electroanalytical methods

CH 3001 Electroanalytical Methods and

Chemical Sensors

• Introduction

• Voltametry

• Potentiometry

• Coulometry and Electrogravimetry

• Conductimetry

• Electrochemical Sensors in brief

Mass transfer

M+n

mass transport occurs by:

1. diffusion = movement due to

concentration difference

2 migration of charged species

due to potential gradient

3. convection (mechanical

stirring or agitation)

Electrolysis

M+n

A-m

(-) (+)

anodeoxidationmeAA

cathodreductionMneMm

n

Faradic Current (Non-Charging Current)

M+n

e

(-) Current due to the transfer of electron from the electrode to the ion makes the ion to get reduced and the current produced by such processes is known as Faradic Current. This current obeys the Faraday’s law. This is also known as “non-charging current”.

Charging Current (Non-Faradic Current)

+ +

+ +

+ +

+ +

+ +

+ +

+ +

+ +

+ +

- -

- -

- -

- -

- -

- -

- -

- -

- -

- -

- -

- -

Met

alM+n

Built-up of electrical charge in the form of a double layer. Current due to the migration of ions through the double layer is known as “Charging Current”. This current does not follow the Faraday’s law. Therefore it is known as “Non-Faradic Current”.

Voltammetry

• Measurement of current as a function of potential.

• Measurement of current under condition of complete concentration polarization.

• A minimal consumption of analyte.

• Use of mercury as a electrode in this method is known as “Polarography”.

Voltammetry and Polarography

Voltammetry is the electrochemical technique in which the current at an electrode is monitored as a function of potential applied to that electrode.

The plot between current and the measured potential is known as “Voltamograme”.

In this technique, three-electrode-system is used.Working Electrode: Where the redox reaction takes place: usually-> Hg electrodeReference Electrode: Used to measure the potential of the working electrodeAuxiliary Electrode: Used to measure the current at the working electrode

A Modern Polarograph

Principle in Polarograpy

M+

Charging Reduction

transfereargChMneM

transferMassMM

electroden

electrod

nelectrode

n)bulk(

Current

Time

e

Faradaic Cureent

Charging Current

charged

Three ways of mass transfer when a potential is applied to the working

electrode:

1. Migration of ions under the influence of the electric field – Migration Current, imig

2. Convection current due to the movement of ions under the influence of hydrodynamic forces – Convection Current, icon

3. Diffusion of ions from higher to lower concentration – Diffusion Current, idif

inet = imig + icon + idif

Diffusion rate of the ions Concentration of the ions

diffusion current inet

DC Polarography

Here, the applied potential is DC voltage

The plot between current and the applied potential is known as a Polarogrph

il

EappE1/2

i

Powerful Variations of Voltametric Methods:

• AC Polarography • Normal-Pulse Polarography (NPP)• Differential Pulse Polarography

(DPP) • Anodic Stripping Voltammetry

(ASV) • Cyclic Voltametry (CV)

Faradic Current (Non-Charging Current)

M+n

e

(-) Current due to the transfer of electrons from the electrode to the ion makes the ion to get reduced and the current produced by such processes is known as Faradic Current. This current obeys the Faraday’s law. This is also known as Non-Charging Current.

Charging Current (Non-Faradic Current)

+ +

+ +

+ +

+ +

+ +

+ +

+ +

+ +

+ +

- - -

- -

- -

- -

- -

- -

- -

- -

- -

- -

- -

-

Met

alM+n

Built-up of electrical charge in the form of a double layer. Current due to the migration of ions through the double layer is known as charging current. This current does not follow the Faraday’s law. Therefore known as Non-Faradic Current.

Polarography Instrument

DC Polarograph

Oxygen Interferences

How to overcome oxygen interference

• Bubbling of solution with pure nitrogen gas.

• Bubbling should be done before the commencement of the analysis.

• Through out the analysis nitrogen atmosphere is maintained.

