Electrochemistry 1

Electrochemistry

Electrochemical Cells

Consists of two electrodes Anode and Cathode immersed in a suitable ElectrolyteElectrodes are connected via a conductorthey permit movement of ions in solutionelectron transfer reactions can occur

Conduction in a CellGalvanic Cells and Electrolytic CellsAnodes and Cathodes

Assignment: Chapter 22 Skoog and West

4

Cell Diagrams – Conventions

Place the anode on the left side of the diagram

Place the cathode on the right side of the diagramUse a single vertical line ( | ) to represent the boundary between different phases, such as between an electrode and a solution

Use a double vertical line ( || ) to represent a salt bridge or porous barrier separating two half-cells

Chapter 18: Electrochemistry 5

An Example Cell Diagram

EOS

Chapter 18: Electrochemistry 13

Standard Hydrogen ElectrodeIn the standard hydrogen electrode, hydrogen gas at exactly 1 bar pressure is bubbled over an inert platinum electrode and into an aqueous solution with the concentration adjusted so that the activity of H3O+ is exactly equal to one EOS

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Standard Electrode Potentials

A standard electrode potential, Eo, is based on the tendency for reduction to occur at the electrode

The cell voltage, called the standard cell potential (Eo

cell), is the difference between the standard potential of the cathode and that of the anode

Voltaic cells can produce electrical workw = –n F Ecell

According to IUPAC electrode potential is +ve if electrode reaction involves reduction and -ve if oxdn

EMF and Equm Const of Cell Reactions

aA + bB cC + dD

0 lnG G RT Q

Consider the reaction

For a forward reaction free energy should be negative given by

Where Q is the activity of the products and the reactants. If E is the EMF of the cell in volts and the cell reaction involves n faradays, ie nF coulombs, the electrical energy produced is nFE volt coulombs or joules

0

0

ln

ln '

( ) ( )ln ln( ) ( )

c dC D

a bA B

nFE G RT Q

G RT K van t Hoffs reaction isotherm

a anFE RT K RTa a

2.303 2.303 [ ] [ ]log log[ ] [ ]

c d

a b

RT RT C DE KnF nF A B

0.0591 0.0591 [ ] [ ]log log[ ] [ ]

c d

a b

C DE Kn n A B

Standard EMF of a cell is the EMF when the activity or concentration of the reactants and products become unity.

• 1. a sensing part of electrode, a bulb made from a specific glass There are two main glass-forming systems:

• silicate matrix based on molecular network of silicon dioxide (SiO2) with additions of other metal oxides, such as Na, K, Li, Al, B, Ca, etc.

• chalcogenide matrix based on molecular network of AsS, AsSe, AsTe.

2. sometimes the electrode contains a small amount of AgCl precipitate inside the glass electrode

3. internal solution, usually 0.1M HCl for pH electrodes or 0.1M MCl for pM electrodes

4. internal electrode, usually silver chloride electrode or calomel electrode

5. body of electrode, made from non-conductive glass or plastics.

6. reference electrode, usually the same type as 47. junction with studied solution, usually made from

ceramics or capillary with asbestos or quartz fiber.

Working: When glass is immersed in a solution the potential developed is a linear function to H+ ion concentration

The potential between HCl/inner glass wall and Ag/AgCl electrode remains constant, and the variable factor is the potential out side wall and H+ ion conc

Corning 15 : SiO2 72%, Na2O 22%, CaO 6% could reach up to PH 9, above which it showed alkaline error

Since the glass also was sensitive towards presense of more sodium, all Na in the glass was replaced by lithium to get better results

SiO2 63%, Li2O 28%, Cs2O 2% BaO 4% and La2O3 3% could reach up to 12.8 PH with less error. For PH indicator glass electrode, the glass should be hygroscopic. H+ + Na+Gl↔ Na+ + HGlAg,AgCl|Hcl(0.1M)|glass|Test Solution|Calomel electrode E = K+RT/F ln(aH+) K is a constant

dependent upon the nature of construction of the glass

Ion Selective Electrodes

An Ion-selective electrode (ISE) (also known as a specific ion electrode (SIE)) is a transducer (sensor) which converts the activity of a specific ion dissolved in a solution into an electrical potential which can be measured by a voltmeter or pH meter. The voltage is theoretically dependent on the logarithm of the ionic activity, according to the Nernst equation . The sensing part of the electrode is usually made as an ion-specific membrane, along with a reference electrode . Ion-selective electrodes are used in biochemical and biophysical research, where measurements of ionic concentration in an aqueous solution are required, usually on a real time basis. They are also known as P ion indicators.

The main aspect is the type of membrane which is usedA. Crystalline membranes: single crystals, LaF3 for F-B. Polycrystalline membranes : Ag2S for S2- and Ag+C. Non Crystalline membranes 1. glass: Silicate Glasses2. Liquid: Liquid ion exchangers3. Immobilized polymer or enzyme

4. Minimal solubility, Stability, Electrical Conductivity, and selective reactivity with analyte

capability of selective binding to analyte ion. Ion exchange, complexation or crystallization. For PH indicator glass electrode, the glass should be hygroscopic. H+ + Na+Gl↔ Na+ + HGlTo serve as an indicator for cations, the glass membrane should conduct electricity

Alkalimetal Ion selectivity can be achieved by various concentrations of Na, Al, and Si oxidesOther Solid membranes: Iodide selective electrode: AgI dispersed In Silicone rubber monomer and then carrying out

polymerization a glass tubes lower end is sealed with the said polymer and KI solution is filled in it with a Ag wire.

LaF3 electrode for Flouride ionIon exchange membranes: Valinomycine, Dialkyl phosphates, bis thiourea derivativesCation Exchangers, neutral Exchangers, Anion ExchangersEnzyme based electrodes: Urea electrode: Urease is employed to effect hydrolysis of urea which

is further analyzed by glass electrode sensitive to NH4+ ionsEnzyme Pencillinasese to destroy pencillin to produce H+ ions then estimated via glass electrodeGas Sensitive electrodes: Gas Dissolved in water and then estimated

Enzymes are immobilized or trapped in polymers, three dimentional graphite or epoxy matrices or co deposition with metals like Pt or Rh