POTENTIOMETRY NOTES: 2019 - Destination...

POTENTIOMETRY NOTES: 2019

FREE NOTES AND STUDY MATERIAL FOR GPAT



Electrochemical cells

Electrochemical cells can be conveniently classified as galvanic if they are employed to produce

elcctrical energy and electrolytic when they consume electricity from an external source.

Electrochemical Cell Components

a) Electrode

b) Electrolytic solution

c) Saturated KCl Solution

An electrochemical cell consists of two metallic conductors called electrodes, each immersed in a

suitable electrolyte solution.

For electricity to flow, it is necessary:

(1) the electrodes be connected externally by means of a metal

conductor

(2) the two electrolyte solutions be in contact to permit

movement of ions from one to the other.

Conduction in EIectrochemical CeIl



Electricity is conducted by three distinct processes in various parts of the galvanic ceil shown in Figure

In the copper and zinc electrodes, as well as in the external conductor, electrons serve as carriers,

moving from the zinc through the conductor to the copper.

Within the two solutions the flow of electricity involves migration of both cation & anion, the former

away from the zinc electrode toward the copper and the latter in the reverse direction.

All ions in the both solutions (Electrolytic solution and Saturated KCl solution) participate in this process.

A third type of conduction occurs at the two electrode surfaces. Here, an oxidation or a reduction

process provides a mechanism whereby the ionic conduction of the solution is coupled with the electron

conduction of the electrode to provide a complete circuit for a current.

The two electrode processes are described by the equations

Zn(s) Zn++ + 2e

Cu++ + 2e Cu(s)

Anode & Cathode

By definition, the cathode of an electrochemical cell is the electrode at which reduction occurs, while

the anode is the electrode where an oxidation takes place.

These definitions apply to both galvanic and electrolytic cells.

The copper electrode is the cathode and the zinc electrode is the anode.

Note that this cell could be caused to behave as an electrolytic cell by imposing a sufficiently large

potential from an external source.

What is Electric Potential?

An electric potential (also called the electric field potential, potential drop or the electrostatic potential)

is the amount of work needed to move a unit of charge from a reference point to a specific point inside

the field.

What is electric potential energy?

It is the energy that is needed to move a charge against an electric field. You need more energy to move

a charge further in the electric field, but also more energy to move it through a stronger electric field.

What is Electric Potential Difference?



The potential difference between two bodies is defined as the work required to be done for bringing a

unit positive charge from one point to other point.

When a body is charged, it can attract an oppositely charged body and can repulse a similar charged

body. That means, the charged body has ability of doing work. That ability of doing work of a charged

body is defined as electrical potential of that body.

If two electrically charged bodies are connected by a conductor, the electrons starts flowing from lower

potential body to higher potential body, that means current starts flowing from higher potential body to

lower potential body depending upon the potential difference of the bodies and resistance of the

connecting conductor

Potentiometry

Potentiometry generally refers to the analytical methods, which are based on the measurement of

potential of a galvanic cell in the absence of current state by using voltameter (high impedence).

Use of a high impedance voltmeter in important, because it ensures that current flow is negligible. Since

there is no net current, there are no net electrochemical reactions, hence the system is in equilibrium.

In potentiometric measurements, the potential between two electrodes is measured using a high

impedence voltmeter. It is usually employed to find the concentration of a solute in solution.

Thus it deals with the chemical transformations produced by the passage of electricity in determinate

chemical systems, and the production/storage of electricity by means of chemical transformations.

The cell potential is

Ecell = Eind – Eref + Ejunc



How Potentiometer determines Potential value? and How potential value is correlated with

concentration or how concentration is determined with the help of potential difference value?

For this first we have to understand the construction and working of potentiometer!

Instrumentation typically used in potentiometry includes the 1Reference electrode, with a known

potential, constant over time and independent of the composition of the solution containing the analyte

in which it is immersed, and an 2Indicator (or working) electrode, whose response depends on the

concentration of the analyte, and finally an instrument for measuring potential 3‘voltmeter’.

Construction of Potentiometer:

At its most fundamental level, a potentiometer consists of two electrodes inserted in two solutions

connected by a salt bridge (see diagram below). The voltmeter is attached to the electrodes to measure

the potential difference between them.

One of the electrodes is a reference electrode, whose electrode potential is known and remains

constant when dipped into respective solution.

The other electrode is the indicator electrode. The indicator electrode is immersed in a solution, whose

concentration you want to determine. Indicator electrode potential is dependent on activity of ions into

solution, activity is nothing but concentration of active ions of solute in solution thus this potential

directly indicates the concentration of solution.

What exactly is electrode potential?

To carry out the process of electrolysis, we need electricity to break the constituent particles in the

electrolyte.