Advantages of DME (compared to planar electrodes)

• clean surface generated

• rapid achievement of constant current during drop growth

• remixing of solution when the drop falls

• high Hg overvoltage means even metals with high -ve E0 can be measured without H2 formation

Disadvantages of DME:

• Hg easily oxidized, limited use as anode (E< +0.4 V)

• 2Hg + 2Cl- Hg2Cl2 + 2e-

• nonfaradaic residual currents limit detection to >10-5 M

• cumbersome to use (toxic mercury)

• sometimes produce current maxima for unclear reasons (use maxima suppressor)

Polarographic Maxima

• sometimes produce current maxima

• Can be overcome by using maxima suppressor

Powerful improvement of polarography

• AC polarogrphy

• Normal pulse polarography

• Differential pulse polarography

• Stripping voltametry

• Cyclic voltametry

AC polarogrphy

• Instead of DC scan an AC scan is used.

Peaks - the selectivity is higher

Normal pulse polarographyIn Normal pulse voltammetry (polarography) - a potential wave is applied – an HME is used

The sensitivity is higher due to the absence of charging current

Differential pulse polarographyIn DifferentialDifferential pulse voltammetry (polarography) - a differential potential wave is applied – an HME is used

Both sensitivity and selectivity are higher due to the absence of charging current and peaks

Differential pulse polarography

pH (Glass Membrane) Electrodes

• One of the simpler ion-selective electrodes (ISE)

• Hydrogen Ion imparts a charge across a hydrated glass membrane

• Generally include an internal reference electrode (Ag/AgCl) and a separate Ag/AgCl electrode for sensing the charge imparted by the hydrogen ions

• Not as simple to use as you think!

constant pHconstant pH

External solutionExternal solution

glassglassmembrane-membrane-

hydratedhydrated(50 (50 m thick)m thick)

externalexternalreferencereferenceelectrodeelectrode(porous plug)(porous plug)

potentialpotentialdevelops acrossdevelops acrossmembrane duemembrane dueto pH differenceto pH difference

Combination Glass ElectrodeCombination Glass Electrode

solution levelsolution level

internalinternalreferencereferenceelectrodeelectrode

Commercial pH meter

3D network of silicate groups. There are sufficient cations within the interstices of this structure to balance the negative charge of silicate groups. Singly charged cations such as sodium are mobile in the lattice and are responsible for electrical conductance within the membrane.

Structure of membrane Glass

Hygroscopicity of Glass membrane

• Surface of the glass membrane must be hydrated to function as a pH electrode.

• It can lose pH sensitivity on dehydration, but is reversible and can be restored by soaking the electrode in water.

• The hydration involves an ion-exchange between singly charged cations in the glass lattice and protons from the solution.

GlH)solu(NaGlNa)solu(H

Electrical conduction across membranes

Conductance within the hydrated glass membrane involves the movement of Na+ and H+ ions.

Na+ - Charge carriers in the dry interiorH+ - Charge carriers in the gel interface

Gl)solu(HGlH

GlHGl)solu(H

2

1

The positions of these equilibrium depend on the H+ concentration on either side and charge on the glass surface giving potential. The potential difference between the two sides is known as the “Boundary Potential”.

The boundary potential

The potentials associated with each side (solution 1 and solution 2) E1 and E2

solutiondardtansisaa

aln

F

RTEE

)aln(F

RTconstE)aln(

F

RTconstE

22

121

2211

The boundary potential depends only upon the hydrogen ion activity of the external

The asymmetry potential

• When glass electrode’s two sides are in contact with identical solutions, we expect zero potential

• However a small potential known as the “Asymmetric Potential” is encountered.

• This gradually changes with time.• Electrode must be calibrated against one or

more standards.

alnF

RTconstEE 21

Basic Nernst Equation of a pH Electrode

alnF

RTconstEE 21

depends on internal referencedepends on internal referenceelectrode and glass membraneelectrode and glass membranebehavior, behavior, which changes with timewhich changes with time

Must calibrate with a buffer!Must calibrate with a buffer!

How does the measured voltage, E, vary with pH?

EE

pHpH

E ’E ’Two point calibrationTwo point calibration

with bufferswith buffers

E = E’ – 0.0591E = E’ – 0.0591 pH pH

Calibration of pH meter

ISFETA “new”

pH electrode

What does 0.05916 mean?• It is a constant, if there is a

one-electron reaction• It can be considered as the

equivalent of a constant of 59.16 mV

• A pH meter is a high-impedance potentiometer (measures voltage)

• A pH change of “1” imparts a change in 59.16 mV to the potential recorded by the pH meter!

