Lecture 7c- Electrochemical Analysis (1h) (2)

8/11/2019 Lecture 7c- Electrochemical Analysis (1h) (2)

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Lecture 7c

Electrochemical Analysis



7c.1.Introduction

Electrochemistry focuses on the relationshipbetween electrical signal and chemical effects.Many chemical reactions produce a characteristicelectric potential, which can be measured ascurrent.

The magnitude of the current generated may beused to test for the presence of an analyte insolution, or to quantity the concentration of a

specific analyte.

Electrodes placed in the solution monitor thechange in potential of the solution due to chemicalstimuli.



Classification of Electrochem. Methods

Analytical signal: potential; current; charge



Electrochemical Methods: Four main types of electrochemicalmethods:

Conductivity: Measure conductance of a solution, using INERT ELECTRODES,

ALTERNATING CURRENT, AND AN ELECTRICAL NULL CIRCUIT – thereby ensure nonet current flow and no electrolysis. The concentration of ions in the solution isestimated from the conductance.

Coulometry: Electrolysis of a solution and use of Faraday’s Law relating quantity ofelectrical charge to amount of chemical charge (essentially states that it takes

9.65x104 Coulombs of electrical charge to cause electrolysis of 1 mole of a univalentelectrolyte species)

Potentiometry: measure electrical potential developed by an electrode in an

electrolyte solution at zero current flow. Use NERNST EQUATION relating potential to

concentration of some ions in solution

Voltammetry: Determine concentration of ion in dilute solutions from current flow asa function of voltage when polarization of ion occurs around the electrode

(Polarization: Depletion of concentration caused by electrolysis)



c. . o en ome r c me o s oanalysis



Potentiometry:

Electrode potential and electrochemical cells:

Electrode potential: Electrode placed in a solution containing ions which it canexchange acquires +ve or –ve potential relative to solution

Ex: Cu electrodeCu2+ + 2e Cu

Ag/AgCl electrode AgCl + e Ag + Cl-

Can not measure electrode potential

of HALF CELL without perturbing electrodeequilibrium

Using Electronic Voltmeter to measurepotential electrode and now replaces bypotentiometer



Potentiometric Methods 1.) Potentiometric Methods: based on measurements of the potential of

electrochemical cells in the absence of appreciable currents (I = 0)

2.) Basic Components:

a) reference electrode: gives reference for potential measurement

b) indicator electrode: where species of interest is measured

c) potential measuring device



Reference Electrodes

A reference is an electrode that has the half-cell

potential known, constant, and completelyinsensitive to the composition of the solutionunder study. In conjunction with this reference is

the indicator or working electrode , whose responsedepends upon the analyte concentration.

By convention, the reference electrode is taken tobe the anode; thus, the shorthand notation for apotentiometric electrochemical cell is:

Reference || Indicator

The cell potential is: Ecell=Eind – Eref +Elj



Standard hydrogen electrode(SHE



Calomel Electrodes



Silver/Silver Chloride Electrodes



Reference Electrodes

Ideal Reference Electrode:

Is reversible and obeys the Nernst equation

Exhibits a potential that is constant with time Returns to its original potential after being

subjected to small currents

Exhibits little hysteresis with temperaturecycling





Metallic Electrodes of the first kind

Metals are limited to Ag,Bi, Cd, Cu, Hg, Pb, Sn, Tl,and Zn (but Zn is easily

dissolved in acidic media .

A metallic electrode whose

potential is a function ofthe concentration of Mn+

in an Mn+/M redox half-

reaction.



A metallic electrode whose potential is a

function of the concentration of X in anMXn/M redox half-reaction

Eg.:

Metallic Electrodes of the second kind



Metallic Electrodes of the third kind

- Metal electrodes responds to a different cation- Linked to cation by an intermediate reaction- Already saw detection of EDTA by Hg electrode (2nd Kind)

- Can be made to detect other cations that bind toEDTA affecting a Y4-

- Example: Detect Ca2+ by complex with EDTA

equilibrium reaction: CaY 2-

Ca2+

+ Y 4-

Where:

Eind = 0.21 – (0.0592/1) log a Y4-/aHgY2-

CaY

Y Ca

f a

aa K

42



Redox Electrodes

An inert electrode that serves as a source or sinkfor electrons for a redox half-reaction.

The Pt cathode is sink in a solution of Fe3+/Fe2+.

its potential is determined by the concentrationsof Fe2+ and Fe3+ in the indicator half-cell.

Note: the potential of a redox electrode generallyresponds to the concentration of more than oneion, limiting their usefulness for directpotentiometry.

