ANALYTICAL CHEMISTRY CHEM 3811

CHAPTER 15

DR. AUGUSTINE OFORI AGYEMANAssistant professor of chemistryDepartment of natural sciences

Clayton state university

CHAPTER 15

ELECTRODE MEASUREMENTS

INDICATOR ELECTRODES

Chemically Inert Electrodes

- Do not participate in the reaction

ExamplesCarbonGold

PlatinumITO

INDICATOR ELECTRODES

Reactive Electrodes

- Participate in the reaction

ExamplesSilver

CopperIronZinc

INDICATOR ELECTRODES

- Respond directly to the analyte

Two Classes of Indicator Electrodes

- Metal Electrodes

- Surfaces on which redox reactions take place

ExamplesPlatinum

Silver

INDICATOR ELECTRODES

- Respond directly to the analyte

Two Classes of Indicator Electrodes

- Ion-Selective Electrodes

- Selectively binds one ion (no redox chemistry)

ExamplespH electrode

Calcium (Ca2+) electrodeChloride (Cl-) electrode

DOUBLE-JUNCTION REFERENCE ELECTRODES

- With the use of reference electrodes

- KCl solution may slowly leak into solution through the porous plug (salt bridge)

- Cl- may introduce errors(e.g. consumes Ag+ when reagent is Ag+ solution)

- Double-junction reference electrode prevents direct leakage into reagent

JUNCTION POTENTIAL

- When two dissimilar electrolyte solutions come in contact

- Potential difference develops at the interface

- Voltage is very small usually in millivolts

- Very common at the ends of salt bridges

- Observed voltage measurements may include junction potential

JUNCTION POTENTIAL

Eobserved = Ecell + Ejunction

- A result of unequal ion mobilities

- K+ and Cl- have similar mobilities

- Reason why KCl is used in salt bridges

POTENTIOMETRY

- The use of voltage measurements for quantification

Direct Potentiometric Method- Measures absolute potential (concentration)

- A metal in contact with a solution of its cation- Associated with errors due to junction potentials

Examples- Silver wire for measuring [Ag+]

- Potassium ion-selective electrode for measuring [K+]- pH electrode for measuring [H+]

POTENTIOMETRY

- The use of voltage measurements for quantification

Relative Potentiometric Method- Measures changes in potential (concentration)

- Relatively precise and accurate

Example- Measuring changes in potential during titration

ION-SELECTIVE ELECTRODES

- Responds preferentially to one species in solution

Internal reference electrode

Ion-selective membrane

Filling solution

- Selective (preferential) ion is C+

- Membrane is made of poly(vinyl chloride)

- 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

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

- Produces a potential difference across the membrane

ION-SELECTIVE ELECTRODES

inner

outer

][C

][Clog

n

0.05916E

- n is the charge on the selective ion (negative for anions)

n = +1 for K+

n = +2 for Ca2+

n = -2 for CO32-

at 25 oC

pH GLASS ELECTRODE

- The most widely used

- Selective ion is H+

- Glass membrane (bulb) consists of SiO4

- 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

Glass Electrode Response at 25 oC

E = constant + β(0.05916)ΔpH

ΔpH = pH difference between inside and outside of glass bulb

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

constant = assymetry potential

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)

COMPOUND ELECTRODE

- Electrode surrounded by a membrane

- Membrane isolates the analyte to which the electrode responds

Examples- Gas sensing electrodes

NH3, CO2, NOx, H2S, SO2

- Enzyme electrodes (highly selective)

ELECTROCHEMICAL METHODS

Applications

- Biosensors (analyte sensors)(Glucose sensors)

- Chromatography detectors- Solar energy storage systems

- Microelectronics- Electrocatalysis of fuel cells and batteries

Electrogravimetric Analysis

- Chemically inert cathode with large surface area is used(in the form of gauze)

- Analyte is electroplated (deposited) on a preweighed cathode

- Cathode is weighed again

- Mass of analyte is determined by difference

Cu2+(aq) + 2e- → Cu(s) (deposited on cathode)

ELECTROCHEMICAL METHODS

Coulometric Analysis

- Amount of analyte is determined from electron count

- Electric current and time required to generate product are measured

- Number of electrons is determined from current and time

- Number of moles of analyte is determined from electron count

Reaction of I2 and H2SI2 + H2S → S(s) + 2H+ + 2I-

ELECTROCHEMICAL METHODS

Three Electrode Cells

- Reference electrode- Working (indicator) electrode- Auxiliary (counter) electrode

- Current flows between working and auxiliary electrodes

- Voltage is measured between working and reference electrodes

ELECTROCHEMICAL METHODS

Amperometry

- The electric current between the pair of electrodes is measured

- Voltage is fixed

- Current is proportional to the concentration of analyte

Biosensors(glucose monitors)

ELECTROCHEMICAL METHODS

Voltammetry

- Voltage between two electrodes is varied as current is measured

- Oxidation-reduction takes place at or near the surface of the working electrode

- Graph of current versus potential is obtained(called voltammogram)

- Peak current is proportinal to concentration of analyte

ELECTROCHEMICAL METHODS

Voltammetry

Polarography- Uses dropping-mercury electrode

Square Wave Voltammetry- Uses waveform which consists of square wave

superimposed on a staircase

ELECTROCHEMICAL METHODS

Voltammetry

Stripping Voltammetry- Analyte is concentrated into a drop of Hg by reduction

- Analyte is reoxidized by making potential more positive- Current is measured during oxidation

Cyclic Voltammetry (CV)- Electrode potential versus time is linear

- Current versus applied voltage gives a cyclic voltammogram trace- Used to study electrochemical properties of analytes

ELECTROCHEMICAL METHODS