• 1 pH unit change= 59.16 mV

mV (relative readings)

pH

100.00 2.00

159.16 3.00

218.32 4.00

277.48 5.00

336.64 6.00

395.80 7.00

Errors in pH Measurement…1. Uncertainty in your buffer pH due to normal weighing, diluting

errors etc.

2. Junction Potential due to the salt bridge and differences in Junction Potentials over time due to contamination of the junction

3. Overcome by regular recalibration

4. Sodium Error will result in high concentration of sodium solutions. The sodium can also impart a charge across the glass membrane.

5. Acid Error (strong acids) can saturate or contaminate the membrane with hydrogen ions!

6. Equilibration Error is overcome by letting the electrode equilibrate with the solution

7. Dried out glass membrane (ruins electrode)

8. Temperature. Since temperature affects activities, it is best to have all solutions at the same, constant temperature!

9. Strong bases. Strongly basic solutions (>pH 12) will dissolve the glass membrane!

Sodium Electrode•The change in the composition of the glass membrane permits the determination of cations other than H+.

• By modification of the glass membrane, you can make an electrode that is more sensitive to Na+ compared to H3O+.

• The modification can be done by incorporating Al2O3 or B2O3.

Ion Selective Electrodes…

• Selectivity Coefficient– Defines how an ISE responds to the species of

interest versus some interfering species• Interferences cause a signal (voltage) to be imparted

on the electrode that is NOT the result of the ion or chemical species of interest

– You want the selectivity coefficient to be as SMALL as possible

)),(

),(

XXAA

XA

AkA

A X

AX

k

( log n

0.05916 constant E

analyte the is and ceinterferen the is where

to response electrode to response electrode

Examples of Electrodes / Interferences

Solid State Electrodes

• Uses a small amount of a doped crystal to transport charge from the solution to an inner electrode

• The inner (sensing) electrode can be a Ag/AgCl electrode, and a separate Ag/AgCl electrode can be present– Combination electrode

• You can use a separate reference electrode also.

• Typical equation (Fluoride Electrode):

• Use:– Prepare calibration and sample solutions to

similar ionic strengths and temperatures– Connect ISE and reference electrode to

potentiometer (pH meter in mV mode)– Record potentials of calibration solutions– Prepare calibration curve– Measure potential of sample solutions and

calculate fluoride activity!

)( log x 0.05916 x - constant E outside -FA

Liquid membrane electrodes

• Similar to pH electrode except the membrane is an organic polymer saturated with liquid ion exchanger.

• Interaction of this exchanger with target ions resulted in a potential across the membrane that can be measured.

Liquid Membrane ISE’s• Replace the solid

state crystal with a liquid ion-exchanger filled membrane

• Ions impart a charge across the membrane

• The membrane is designed to be SELECTIVE for the ion of interest…

Liquid membrane electrodes• The reservoir forces exchanger into

membrane.• The exchanger forms complexes with species

of interest.• This results in a concentration difference and

the resulting potential difference can be measured.

Gas Sensing Electrodes• Still considered “ion-

selective”• Works by the permeation

of gas across a selective membrane

• Also called compound electrodes

• The gas changes the pH inside the electrode (on the inside of the membrane) and this signal is proportional to the gas concentration.

Gas Sensing Electrodes

Enzyme Electrodes

Enzyme Electrodes

Enzyme Electrodes

EE transducer

Analyte signalrecognition

Enzyme Electrodes

EE transducer

Analyte No signalNo recognition

Potentiometric Titrations

ECell

VTitrantEnd Point

Potentiometric Titration

• How does the pH change during an acid-base neutralization reaction?

• Measure pH as the titrant is added.

pHpH

Volume of titrant (mL NaOH)Volume of titrant (mL NaOH)

HCl + NaOH HCl + NaOH NaCl + H NaCl + H22OO

pH = 7pH = 7

Equivalence point pH Equivalence point pH

How do we locate the equivalence point?How do we locate the equivalence point?

pHpH

Volume of titrant (mL NaOH)Volume of titrant (mL NaOH)

HCl + NaOH HCl + NaOH NaCl + H NaCl + H22OO

pH = 7pH = 7

Just acidJust acid

Just saltJust salt

Both acid & saltBoth acid & salt

xs NaOH & saltxs NaOH & salt

What is present at various points?What is present at various points?

pHpH

Volume of titrant (mL NaOH)Volume of titrant (mL NaOH)

HCl + NaOH HCl + NaOH NaCl + H NaCl + H22OO

The slope between each pair of data points.The slope between each pair of data points.