M b El d



Membrane Electrodes

Ion-selective electrodes (ISEs)



Types of Ion- Selective membrane

Electrodes

A.Crystalline Membrane Electrodes1. Single crystal ( eg.: LaF3 for F- )

2. Polycrystalline & mixed crystal(eg.: Ag 2S for S2- and Ag + )

B. Non Crystalline Membrane Electrodes1. Glass (eg.: silicate glassed for Na+ and H+ )

2. Liquid (eg.: liquid ion exchanges for Ca2+ and neutral carriers for K + )

3. Immobillized liquid in a rigid polymer

( eg.: polyvinyl chloride matrix for Ca2+ and NO3- )



Glass Electrodes:



Membrane Indicator Electrodes

Glass Electrodes: Potential

The boundary potential.

The potential of the internal Ag/AgCl referenceelectrode.

A small asymmetry potential.



Membrane Indicator Electrodes

Fluoride Electrode:



Instruments for Measuring Cell

Potentials Direct-Reading Instruments

Commercial Instruments

Utility

General-purpose

Expanded-scale

Research



Direct Potentiometric

Measurements The Sign Convention and Equations for Direct

Potentiometry

The Electrode Calibration Method

Inherent Error in the Electrode Activity Versus Concentration

Calibration Curves for Concentration Measurement

Standard Addition Method

Potentiometric pH Measurements with a GlassElectrodes

Summary of Errors Affecting pH Measurements with the Glass Electrode

The O erational Definition of H



Potentiometric Titrations

- Use different Indicator Electrode

+ Acid- Base Titr.:

glass Electrode

+ Ag+ Titration:First type Electrode+ Redox Titration:

Redox Electrode

+ Complexon Titr.:

Third Type Electrode



When potentiometrics is used

• It is used when the endpoints are very

difficult to determine, either when:

1- very diluted solution.

2-coloured and turbid solution

3-absence of a suitable indicator



Data treatment in potentiometric

Titration

0.00 5.00 10.00 15.00 20.00 25.00

2.00

4.00

6.00

8.00

10.00

12.00

13.0 13.2 13.4 13.6 13.8 14.0 14.2 14.4 14.6 14.8 15.0

0.00

2.00

4.00

6.00

8.00

10.00

12.00

14.00

16.00

18.00

pH

V titrant

pH/ V

V titrant

7 3 C l t i M th d



A.) Introduction:

1.) Coulometry: electrochemical method basedon the quantitative oxidation or reduction

of analyte

- measure amount of analyte by measuring amountof current and time required to complete reaction

charge = current (i) x time in coulombs

Q=It- electrolytic method external power added

to system

7c.3.Coulometric Methods



Coulometric Methods

2.) Example:

- Coulometric Titration of Cl-

- use Ag electrode to produce Ag+

Ag (s) Ag+

+ e- Ag+ + Cl- AgCl (ppt.)

- measure Ag+ in solution by 2nd electrode

- only get complete circuit when Ag+ exists in solution- only occurs after all Cl- is consumed

- by measuring amount of current and time required

to complete reaction can determine amount of Cl-



3.) Based on Measurement of Amount of Electricity

(or charge, in coulombs) Required to Convert

Analyte to Different Oxidation State

- Q = It for constant current with time

where: Q = charge required (coulombs = amp . sec)

I = current (amp.)

t = time of current (sec)

for variable current with time:

Q = Idt

Relate charge (coulombs, C) to moles of e- passing

electrode by Faraday constant

Faraday (F) = 96,485 Coulombs (C)/mole e-

If know moles of e- produced and stoichiometry of ½ cell

reaction: Ag (s) Ag+ + e- (1:1 Ag+/e-)

gives moles of analyte generated, consumed, etc.

0

t



Example : Constant current of 0.800 A

(amps.) used to deposit Cu at the

cathode and O2 at anode of anelectrolytic cell for 15.2 minutes. What

quantity in grams is formed for each

product?



Amperostatic Methods



Amperostatic Methods

(Coulometric Titrations)

•

1.) Basics: titration of analyte in solution by usingcoulometry at constant current to generate aknown quantity of titrant electrochemically

- potential set by contents of cell- Example:

Ag (s) Ag+ + e- for precipitation titration of Cl-

- To detect endpoint, use 2nd electrode to detectbuildup of titrant after endpoint.