This application of electricity creates potential difference across the electrolytic cell depending upon the

nature and construction of electrode, again what is this potential difference, it is nothing but the ability

to carry out electrolysis here. This potential difference is created as a result of the difference between

individual potentials of the two metal electrodes with respect to the electrolytes.



After application of electricity the tendency of an electrode to lose or gain electrons when placed in

solution is known as electrode potential.

Here in Potentiometry instrument we have two electrodes : Reference Electrode and Indicator

Electrode.

Now let’s discuss Reference Electrode first!!

Reference Electrode

Potential Value of reference electrode is independent of:

1) Analyte Concentration

2) Temprature of Solution

Some Reference electrodes includes:

1) Standard hydrogen electrode

2) Saturated calomel electrode

3) Silver-Silver Chloride electrode

Standard Hydrogen Electrode:

Construction: Hydrogen electrode consists of two main components: enclosing tube and conductor.

Enclosing tube – It is made from glass material and it consist of conductor wire (platinum). Through this

hydrogen gas passed under 1 atmospheric pressure which reacts at conductor surface.

Conductor – It is made from platinum foil which is coated with platinum black (means it is platinized)

and attached to platinum wire which is connected with high impedence voltmeter. Because of finely

divided platinum present at electrode surface rapid reaction occurs.

Dig:: Standard Hydrogen Electrode



Working: When the circuit is formed hydrogen is either oxidized to Hydrogen ion (if electrode is anode)

or it is reduced to hydrogen gas (if electrode is cathode).

Potential of SHE: Potential value depends upon three factors such as follow:

1) Temprature of Pt/H2

2) Hydrogen ion activity in solution

3) Pressure of Hydrogen at electrode surface (Here, Hydrogen gas must be at 1 atm pressure)

Representation of this SHE based cell will be as follow:

Pt(s) H2 (g) I H+(aq)II indicator electrode

Standard-state potential for the reaction

Disadvantage includes:

1) Open at bottom and can be easily poisoned

2) Can’t be used in presence of strong oxidizing or reducing agent

Despite its importance as the fundamental reference electrode against which we measure all other

potentials, the SHE is rarely used because it is difficult to prepare and inconvenient to use.

Calomel Electrode:

It consists of mercury in contact with solution that is saturated with mercurous chloride (Hg2Cl2) and

known concentration of KCl.

Depending on the concentration of KCl these calomel electrodes are divided into three types:

Decimolar Calomel Electrode (KCl conc: 0.1M)

Molar Calomel Electrode (KCl conc: 1M)

Saturated Calomel Electrode (KCl conc: Saturated, above 4.5M )

Construction:

It consist of Inner tube and Outer tube.

Inner tube : Inner tube filled with Hg(l) and Hg2Cl2, KCl. Small hole present at bottom of inner tube which

connects it to outer tube.

Outer tube: outer tube are made from glass material and it possesses Porous wick at bottom which

connects it with analyte solution and acts as small bridge.



As shown in above Figure, in saturated calomel electrode (SCE) the concentration of Cl– is

determined by the solubility of KCl.

The electrode consists of an inner tube packed with a paste of Hg, Hg2Cl2, and KCl, situated

within a second tube containing a saturated solution of KCl.

A small hole connects the two tubes and a porous wick serves as a salt bridge to the solution in

which the SCE is immersed.

A stopper in the outer tube provides an opening for adding addition saturated KCl.

Dig:: Calomel Electrode

Working:

The potential of a calomel electrode, therefore, is determined by the activity of Cl– in equilibrium with

Hg and Hg2Cl2.

The short hand notation for this cell is

Because the concentration of Cl– is fixed by the solubility of KCl, the potential of an SCE remains

constant even if we lose some of the solution to evaporation. A significant disadvantage of the SCE is

that the solubility of KCl is sensitive to a change in temperature. At higher temperatures the solubility of

KCl increases and the electrodes potential decreases.

For example, the potential of the SCE is +0.2444 V at 25 oC and +0.2376 V at 35 oC.

The potential of a calomel electrode containing an unsaturated solution of KCl is less temperature

dependent, but its potential changes if the concentration, and thus the activity of Cl–, increases due to

evaporation.

Half cell presented as follow:

Hg(l) I Hg2Cl2(s) KCl(aq) II indicator electrode



Calomel reference electrodes are based on the following redox couple between Hg2Cl2 and Hg,

The reaction can be presented as follow:

for which the Nernst equation is

Potential value of electrode are dependent on activity of Cl– which is in equilibrium with Hg and Hg2Cl2

(determined at 25 °C)

Decimolar Calomel Electrode (KCl conc: 0.1M) Potential Value: 0.3358 V

Molar Calomel Electrode (KCl conc: 1M) Potential Value: 0.2824 V

Saturated Calomel Electrode (KCl conc: Saturated, above 4.5M) Potential Value: 0.2444 V

Silver-Silver Chloride Electrode

Construction: It consists of glass tube, in which silver coated wire is dipped into the solution of KCl of

known concentration which is saturated with Silver chloride.

