ELECTROCHEMISTRY

CHEM 4700

CHAPTER 5

DR. AUGUSTINE OFORI AGYEMANAssistant professor of chemistryDepartment of natural sciences

Clayton state university

CHAPTER 5

POTENTIOMETRY

- Based on static (zero-current) measurements

- Used to obtain information on the composition of an analyte

- Potential between two electrodes is measured

Applications- Environmental monitoring

- Clinical diagnostics (blood testing, electrolytes in blood)- Control of reaction processes

POTENTIOMETRY

- Also known as indicator electrodes

- Respond directly to the analyte

- Used for direct potentiometric measurements

- Selectively binds and measures the activity of one ion (no redox chemistry)

ExamplespH electrode

Calcium (Ca2+) electrodeChloride (Cl-) electrode

ION-SELECTIVE ELECTRODES (ISE)

Advanteages

- Exhibit wide response

- Exhibit wide linear range

- Low cost

- Color or turbidity of analyte does not affect results

- Come in different shapes and sizes

ION-SELECTIVE ELECTRODES (ISE)

- Made from a permselective ion-conducting membrane(ion-exchange material that allows ions of one electrical

sign to pass through)

- Reference electrode is inbuilt

- Internal solution (solution inside electrode) contains ion of interest with constant activity

- Ion of interest is also mixed with membrane

- Membrane is nonporous and water insoluble

ION-SELECTIVE ELECTRODES (ISE)

- Responds preferentially to one species in solution

Internal reference electrode

Ion-selective membrane

Internal (filling) solution

ION-SELECTIVE ELECTRODES (ISE)

- Selective (preferential) ion is C+

- Membrane is made of poly(vinyl chloride) (PVC)

- Membrane is impregnated with nonpolar liquid

- Membrane contains ligand L (ion-selective ionophore)

- Membrane contains the complex LC+

- Membrane contains hydrophobic anion R- (ion exchanger)

ION-SELECTIVE ELECTRODES (ISE)

- [C+] inside the electrode ≠ [C+] outside the electrode

- Results in a potential difference across the membrane

Generally (at 25 oC)- 10-fold change in activity implies 59/zi mV change in E- zi is the charge on the selective ion (negative for anions)

- zi = +1 for K+, zi = +2 for Ca2+, zi = -2 for CO32-

ION-SELECTIVE ELECTRODES (ISE)

inner

outer

i ][C

][Cln

Fz

RTE

inner

outer

i

o

][C

][Clog

z

0.05916EC, 25At

- Let ci = molarity of C+

- Activity (ai) rather than molarity is measured by ISEs

- Activity is the effective (active) concentration of analyte(effective concentration decreases due to ionic interactions)

- zi = ionic charge (±)

ai = γici

where γi = activity coefficient (between 0 and 1)

ION-SELECTIVE ELECTRODES (ISE)

Debye-Hückel Equation

- Relates activity coefficients to ionic strength (at 25 oC)

α = size of ion in picometers (1 pm = 10-12 m)

µ = ionic strength

/305)μ(α1

μ0.51zγlog

2i

ION-SELECTIVE ELECTRODES (ISE)

Ionic strength- A measure of the concentration of all ions in solution

with their charges taken into account

.........zczczc2

1zc

2

1μ 2

ii2ii

2ii

i

2ii

ci = the concentration of the ith species

Ionic strength of electrolytes1:1 electrolytes (NaCl) µ = molarity

2:1 electrolytes (CaCl2) µ = 3 x molarity3:1 electrolytes (AlCl3) µ = 6 x molarity2:2 electrolytes (MgSO4) µ = 4 x molarity

ION-SELECTIVE ELECTRODES (ISE)

- For very dilute solutions ai ≈ ci

- Activity coefficient decreases as ionic strength increases

For zi = 1- 1 mV change in potential implies 4% change in activity

For zi = 2- 1 mV change in potential implies 8% change in activity

- This is known as Nernstian behavior

ION-SELECTIVE ELECTRODES (ISE)

Selectivity Coefficient (k)

- A measure of the ability of ISE to discriminate against an interfering ion

- It is assumed that ISEs respond only to ion of interest

- In practice, no electrode responds to only one specific ion

- The lower the value of k the more selective is the electrode

- k = 0 for an ideal electrode (implies no interference)

ION-SELECTIVE ELECTRODES (ISE)

Selectivity Coefficient (k)

For k > 1- ISE responds better to the interfering ion than to the target ion

For k = 1- ISE responds similarly to both ions

For k < 1- ISE responds more selectively to ion of interest

ION-SELECTIVE ELECTRODES (ISE)

Empirical Calibration PlotP

oten

tial

(m

V)

p[C+]

Slope = 59/zi mV

zi = charge of ion

Called Nernstian slope

- Used to determine the unknown concentration of analytes

- Departure from linearity is observed at low concentrations

ION-SELECTIVE ELECTRODES (ISE)

Three groups of ISEs

- Glass electrodes

- Liquid electrodes

- Solid electrodes

ION-SELECTIVE ELECTRODES (ISE)