2 .) Applications of coulometric titration

a) Can be used for Acid-Base Titrations- Acid titration

2H2O + 2e- 2OH- + H2 titrant generation reaction

- Base titration

H2O

2H+ + ½ O2 + 2e titrant generation reaction

b.) Can be used for Complexation Titrations

(EDTA )

HgNH3Y

2-

+ NH4

+

+ 2e-

Hg + 2NH3 +HY

3-

HY3- H+ + Y4-

c.) Can be used for Redox Titrations

Ce3+ Ce4+ + e-

Ce4+ + Fe2+

Ce3+ + Fe3+



3.) Compar ison o f Coulometr ic and Volumetr ic Ti t rat ion

a) Both Have Observable Endpoint

- Current (e- generation)

-serves same function as a standard titrant solution

- Time

- serves same function as volume delivered

- amount of analyte determined by combining capacity

- reactions must be rapid, essentially complete and freeof side reactions

b.) Advantages of Coulometry

- Both time and current easy to measure to a high accuracy

- Don’t have to worry about titrant stability

- easier and more accurate for small quantities of reagent

-small volumes of dilute solutions problem with volumetric

- used for precipitation, complex formation

oxidation/reduction or neutralization reactions

- readily automated



4.) Change in Potential During Amperostatic Methodsa) In constant current system, potential of cell will vary with time as

analyte is consumed.

- Cell “seeks out” electrochemical reactions capable of carrying the

supplied currentCu2+ + 2e- Cu (s) initial reaction

- Nernst Equation

Ecathode = EoCu2+/Cu – 0.0592/2 log (1/aCu2+)

Note: Ecathode depends on aCu2+.

As aCu2+ decreases (deposited

by reaction) Ecathode decreases.

C) Potentiostatic Coulometry



C) Potentiostatic Coulometry

1.) Basics:

-detection of analyte in solution by using Coulometry at

fixed potential to quantitatively convert analyte to a givenform

-current controlled by contents of cell.

7 4 V lt mm tr



7c.4. Voltammetry

Voltammetry techniques measure current as a function of

applied potential under conditions that promote polarizationof a working electrode

A plot of current as a function of applied potential iscalled a voltammogram and is the electrochemical

equivalent of a spectrum in spectroscopy, providingquantitative and qualitative information about thespecies involved in the oxidation or reduction reaction

Polarography: Invented by J. Heyrovsky (Nobel Prize 1959).

Differs from voltammetry in that it employs a droppingmercury electrode (DME) to continuously renew theelectrode surface.

Amperometry: current proportional to analyte

concentration is monitored at a fixed potential



Voltametric Measurements

Three electrode system potentiostatmentioned earlier is used as a device

that measures the current as a function

of potential Working electrodes used: Hg, Pt, Au,

Ag, C or others

Reference electrode: SCE or Ag/ AgCl;

Auxiliary electrode: Pt wire



Polarization Some electrochemical cells have significant currents.

Electricity within a cell is carried by ion motion

When small currents are involved, E = IR holds

R depends on the nature of the solution (next slide)

When current in a cell is large, the actual potentialusually differs from that calculated at equilibriumusing the Nernst equation

This difference arises from polarization effects The difference usually reduces the voltage of a galvanic

cell or increases the voltage consumed by an electrolyticcell



Ohmic Potential and the IR Drop

To create current in a cell, a driving voltage is

needed to overcome the resistance of ions to movetowards the anode and cathode

This force follows Ohm’s law, and is governed by

the resistance of the cell:

IR E E E left right cell

Electrodes

IR Drop



More on Polarization

Electrodes in cells are polarized over certain

current/voltage ranges “Ideal” polarized electrode: current does not vary with potential



Overvoltage and Polarization Sources

Overvoltage: the difference between the

equilibrium potential and the actual potential

Sources of polarization in cells:

Concentration polarization: rate of transport toelectrode is insufficient to maintain current

Charge-transfer (kinetic) polarization: magnitude of

current is limited by the rate of the electrode reaction(s)(the rate of electron transfer between the reactants andthe electrodes)

Other effects (e.g. adsorption/desorption)

Polarography



Polarography

In polarography, the current flowing through the cell ismeasured as a function of the potential of the working

electrode. Usually this current is proportional to the concentration

of the analyte.

Apparatus for carrying out pol. is shown below.

The working electrode is a dropping mercury electrodeor a mercury droplet suspended from a bottom of aglass capillary tube.

Analyte is either reduced (most of the cases) or

oxidized at the surface of the mercury drop. The current – carrier auxiliary electrode is a platinum

wire.

SCE or Ag/AgCl reference electrode is used.

The potential of the mercury drop is measured with



T i l l t h i l



Typical electrochemical

cell used in polarography



Wh D i M El t d ?



Why Dropping Mercury Electrode?

Hg yields reproducible current-potential data.

This reproducibility can be attributed to thecontinuous exposure of fresh surface on the growingmercury drop.

With any other electrode (such as Pt in various forms),

the potential depends on its surface condition andtherefore on its previous treatment.

The vast majority of reactions studied with the mercuryelectrode are reductions.

At a Pt surface, reduction of solvent is expected tocompete with reduction of many analyte species,especially in acidic solutions.

The high overpotential for H+ reduction at the mercurysurface. Therefore, H+ reduction does not interfere

with many reductions.

Problems with mercury electrode



Problems with mercury electrode A mercury electrode is not very useful for performing oxidations,

because Hg is too easily oxidized.

