ANALYTICAL CHEMISTRY CHEM 3811 CHAPTER 15 DR. AUGUSTINE OFORI AGYEMAN Assistant professor of...

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