GLASS ELECTRODES

- Responsive to univalent cations

- Employs thin ion-selective glass membrane

pH GLASS ELECTRODE

- The most widely used

- For pH measurements (selective ion is H+)

- Response is fast, stable, and has broad range

- pH changes by 1 when [H+] changes by a factor of 10

- Potential difference is 0.05196 V when [H+] changes by a factor of 10

For a change in pH from 3.00 to 6.00 (3.00 units)Potential difference = 3.00 x 0.05196 V = 0.177

pH GLASS ELECTRODE

- Thin glass membrane (bulb) consists of SiO4

- Most common composition is SiO2, Na2O, and CaO

Glass membrane contains - dilute HCl solution

- inbuilt reference electrode (Ag wire coated with AgCl)

pH GLASS ELECTRODE

Glass Electrode Response at 25 oC (potential across membrane with respect to H+)

ΔpH = pH difference between inside and outside of glass bulb

β ≈ 1 (typically ~ 0.98)(measured by calibrating electrode in solutions of known pH)

K = assymetry potential (system constant, varies with electrodes)

ΔpHβ(0.05916)KE

)(a0.05916log -KEH

pH GLASS ELECTRODE

- Equilibrium establishes across the glass membrane with respect to H+ in inner and outer solutions

- This produces the potential, E

- Linearity between pH and potential

- Calibration plot yields slope = 59 mV/pH units

- Electrode is prevented from drying out by storing in aqueous solution when not in use

pH GLASS ELECTRODE

Sources of Error

- Standards used for calibration- Junction potential- Equilibration time

- Alkaline (sodium error)- Temperature- Strong acids

- Response to H+ (hydration effect)

OTHEER GLASS ELECTRODES

Glass Electrodes For Other CationsK+ -, NH4

+-, Na+-selective electrodes- Mechanism is complex

- Employs aluminosilicate glasses (Na2O, Al2O3, SiO2)- Minimizes interference from H+ when solution pH > 5

pH Nonglass Electrodes- Quinhydrone electrode (quinone – hydroquinone couple)

- Antimony electrode

LIQUID MEMBRANE ELECTRODES

- Employs water-immiscible substances impregnated in a polymeric membrane (PVC)

- For direct measurement of polyvalent cations and some anions

- The inner solution is a saturated solution of the target ion

- Hydrophilic complexing agents (e.g. EDTA) are added to inner solutions to improve detection limits

- Inner wire is Ag/AgCl

Ion-Exchange Electrodes

- The basis is the ability of phosphate ions to form stable complexes with calcium ions

- Selective towards calcium

- Employs cation-exchanger that has high affinity for calcium ions(diester of phosphoric acid)

- Inner solution is a saturated solution of calcium chloride

- Cell potential is given by )log(a2

0.05916KE

aC

LIQUID MEMBRANE ELECTRODES

Other Ion-Exchange Electrodes

- Have poor selectivity and are limited to pharmaceutical formulations

Examples- IEE for polycationic species (polyarginine, protamine)

- IEE for polyanionic species (DNA)- IEE for detection of commonly abused drugs

(large organic species)

LIQUID MEMBRANE ELECTRODES

Neutral Carrier Electrodes

- Employs neutral carriers such as crown ethers and cyclic polyesters

- Carriers envelope target ions in their pockets

Used for clinical analysis- detection of blood electrolytes

- detection alkali and alkaline earth metal cations

LIQUID MEMBRANE ELECTRODES

Neutral Carrier Electrodes

Examples of Carriers- Monensin for sodium

- Macrocyclic thioethers for Hg and Ag- Valinomycin for potassium ions- Calixarene derivatives for lead- 14-crown-4-ether for lithium

LIQUID MEMBRANE ELECTRODES

Anion-Selective Electrodes

- For sensing organic and inorganic anions

Examples of Anions- Phosphate- Salicylate

- Thiocyanate- Carbonate

LIQUID MEMBRANE ELECTRODES

SOLID-STATE ELECTRODES

- Solid membranes that are selective primarily to anions

Solid-state membrane may be - single crystals

- polycrystalline pellets or

- mixed crystals

SOLID-STATE ELECTRODES

Examples - Most common is fluoride-ion-selective electrode

(limited pH range of 0-8.5)(OH- is the only interfering ion due to similar size and charge)

- Iodide electrode (high selectivity over Br- and Cl-)

Chloride electrode (suffers interference from Br- and I-)

Thiocynate (SCN-) and cyanide (CN-) electrodes

OTHER ELECTRODES

- Coated-wire electrodes (CWE)

- Solid-state electrodes without inner solutions

- Made up of metallic wire or disk conductor (Cu, Ag, Pt)

- Mechanism is not well understood due to lack of internal reference

- Usually not reproducible

For detection ofamino acids, cocaine, methadone, sodium

APPLICATIONS OF ISEs

- Used as detectors for automated flow analyzers(flow injection systems)

- High-speed determination of blood electrolytes in hospitals(H+, K+, Cl-, Ca2+, Na+)

- For measuring soil samples (NO3-, Cl-, Li+, Ca2+, Mg2+)

- Coupling ion chromatography with potentiometric detection

- Micro ISEs as probe tips for SECM

- Column detectors for capillary-zone electrophoresis