In a noncomplexing medium, Hg is oxidized near + 0.25 V(versus S.C.E.).

For most oxidations, some other working electrode must beemployed.

Pt electrode Vs SCE; works for a range of +1.2 to – 0.2 in

acidic solution +0.7 V to –

1 V in basic solution. Carbon pasteelectrode is also used in voltammetry

Mercury is toxic and slightly volatile, and spills are almostinevitable. a good vacuum cleaner.

To remove residual mercury, sprinkle elemental zinc powder onthe surface and dampen the powder with 5% aqueous H2S04

Mercury dissolves in the zinc. After working the paste intocontaminated areas with a sponge or brush, allow the paste todry and then sweep it up. Discard the powder appropriately as

contaminated mercury waste

C t i V lt t



Current in Voltammetry When an analyte is oxidized at the working

electrode, a current passes electrons through theexternal electric circuitry to the auxiliary electrode.

This current flows from the auxiliary to the workinelectrode, where reduction of the solvent or other

components of the solution matrix occurs . The current resulting from redox reactions at the

working and auxiliary electrodes is called a faradaicurrent.

Sign Conventions A current due to the analyte'sreduction is called a cathodic current and, byconvention, is considered positive. Anodic currentsare due to oxidation reactions and carry a negative

value.

I fl f li d t ti l



Influence of applied potential

on the faradaic current

When the potential applied to the working

electrode exceeds the reduction potential of

the electroactive species, a reduction will

take place at the electrode surface Thus, electroactive species diffuses from the

bulk solution to the electrode surface and

the reduction products diffuse from theelectrode surface towards the bulk solution.

This creates what is called the faradaic

current.





The magnitude of the faradaic current isdetermined by the rate of the resulting

oxidation or reduction reaction at theelectrode surface.

Two factors contribute to the rate of theelectrochemical reaction:

the rate at which the reactants and products are transported to and from thesurface of the electrode (mass transport)

and the rate at which electrons passbetween the electrode and the reactantsand products in solution. (kinetics ofelectron transfer at the electrode surface)



SH PE OF THE POL ROGR M

A graph of current versus potential in a polarographic

experiment is called a polarogram.

Cd2+ + 2e Cd



Effect of Dissolved Oxygen



yg

Oxygen dissolved in the solution will be reduced at theDME leading to two well defined waves which wereattributed to the following reactions:

O2(g) + 2H+ + 2e- < ==== > H2O2; E1/2 = - 0.1V

H2O2 + 2H+ +2e- < ==== > 2H2O; E1/2 = - 0.9V

E1/2 values for these reductions in acid solution correspond

to -0.05V and -0.8V versus SCE. This indicates that dissolved oxygen interferes in the

determination of most metal ions.

Therefore, dissolved O2 has to be removed by bubblingnitrogen free oxygen into the solution before recording the

polarogram.





Voltammetric Techniques

Normal Polarography The earliest voltammetric experiment was

normal polarography at a dropping mercury

electrode. In normal polarography the potential is linearly scanned, producing voltammograms ( polarograms) such as that

shown in Figure above. This technique is discussed above and

usually called Direct Current (DC) polarography

Differential Pulse Polarography



Differential Pulse Polarography In direct current polarography, the voltage

applied to the working electrode increases

linearly with time, as shown above. The currentis recorded continuously, and a polarogramsuch as that shown above results. The shape ofthe plot is called a linear voltage ramp.

In differential pulse polarography, small voltage pulses are

superimposed on the linear voltage ramp, as inthe Figure below.

The height of the pulse is called its modulationamplitude.

Each pulse of magnitude 5-100 mV is applied

during the last 60 ms of the life of each mercury



The drop is then mechanically dislodged.

The current is not measured continuously.Rather, it is measured once before the pulseand again for the last 17 ms of the pulse.

The polarograph subtracts the first currentfrom the second and plots this difference versus the applied potential (measured justbefore the voltage pulse).

The resulting differential pulse polarogramis nearly the derivative of a direct current polarogram, as shown in the Figure below









Stripping Analysis



Stripping Analysis The analyte from a dilute solution is first concentrated

in a single drop of Hg (or any micro-electorde) byelectroreduction or electro-oxidation.

The electroactive species is then stripped from theelectrode by reversing the direction of the voltage sweep.

The potential becomes more positive, oxidizing thespecies back into solution (anodic stripping volt.) ormore negative reducing the species back into solution(cathodic stripping voltammetry)

The current measured during the oxidation or reductionis related to the quantity of analyte

The polarographic signal is recorded during theoxidation or reduction process.

The deposition step amounts to an electrochemical preconcentration of the analyte; that is, the concentration

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Lecture 7c- Electrochemical Analysis (1h) (2)