Overview
Electrochemistry is the study of the interchange between chemical energy and electrical energy. Many important biochemical reactions are electrochemical in nature, involving the transfer of electrons between atoms or molecules.
This lesson covers oxidation-reduction fundamentals, electrochemical cells, electrode potentials, the Nernst equation, and the clinically important electrochemical devices used in the laboratory — ion selective electrodes, pH electrodes, the Clark pO2 electrode, biosensors, and electrochemical HPLC detectors.
Learning Objectives
After this lesson you will be able to…- Describe the basic concepts of oxidation and reduction reactions.
- Explain electrochemical cells and how to represent them in cell notation.
- Describe electrochemical (electrode) potentials and the Nernst equation.
- Explain the principle of ion selective electrodes, pH electrodes, and the Clark pO2 electrode.
- Describe the application of electrochemistry through biosensors and electrochemical detectors.
Clinical Story
Why This MattersA diabetic patient uses a home glucose monitor before dinner. Behind that simple finger-prick reading lies an electrochemical biosensor: glucose oxidase reacts with blood glucose, and the resulting electron transfer generates a measurable current proportional to glucose concentration — a direct clinical application of the electrochemistry principles in this lesson.
Core Concepts
Oxidation is the loss of electrons (charge becomes more positive), e.g. Fe²⁺ → Fe³⁺ + e⁻. Reduction is the gain of electrons, e.g. Zn²⁺ + 2e⁻ → Zn. A redox reaction combines both simultaneously — electrons lost in oxidation are gained in reduction. Direct redox reactions occur in the same vessel (chemical energy → heat); indirect redox reactions occur in separate vessels connected by a circuit (chemical energy → electrical energy) — the basis of electrochemical cells.
An electrochemical cell converts chemical energy to electrical energy via an indirect redox reaction. Galvanic cells produce power spontaneously; electrolytic cells consume power to drive a non-spontaneous reaction; reversible cells carry no current. Oxidation occurs at the negatively charged anode; reduction occurs at the positively charged cathode. Electrons flow from anode to cathode, while conventional current flows in the opposite direction. A salt bridge maintains electrical neutrality and completes the circuit. Cell notation places the anode on the left and cathode on the right, e.g. Cu(s)/Cu⁺//Zn²⁺/Zn(s).
Electrode potential is the potential difference between an electrode and its electrolyte, reflecting its tendency to gain or lose electrons. The Standard Hydrogen Electrode (SHE) is the universal reference, assigned zero potential. Cell potential E_cell = E_cathode − E_anode. Under non-standard conditions, the Nernst equation adjusts for temperature and concentration: E = E° − (RT/nF)lnQ. Electrode potential increases with increasing electrolyte concentration and decreases with decreasing temperature.
An Ion Selective Electrode (ISE) uses a membrane that selectively transports one ion, generating a potential difference proportional to ion concentration. Types include glass membrane (H⁺), solid-state (F⁻), liquid-based (Ca²⁺), and compound (CO2) electrodes. ISEs offer linear response, are non-destructive and non-contaminating, and are unaffected by colour or turbidity, but have limited precision (~1%) and can be fouled by proteins. The pH electrode is a glass membrane ISE paired with a reference electrode (commonly silver/silver chloride), with the potential converted to pH via the Nernst equation.
The Clark (pO2) electrode measures oxygen tension amperometrically — a current proportional to pO2 is generated as oxygen diffuses through a semi-permeable membrane and is reduced at the cathode. Biosensors use a biocatalyst (enzyme, cell, or tissue) coupled to a transducer to convert a biochemical signal into an electrical one, classified as first, second, or third generation based on the intimacy between biocatalyst and transducer; glucose sensors are the classic clinical example. Electrochemical detectors (ECD) coupled to HPLC measure current generated by oxidation/reduction of eluting compounds, offering extremely sensitive detection (down to 50 pmol/L) for catecholamines, neurotransmitters, and drugs.
Laboratory Principle
Electrochemical measurement devices exploit the fact that ion transport across a selective membrane, or an oxidation/reduction reaction at an electrode surface, generates a measurable potential difference or current. This signal is directly proportional (per the Nernst equation, or amperometric current laws) to the concentration or partial pressure of the analyte — allowing quantification of ions (Na⁺, K⁺, Ca²⁺, H⁺), gases (pO2), and biomolecules (glucose, urea, glutamate) via biosensors.
Equipment Required
Reagents & Materials
| Reagent / Material | Concentration / Grade | Purpose | Storage |
|---|---|---|---|
| Potassium chloride (KCl) electrolyte | 0.1 M | Clark pO2 electrode electrolyte | Room temperature |
| Silver/Silver chloride (Ag/AgCl) | Reference electrode coating | Reference electrode system for pH and pO2 electrodes | Room temperature, dry |
| Buffer solutions (pH 4, 7, 10) | Certified calibration standards | pH electrode calibration | Room temperature, sealed |
| Glucose oxidase | Enzyme reagent (biosensor strip) | Biocatalyst for glucose biosensors | 2–8°C, protect from moisture |
| Agar-agar / gelatine salt bridge | Inert electrolyte gel | Maintains electrical neutrality between half cells | Room temperature |
Step-by-Step Procedure
Calibrate the pH or ion selective electrode using certified buffer solutions (e.g. pH 4, 7, 10) before sample measurement.