Schematic diagram shows Ag / AgCl electrode. Because the electrode does not contain solid KCl, this is

an example of an unsaturated Ag / AgCl electrode.

Porous plug is present at bottom of tube allowing electrical contact of electrode.

Working: based on the redox couple between AgCl and Ag.

The activity of Cl– determines the potential of the Ag/AgCl electrode; thus Nernst Equation is:



Potential value of electrode are dependent on activity of Cl– which is in equilibrium with KCl (determined

at 25 °C)

Unsaturated Silver-Silver Chloride Electrode: (KCl conc: 3.5M) Potential Value: 0.205 V

Saturated Silver-Silver Chloride Electrode: (KCl conc: Sat’d, above 4.5M) Potential Value: 0.197 V

The electrodes short hand notation is

Indicator Electrode

Potential of an indicator electrode depends mainly on the concentration of the analyte ions.

Glass Electrode:

These electrodes have thin glass membrane fused to the end of a glass or plastic body. This

electrode mainly responds to activity of [H+] ions.

Construction: The electrode consists of a thin layer of glass, typically about 50 μm thick.

Materials used in construction are SiO2, Al2O3, LiO, NaO. The main body of the electrode contains an

internal reference electrode typically Ag / AgCl and

is filled with a aqueous HCl solution of concentration around 1.0 mol / dm3.

The samples reference electrode is a Ag/AgCl electrode in a solution of KCl (which may be

saturated with KCl or contain a fixed concentration of KCl). A porous wick serves as a salt bridge

between the sample and its reference electrode.

Working: The pH electrode responds to hydrogen ions as a result of the thin ion-exchange sites

on the surface of a hydrated glass membrane. For this ion exchange sites to operate hydration is

necessary. Charge is transported across the membrane by sodium or lithium ions within the

glass. The surface layer of the glass consist of silicate group associated with sodium ion (- Si− +

ONa). When this electrode is hydrated properly, the sodium ions exchanged with the protons in

water.



Schematic diagram showing a combination glass electrode for measuring pH

Electrical Conduction across the glass membrane: to serve as an indicator electrode, it must

conduct within hydrated layers which involve movement of Hydrogen ions. Hydrogen are

charge carrier in the dry interior of membrane.

There are two surfaces: Outer Surface and Inner Surface

Outer Surface:

H+ + SiO-Na+ SiO- H+ + Na+

Hydrogen replaces sodium of glass membrane, this sodium ion is released into solution.

Inner Surface:

SiO- H+ H+ + SiO-



Both above mentioned reactions at outer and inner electrode surfaces determine the

equilibrium of Hydrogen ion concentration associated with the electrode. This Hydrogen ion

concentration intern determines the potential of glass electrode.

If glass electrode is placed in a test solution its glass membrane will have an inner and outer

hydrated layers and potential difference is developed due to the difference in hydrogen ion

activities between test solution and outer hydrated surface of glass electrode as well as inner

solution and inner hydrated surface. This potential is called boundary potential and it varies

with the activity or pH of the solution. Overall boundary potential is the potential difference

between both the boundary potentials.

Thus, the boundary potential is a measurement of the hydrogen ion activity or the pH of the

external solution.

Disadvantages

The glass membrane being very fragile, it requires great care while using.

The ordinary potentiometer cannot be used for measuring the potential of the glass

electrode.

Cannot be employed in pure ethyl alcohol, acetic acid and gelatin

Require frequent standardization

Features:

It may be used in the presence of strong oxidizing and reducing solutions in viscous

media and in presence of proteins which interfere with operation of other electrodes.

It can be used for solutions having pH values 2 to 10 with some special glass,

measurements can be extended to pH values greater than 10.

It is simple to operate and immune to poisoning.

The equilibrium is reached quickly

Factors affecting accuracy are:

The alkaline error

The acid error

Variation in junction-potential

Error in the pH of the standard buffer



Temperature: Calibration procedures

Membrane Electrodes

The underlying principle behind these types of electrode is potential developed on different surfaces of

electrode due to unequal charge distribution. The resulting charge at each membrane is exclusively

controlled and monitored by position of equilibrium which inturn dependent on the concentration of ion

present in the solution.

There are two major types of membrane selective electrode :

Ion selective electrode

Quinhydrone electrode

Ion selective electrode:

Eg : single crystal electrode: fluoride electrode

Lanthanum fluoride a neutral conductor is used to construct electrode. It is nearly ideal electrode for

constructing crystalline membrane electrode which is used for determination of fluoride ions

Although it is neutral conductor its conductivity is greatly enhanced by doping wih europium.

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