How is the slope changing?How is the slope changing?

increasing slopeincreasing slope

decreasing slopedecreasing slope

77

inflection pointinflection point - - changed changed from increasing to decreasingfrom increasing to decreasing

Using the derivative toUsing the derivative tolocate the equivalence pointlocate the equivalence point

0

2

4

6

8

10

12

14

16

0 5 10 15 20

volume of NaOH

pH

0

5

10

15

20

25

30

35

40

45

50

deri

vati

ve

pKpKaa

The maximum of the derivative locates the equivalence point.The maximum of the derivative locates the equivalence point.

Electrogravimetry & coulometry Both are electrolysis processes

Constant potential electrolysis – potential of the

working electrode is held constant

Constant current electrolysis – current at the

working electrode is held constant

Complete electrolysis

Amount of material discharge – Electrogravimetry

Amount of electricity required - Coulometry

Electrogravimetry

• Mostly used in metal ion analysis

• Deposits on cathode as the metal

• Weight of the cathode before and after the complete electrolysis

• Electrodes used in the electrolysis must be inactive

Decomposition potentialIncrease of the potential at the working electrode leads to give a current at a certain potential.

Current

potentialDecomposition potential

Factors effecting decomposition potential

• Equilibrium potential• Ohmic potential• Overpotential –

activation and concentration

copaopopeqd EEEEE

Selectivity in electrolysis

Current

potentialE1 E1

Applied potential between E1 and E2 only one metal ion gets reduced.

The electrolysis under constant potential is selective.

The electrolysis under constant current is non-selective.

Selectivity in electrolysis cont...

Apparatus for electrogravimetry

Depolarization of electrodes

• As the current decreases, the potential needs to be increased in order to offset IR drop and then excess ions present in the solution may start to discharge. Under these circumstance, the electrode is said to be depolarized. This will lead to co-deposit and can start before the ion interested to complete the deposition. This will lead to interferences.

Interferences in ElectrolysisExample:

Mixture of Cu(II) and Pb(II)

Deposition of Cu(II) starts at 0.2 V. Due to the IR drop, in order to keep the current constant,potential should be increased.

In acidic solutions reduction of H+ occurs and deposition may not adhere to the electrode.

Depolarizers in electrolysis

• In order to avoid H2 evolution from cathode, we add nitrate ions into the solution.

OH3NHe8H10NO

H2

1eHofInstead

243

2

• Here, the nitrate ion is called depolarizer which avoids the reduction of H+.

Electrolysis under constant current

• This is lack of selectivity and do not have any practical importance when a mixture of ions are present in the solution.

• However, if a single ion is present, this method is more advantages since the time necessary for complete electrolysis can be controlled.

Electrolysis under constant potential

• The potential of the working electrode can be controlled and kept at a constant value. However as the electrolysis is going on the current is continuously dropping (the rate of discharge is dropping).

• It takes a longer time for complete electrolysis, but however interferences can be avoided.

• Mixture of Cu(II) and Pb(II)

• The deposition potential for Cu(II) – 0.2 V

• The deposition potential for Pb(II) - -0.15 V

• By controlling the potential, Cu can be deposited first and then Pb can be determined by taking the weight of the cathode at each occasion.

Electrolysis under constant potential

The quantity of electrical charge

• Under constant current Q = it

• Under constant potential

• By measuring the amount of electricity one can calculate the amount of material discharge.

t

o

idtQ

Coulometry

• Two types of methods

• Constant potential coulometry - selective• Constant current coulometry- non-selective

• Coulometry is same as electrogravimetry but instead of measuring the weight of discharge material, here, the amount of electricity is measured which can be related to the amount of material according to Faraday’s law.

Coulometric Titration

• Here the titrant is electrically generated.

• Example: Analysis of As(III)

• The solution containing As(III) was added to excess I- and electrolysis carried out.

• The I2 produced will react with As(III)

I2e2I

e2)V(As)III(As

2