. The cell structure used

is often referred to as an electrode even though it

actually contains two electrodes: an indicator

electrode and a reference electrode.

Potentiometry usually uses electrodes made

selectively sensitive to the ion of interest,

such as a fluoride-selective electrode. The

most common potentiometric electrode is the

glass-membrane electrode used in a pH meter.

2. Voltammetry method: is based on the applies a

constant and/or varying potential at an

electrode's surface and measures the resulting

current with a three electrode system.

Voltammetry, with its variety of methods,

constitutes the largest group of electrochemical

methods of analysis and is commonly used for

the determination of compounds in solutions

(for example, polarography and amperometry).

3. Coulometry methods: based on the

measurement of the amount of material

deposited on an electrode in the course of an

electrochemical reaction in accordance with

Faraday’s laws. A distinction is made between

coulometry at constant potential and

coulometry at constant current.

Coulometry uses applied current or potential to

completely convert an analyte from one

oxidation state to another. In these experiments,

the total current passed is measured directly or

indirectly to determine the number of electrons

passed. Knowing the number of electrons passed

can indicate the concentration of the analyte or,

when the concentration is known, the number of

electrons transferred in the redox reaction.

4. Conductometry methods: in which

the electrical conductivity of electrolytes

(aqueous and non-aqueous solutions, colloid

systems and solids) is measured

It is based on the change in the concentration

of a compound or the chemical composition

of a medium in the interelectrode space;

Potentiometric titration

Is a technique similar to direct titration of a

redox reaction. No indicator is used, instead

the potential across the analyte, typically an

electrolyte solution is measured. To do this,

two electrodes are used, an indicator

electrode and a reference electrode.

In potentiometry we measure the potential of

an electrochemical cell under static conditions.

Because no current—or only a negligible

current—flows through the electrochemical

cell, its composition remains unchanged. For

this reason, potentiometry is a useful

quantitative method.

Potentiometric Measurements

A potentiometer is used to determine the

difference between the potential of two

electrodes. The potential of one electrode—the

working or indicator electrode—responds to

the analyte’s activity, and the other electrode—

the counter or reference electrode—has a

known, fixed potential.

Potentiometric Electrochemical Cells

The electrochemical cell consists of two half-

cells, each containing an electrode immersed in

a solution of ions whose activities determine

the electrode’s potential. A salt bridge

containing an inert electrolyte, such as KCl,

connects the two half-cells.

The ends of the salt bridge are fixed with

porous frits, allowing the electrolyte ions to

move freely between the half-cells and the salt

bridge. This movement of ions in the salt

bridge completes the electrical circuit as

shown in the Figure below.

By convention, we identify the electrode on the

left as the anode and assign to it the oxidation

reaction; thus

Zn(s) ↔ Zn2(aq) +2e −

The electrode on the right is the cathode, where

the reduction reaction occurs

Ag +(aq) + e− ↔ Ag (s)

Advantages of potentiometric titrations over

'classical' visual indicator methods are:

1. Can be used for coloured, turbid or

fluorescent analyte solution.

2. Can be used if there is no suitable indicator

or the colour change is difficult to ascertain.

3. Can be used in the titration of polyprotic

acids, mixtures of acids, mixtures of bases or

mixtures of halides.

Types of Potentiometric Titration

Depending on the type of the reactions involved to

which potential measurement can be applied for end

point detection, potentiometric titrations can be

classified into followings:

(a) A cid-Base Titration

(b) Complexometric Titration

(c) Oxidation-Reduction Titration

(d) Precipitation Titration

Location of the End Point

Titration Curve: It is obtained by plotting the

successive values of the cell emf on y=axis and

corresponding values of volume of titrant added

on the x-axis. This gives an S-shaped curve. The

central portion of this curve which shows the

steeply rising portion corresponds to the volume

for the end point of the titration.

When there is a small potential change at

the end point like in the titration of weak

acid with strong base, titration of very

dilute solution etc, it is difficult to locate

end point by this method.

Titration method of locating end point

b) Analytical or Derivative Method: The

end point can be more precisely located from

the first or second derivative curves. The first

derivative curve involves the plot of slope of

the titration curve (ΔE/ΔV-ration of change

in emf and change in volume added) against

the volume of the titrant added.

Most frequently ΔE/ΔV is plotted against

the average volume of titrant added

corresponding to the values of emf taken.

Volume on the x- axis corresponding to

the peak of the curve is the end point of

the titration.

First derivative Curve

In second derivative curve we plot the slope of

first derivative curve (Δ2E/ΔV) against volume.

The point on volume axis where the curve cuts

through zero on the ordinate gives the end point.

This point corresponds to the largest steepest

point on titration curve and maximum slope of

the ΔE/ΔV curve.

This mentioned methods need values of

potential corresponding to very small change

in volume of titrant added near the end point

for good result. In the immediate area of the

end point the concentration of the original

reactant becomes very small, and it usually

becomes impossible for the ions to control

the indicator electrode potential.

Second derivative curve