Place the combination electrode (measuring + reference) into the patient sample, ensuring the porous junction is fully submerged.
Wait for the potential reading to stabilize as ion diffusion across the selective membrane reaches equilibrium.
Record the measured potential (for ISE/pH) or current (for amperometric pO2/biosensor devices).
Apply the Nernst equation or the instrument's calibration curve to convert the raw signal into the reported analyte concentration or partial pressure.
Flow Diagram
Quality Control
Calibrate pH and ISE electrodes daily with certified buffers/standards. Run normal and abnormal aqueous controls on blood gas analyzers to verify pO2 electrode accuracy. Replace reference electrolyte and check membrane integrity regularly, as fouling by proteins reduces precision.
Participate in external proficiency testing for blood gas and electrolyte analyzers to confirm inter-laboratory accuracy of electrochemical measurements.
Reference Values
Key Electrochemical Constants⚠️ These are technical/physical constants, not patient reference ranges.
Clinical Interpretation
| Finding | Possible Significance | Action / Follow-up |
|---|---|---|
| Drifting or unstable ISE reading | Membrane fouling by proteins or interfering ions | Clean or replace electrode membrane; recalibrate |
| Low pO2 electrode current | Reduced ambient oxygen tension, or a failing membrane | Check membrane integrity; correlate with clinical oxygenation status |
| Glucose biosensor reading inconsistent with lab value | Strip degradation, interfering substances, or haematocrit effect | Repeat with fresh strip; confirm with laboratory glucose method |
Common Errors & How to Avoid Them
Cause: Proteins and organic solutes coat the selective membrane, reducing sensitivity and precision.
Prevention: Clean electrodes per manufacturer protocol and replace membranes on the recommended schedule.
Cause: Trapped air prevents proper oxygen diffusion, giving falsely low pO2 readings.
Prevention: Ensure the membrane is bubble-free and properly seated during electrode maintenance.
Cause: Skipping daily calibration with standard buffers leads to systematic pH measurement error.
Prevention: Calibrate with at least two-point (ideally three-point) buffer standards before each use session.
Laboratory Tips from the Bench
Combination electrodes are easier to maintain than separate glass and reference electrodes, but if the two parts have very different life expectancies, separate electrodes may be more cost-effective long-term.
Micro-potentiometric sensors can measure ions in volumes as small as 10 µL, making them ideal for 96-well microtiter plate assays or paediatric samples.
Remember "AN OX, RED CAT" — oxidation occurs at the ANode, reduction occurs at the CAThode.
Important Notes
Due to environmental and safety concerns, mercury/calomel reference electrodes have largely been replaced by silver/silver chloride systems in modern laboratories.
First-generation biosensors allow the biocatalyst and transducer to function separately; second-generation sensors require them to work together; third-generation sensors use integrated biochip technology for even greater sensitivity.
Interactive Quiz
Test Your KnowledgeFlashcards
Tap to flipClick or tap any card to reveal the answer. Use arrow keys to navigate in single-card mode.
Clinical Case Study
Apply Your KnowledgePresents for routine diabetes follow-up. His home glucose biosensor readings have been inconsistently high compared to how he feels, prompting a laboratory confirmation.
The significant discrepancy between the home glucose biosensor and the laboratory enzymatic method, combined with expired test strips, indicates degraded glucose oxidase activity on the biosensor strip causing falsely elevated readings.
- →Biosensor accuracy depends on enzyme activity, which degrades with expiry or improper storage.
- →Significant discrepancies between point-of-care and laboratory methods should always be investigated.
- →Patient education on checking expiry dates is important for reliable home monitoring.
Frequently Asked Questions
Electrode potential is always a relative measurement between two electrodes; a single electrode's absolute potential cannot be measured in isolation, which is why the SHE is used as a universal zero-point reference.
ISEs are non-destructive, non-contaminating, have a short response time, and are unaffected by sample colour or turbidity, making them ideal for rapid electrolyte measurement in blood gas and point-of-care analyzers.
First-generation biosensors allow the biocatalyst and transducer to function independently; second-generation sensors require both to work together; third-generation sensors integrate them into a single biochip for greater sensitivity and miniaturization.
Quick Revision
10-Minute ReviewKey Takeaways
- Oxidation is loss of electrons; reduction is gain of electrons; redox reactions combine both.
- Electrochemical cells convert chemical energy into electrical energy via indirect redox reactions.
- The Standard Hydrogen Electrode is the universal zero-potential reference for all electrode potential measurements.
- The Nernst equation adjusts electrode potential for non-standard temperature and concentration conditions.
- Ion selective electrodes, pH electrodes, and the Clark pO2 electrode are core clinical laboratory electrochemical tools.
- Biosensors and electrochemical HPLC detectors extend electrochemistry principles to highly sensitive clinical and research applications.
Competency Checklist
Track Your MasteryReferences
- NIOS Biochemistry Course Material, Lesson 20: Electrochemistry.
- Tietz Textbook of Clinical Chemistry and Molecular Diagnostics, latest edition.
- Harper's Illustrated Biochemistry, latest edition.