basics of electrochemistry

BASICS OF ELECTROCHEMISTRY

Unit I Electrochemistry

POST GRADUATE & RESEARCH DEPARTMENT OF CHEMISTRY

GOVERNMENT ARTS COLLEGE (Autonomous)

Coimbatore - 641018

Dr. N. VELMANI

Assistant Professor

Ion selective electrodes.

Potentiometric Methods

A.) Introduction:

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

B.) Reference Electrodes:

1.) Need one electrode of system to act as a reference against which potential

measurements can be made → relative comparison.

Desired Characteristics:

a) known or fixed potential

b) constant response

c) insensitive to composition of solution under study

d) obeys Nernest Equation

e) reversible

B.) Reference Electrodes:

2.) Common Reference Electrodes used in Potentiometry

a) Calomel Electrode (Hg in contact with Hg2Cl2 & KCl)

i. Saturated Calomel Electrode (SCE) very widely used

½ cell: Hg/Hg2Cl2 (satd), KCl (xM)||

½ reaction: Hg2Cl2 (s) + 2e- → 2Hg + 2Cl-

Note: response is dependent on [Cl-]

SCE

b) Silver/Silver Chloride Electrode

- most widely used reference electrode system

- Ag electrode in KCl solution saturated with AgCl

½ cell: Ag/AgCl (satd), KCl (xM)||

½ reaction: AgCl (s) + e- → Ag(s) + Cl-

Advantage – one advantage over SCE is that Ag/AgCl

electrode can be used at temperatures > 60oC

Disadvantage – Ag reacts with more ions

c) Precautions in the Use of Reference Electrodes

- need to keep level of solution in reference electrode above

level in analyte solution

- need to prevent flow of analyte solution into reference electrode‚ can result in plugging of electrodeat junction→ erratic behavior

Vycor plug

C.) Indicator Electrodes:

1.) Detects or Responds to Presence of Analyte

Three Common Types:

a) Metallic Indicator Electrodes

b) Membrane Indicator Electrodes

c) Molecular Selective Electrode

2.) Metallic Indicator Electrode (Four Main Types)

a) Metallic Electrodes of the First Kind

i. Involves single reaction

ii. Detection of cathode derived from the metal used in the electrode

iii. Example: use of copper electrode to detect Cu2+ in solution

½ reaction: Cu2+ + 2e- → Cu (s)

Eind gives direct measure of Cu2+:

Eind = EoCu – (0.0592/2) log aCu(s)/aCu2+

since aCu(s) = 1:

Eind = EoCu – (0.0592/2) log 1/aCu2+

or using pCu = -log aCu2+:

Eind = EoCu – (0.0592/2) pCu

iv. Problems:

- not very selective

- many can only be used at neutral pH →metals dissolve in acids

- some metals readily oxidize

- certain hard metals (Fe, Cr, Co, Ni) do not yield reproducible

results

- pX versus activity differ significantly and irregularly from theory


b) Metallic Electrodes of the Second Kind

i. Detection of anion derived from the interaction with metal ion (Mn+)

from the electrode

ii. Anion forms precipitate or stable complex with metal ion (Mn+)

iii. Example: Detection of Cl- with Ag electrode

½ reaction: AgCl(s) + e- → Ag(s) + Cl- EO = 0.222 V

Eind gives direct measure of Cl-:

Eind = Eo – (0.0592/1) log aAg(s) aCl-/aAgCl(s)

since aAg(s) and aAgCl(s)= 1

& Eo = 0.222 V:

Eind = 0.222 – (0.0592/1) log aCl-

iv. Another Example: Detection of EDTA ion (Y4-) with Hg Electrode

½ reaction: HgY2- + 2e- → Hg(l) + Y4- Eo = 0.21 V

Eind responds to aY4-:

Eind = Eo – (0.0592/2) log aHg(l) aY4-/aHgY2-

since aHg(l) = 1 and Eo = 0.21 V:

Eind = 0.21 – (0.0592/1) log aY4-/aHgY2-


c) Metallic Electrodes of the Third Kind

i. Metal electrodes responds to a different cation

ii. Linked to cation by an intermediate reaction

- Already saw detection of EDTA by Hg electrode (2nd Kind)

ii. Can be made to detect other cations that bind to EDTA → affecting aY4-

iv. Example: Detect Ca by complex with EDTA

equilibrium reaction: CaY2- → Ca2+ + Y4-

Where: Kf = &

Eind = 0.21 – (0.0592/1) log aY4-/aHgY2-

aca2+ . aY4-

aCaY2-

ay4- = Kf

. aca2+

aCaY2-

Note: aY4- and Eind now also changes with aCa2+


d) Metallic Redox Indicators

i. Electrodes made from inert metals (Pt, Au, Pd)

ii. Used to detect oxidation/reduction in solution

iii. Electrode acts as e- source/sink

iv. Example: Detection of Ce3+ with Pt electrode

½ reaction: Ce4+ + e- → Ce3+

Eind responds to Ce4+:

Eind = Eo – (0.0592/1) log aCe3+/aCe4+

v. Problems:

- electron-transfer processes at inert electrodes are frequently

not reversible

- do not respond predictably to ½ reactions in tables

3.) Membrane Indicator Electrodes

a) General

i. electrodes based on determination of cations or anions by the selective adsorption

of these ions to a membrane surface.

ii. Often called Ion Selective Electrodes (ISE) or pIon Electrodes

iii. Desired properties of ISE’s

‚ minimal solubility – membrane will not dissolve in solution during

measurement

– silica, polymers, low solubility inorganic compounds

(AgX) can be used

‚Need some electrical conductivity

‚Selectively binds ion of interest

3.) Membrane Indicator Electrodes

b) pH Electrode

i. most common example of an ISE

‚ based on use of glass membrane that preferentially binds H+

ii. Typical pH electrode system is shown

‚ Two reference electrodes here

‚ one SCE outside of membrane

‚ one Ag/AgCl inside membrane

‚ pH sensing element is glass tip of Ag/AgCl electrode

iii. pH is determined by formation of boundary potential across glass membrane

At each membrane-solvent interface, a small local potential

develops due to the preferential adsorption of H+ onto the glass

surface.

Si O-

Glass Surface

iii. pH is determined by formation of boundary potential across glass membrane

Boundary potential difference (Eb) = E1 =E2 where from Nernst

Equation:

Eb = c – 0.592pH -log aH+ (on exterior of probe or

in analyte solution)constant

Selective binding of cation (H+) to glass membrane

iv. Alkali Error

‚ H+ not only cation that can bind to glass surface

- H+ generally has the strongest binding

‚ Get weak binding of Na+, K+, etc

‚ Most significant when [H+] or aH+ is low (high pH)

- usually pH between11-12

At low aH+ (high pH), amount of Na+ or

K+ binding is significant → increases

the “apparent” amount of bound H+

v. Acid Error

‚ Errors at low pH (Acid error) can give readings that are too high

‚ Exact cause not known

- usually occurs at pH # 0.5

c) Glass Electrodes for Other Cations

i. change composition of glass membrane

‚ putting Al2O3 or B2O3 in glass

‚ enhances binding for ions other than H+

ii. Used to make ISE’s for Na+, Li+, NH4+

d) Crystalline Membrane Electrode

i. Fluoride Electrode

‚ LaF3 crystal doped with EuF2

‚ mechanism similar to pH electrode with potential developing at two

interfaces of the membrane from the reaction:

LaF3 » LaF2+ + F-

Solid

(membrane surface)

Solution

ˆ the side of the membrane with the lower aF-

becomes positive relative to the other surface:

Eind = c – 0.0592 pF

e) Liquid Membrane Electrode

‚ “Membrane” usually consists of organic liquid (not soluble in sample) held by

porous disk between aqueous reference solution and aqueous sample solution.

‚ Membrane has ability to selectively bind ions of interest

P

R-OO

R-OO-Ca-O

P

OO-R

O-R

Example: Calcium dialkyl

phosphate Liquid membrane

electrodes

At solution/membrane interfaces:

[(RO)2POO]2Ca » 2(RO2)POO- + Ca2+

Organic

(membrane)

Organic

(membrane surface)

Solution

(aqueous sample)

ˆ the side of the membrane with the lower aCa2+

becomes negative relative to the other surface:

Eind = c – 0.0592/2 pCa

e) Liquid Membrane Electrode

‚ Can design Liquid Membrane Electrodes for either cations or anions

- cations → use cation exchangers in membrane

- anions → use anion exchangers in membrane

f) Molecular Selective Electrodes

i. Electrodes designed for the detection of molecules instead of ions

ii. Gas sensing electrodes (or gas-sensing probes)

‚ Typically based on ISE surrounded by electrolyte solution

- activity of ion measured is affected by dissolved gas

- gas enters interior solution from sample by passing through a

gas permeable membrane

Gas effuses through membrane:

CO2 (aq) » CO2 (g) » CO2 (aq)external membrane internal

solution pores solution

In internal solution, pH changes:

CO2 (aq) + H2O » HCO3- + H+

which is detected by ISE probe

Overall reaction:

CO2 (aq) + H2O » H+ + HCO3-

external internal

solution solution

Eind = c + 0.0592 log [CO2]ext

iii. Enzyme electrodes (or Biocatalytic Membrane Electrodes)

‚ General approach is to use an immobilized enzyme

- enzyme converts a given molecular analyte into a species that

can be measured electrochemically

< enzyme substrate

- Examples:

H+→ pH electrode

CO2 → CO2 gas sensing electrode

NH4+→ NH4

+ ISE

‚ Example – Urea Enzyme Electrode

- Principal: In presence of enzyme urease, urea (NH4)2CO is

hydrolyzed to give NH3 and H+

(NH4)2CO + 2H2O + H+ » 2NH4+ + HCO3

-»

2NH3 + 2H+Monitor amount of NH3 produced

using NH3 gas sensing electrode

Example 18: The following cell was used for the determination of pCrO4:

SCE||CrO42- (xM), Ag2CrO4 (sat’d)|Ag

Calculate pCrO4 if the cell potential is -0.386.

Unit I Electrochemistry

Ionic strength, DHO Equation, Wien Effect & Falkenhagen Effect

POST GRADUATE & RESEARCH DEPARTMENT OF CHEMISTRYGOVERNMENT ARTS COLLEGE (Autonomous)

Coimbatore - 641018

Dr. N. VELMANIAssistant Professor

The first successful attempts to explain the variation ofequivalent conductance of strong electrolytes withdilution was made by Debye and Huckel(1923).

The fundamental idea underlying their work is thatbecause of electrical attraction among the oppositelycharged ions.

+

+

+ +

+

+

+

+

+

+

++

−

−

−

−

−−

−

−

−

−

−

+

−

−

The electrical attractions among the oppositely charged ions which affect the speed of an ion in the electric field are called “interionic effects”.

There are two such effects :-

❖ Relaxation effect or Asymmetry effect

❖ Electrophoretic effect

+

+

+ +

+

++

+

+

+

++

−

−

−

−

−−

−

−

−

−

−

+−

−

Debye-Hückel-Onsager theory

1) Relaxation effect

2) Electrophoretic effect

In 1927, Onsager pointed out thatas the ion moves across the solution,its ionic atmosphere is repeatedlybeing destroyed and formed again.

+−+

−

−

−

−

−

+ +

+

+

+

−−

−

+

+

−

−

The time for formation of a newionic atmosphere (relaxation time)is ca. 10-7s in an 0.01 mol·kg-1

solution.

Under normal conditions, thevelocity of an ion is sufficiently slowso that the electrostatic force exertedby the atmosphere on the ion tendsto retard its motion and hence todecrease the conductance.

RELAXATION EFFECTS OR ASYMMETRY EFFECTS

-

-

+

_

-

-

--

-

--

-

-

--

-

-+

+

(a) (b)

Symmetrical ionic atmosphere around a positive ion

Ionic atmosphere becoming asymmetrical when central ion moves

FIG:1

_

__

_

_ _

_+

FIG:2

Debye and huckel (1923)derived a mathematical expression for thevariation of equivalent conductance with concentration. This equationwas further improved by Onsager(1926-1927) and is known as Debye-Huckel-Onsager equation.

Λc = Λ0-[82.4/(DT)1/2ή +8.20X105/(DT)3/2λ0]√C

Where Λc =Equivalent conductance at concentration c.Λ0 =Equivalent conductance at infinite dilution.D = Diectric constant of the medium.ή =Coefficient of viscosity of the medium.T =Temperature of the solution in degree absolute.c = Concentration of the solution in moles/litre.

As D and ή are constant for a particular solvent.Therefore,at constant temperature, the above equation can be written in the form:

Λc= Λ0-(A+BΛ0)√c where A and B are constants for a particular solvent

VERIFICATION OF THE ONSAGER EQUATION

Two tests can be readily performed to verify the onsager equation.Theseare:-

❖ The plot of Λc vs √c should be linear.❖The slope of the line should be equal to A+B Λ0, calculated by substituting the value of various constants directly.

Eq

uiv

alen

t c

on

du

ctan

ce

√concentration c

HCI Acid

KCl

AgNO3

NaCl

FIG:3 TESTS OF ONSAGER EQUATION

+

+

+ +

+

+

++

+

+

++

−

−

−

−

−−

−

−

−

−

−

+−

−

REGARDING MOVEMENT OF IONS AND INTERACTION

That ‘s All for now

Conductance measurements?

Escape velocity or speeed ?

basics of electrochemistry

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