Electrochemical Methods

Electrochemical Methods

• Overview of techniques used in electrochemistry.

• Two primary methods: Potentiometry and Coulometry.

Electrochemistry (cont’d)

Ion-Selective Electrodes (ISE)

• Designed to be sensitive toward individual ions.

• Types of electrodes:

  • pH Electrodes

  • Indicator electrode

  • Reference electrode

• Importance of liquid junctions

• Readout meter for measurements

• Nernst equation for calculating electrode potentials

• Calibration methods to ensure accuracy

• pH combination electrode features both reference and indicator elements.

Electrochemistry (cont’d)

Gas-Sensing Electrodes

• Purpose: Detect specific gases in solutions.

• Features: Separated from solution by a thin, gas-permeable membrane.

Enzyme Electrodes

• Serve as ISE covered by immobilized enzymes to catalyze specific chemical reactions.

Coulometric Chloridometers

• Chloride ISEs have largely replaced traditional coulometric titrations.

Potentiometry

• Defined as the measurement of potential voltage between two electrodes in a solution.

• Electrical potentials arise from the interface between a metal and its ions in the solution.

• Requires a constant-voltage source as the reference potential.

• Two types of electrodes:

  • Reference electrode (constant voltage)

  • Measuring electrode (indicator electrode)

Potentiometry

• Ion concentration can be deduced by measuring the potential difference between two electrodes.

• Nernst Equation relates cell potential to molar concentration, facilitating predictions of electrochemical cell potential based on concentrations.

Reference Electrodes

Calomel

• Comprised of mercury/mercury chloride in saturated potassium chloride within an inner tube.

Silver/Silver Chloride Electrode

• Consists of a silver electrode immersed in potassium chloride solution saturated with silver chloride.

• Advantage: Usable at temperatures exceeding 60 degrees Celsius.

Ion Selective Electrodes

• Function as membrane-based electrochemical transducers responding to specific ions.

• Potential difference generated by ion transfer from the solution to the membrane phase.

• Membranes made from:

  • Plasticizers

  • Organic solvents

  • Inert polymers

  • Ionophores

Ionophores

• Molecules that enhance the membrane's permeability to specific ions.

Advantages of ISE’s over Wet Chemistry

• Key benefits include:

  • No reagent preparation

  • No standard curve preparation

  • Direct measurement capabilities

  • Cost-effectiveness

  • Rapid response

  • Precision and sensitivity

  • Ease of maintenance

  • Adaptability to automation

pH Electrode

• Glass electrodes are common for measuring hydrogen ion activity.

• Contains:

  • Chloride ion buffer with known hydrogen concentration

  • Internal silver/silver chloride electrode as reference.

Theory Behind pH Electrode

• Sodium ions drift out, replaced by hydrogen ions leading to an increase in external membrane potential difference.

• Chloride ions respond by migrating to the internal glass layer.

• Potential difference corresponds to pH, displayed numerically.

PCO2 Electrode

• pH electrode within a plastic jacket filled with sodium bicarbonate.

• Gas permeable membrane allows CO2 to pass from blood to buffer, leading to a chemical reaction affecting pH.

Reactions in PCO2 Electrode

• Reaction: CO2 + H2O ↔ H2CO3 ↔ HCO3- + H+

Amperometry

• Defined as the measurement of current flow produced by oxidation-reduction reactions.

• Used for PO2 electrode and applicable to chloride detection.

Amperometry and Chloride

• Chloride titrator employs a pair of silver electrodes.

• When chloride is depleted, excess silver ions increase current.

PO2 Gas-Sensing Electrode

• Clark PO2 electrode features a gas-permeable polypropylene membrane, permitting only dissolved oxygen.

• Oxygen interacts with phosphate buffer and reacts at a polarized platinum cathode.

• Electron flow produced is proportional to oxygen concentration in the sampled solution.