Electrochemistry — Redox and Bioelectricity

Electrochemistry studies the relationship between chemical reactions and electrical energy. At its heart is the concept of electron transfer — oxidation-reduction (redox) reactions. Key definitions: Oxidation: loss of electrons (OIL — Oxidation Is Loss) Reduction: gain of electrons (RIG — Reduction Is Gain) The mnemonic: OIL RIG Oxidation states: Every atom in a compound is assigned an oxidation state (oxidation number) — a hypothetical charge if all bonds were ionic. Rules: - Free elements have oxidation state 0 - Oxygen is usually −2 (except in peroxides where it's −1) - Hydrogen is usually +1 (except in metal hydrides where it's −1) - The sum of oxidation states equals the charge of the species Identifying redox: In a redox reaction, one species is oxidised (oxidation state increases) and another is reduced (oxidation state decreases). The oxidising agent accepts electrons and is itself reduced. The reducing agent donates electrons and is itself oxidised. Examples in biology: Cellular respiration is fundamentally a redox process — glucose (C₆H₁₂O₆) is oxidised to CO₂ while O₂ is reduced to H₂O. The electrons flow through the electron transport chain, generating the proton gradient that drives ATP synthesis. NAD⁺ and FAD are electron carriers — they cycle between oxidised and reduced forms (NAD⁺/NADH, FAD/FADH₂).

An electrochemical cell converts chemical energy to electrical energy (or vice versa) using redox reactions. Galvanic (voltaic) cells — spontaneous reactions generating electricity: Two half-cells connected by a salt bridge. In each half-cell, a metal electrode is immersed in a solution of its ions. The more reactive metal is oxidised (anode, negative) and the less reactive metal is reduced (cathode, positive). Standard electrode potential (E°): Each half-cell has a standard reduction potential (E°) measured relative to the standard hydrogen electrode (E° = 0 V). More positive E° = stronger oxidising agent (more easily reduced). The overall cell voltage: E°cell = E°cathode − E°anode. A positive E°cell means the reaction is spontaneous. The standard hydrogen electrode (SHE): H⁺(aq) + e⁻ → ½H₂(g) E° = 0.00 V (reference point for all electrode potentials) Salt bridge: Maintains electrical neutrality between the two half-cells by allowing ion flow without mixing the solutions. Typically contains KCl or KNO₃ in agar. Electrolytic cells — non-spontaneous reactions driven by electricity: External electrical energy drives a non-spontaneous redox reaction. Examples: electroplating, electrolysis of water, aluminium smelting, electrolysis of brine (NaCl solution) to produce NaOH and Cl₂.

Batteries are practical electrochemical cells: Common battery types: Zinc-carbon (dry cell): zinc anode, carbon cathode, 1.5V. Used in household electronics. Alkaline: similar chemistry to zinc-carbon but in alkaline (KOH) electrolyte, higher capacity. Lithium-ion (Li-ion): lithium cobalt oxide cathode, graphite anode, ~3.7V per cell. Used in smartphones and electric vehicles. Very high energy density, rechargeable. Lead-acid: lead anode, lead dioxide cathode, sulfuric acid electrolyte, ~2V per cell. Used in car batteries. Heavy but reliable and very high current capacity. The Nernst Equation: Under non-standard conditions, the cell potential changes with concentration: E = E° − (RT/nF) ln Q Where R = gas constant, T = temperature (K), n = moles of electrons transferred, F = Faraday's constant (96,485 C/mol), Q = reaction quotient. At 25°C, simplified: E = E° − (0.0592/n) log Q Biological significance: The Nernst equation applies directly to membrane potentials in neurons. The equilibrium potential for any ion can be calculated using a version of the Nernst equation, explaining why Na⁺ flows in and K⁺ flows out during an action potential.

Corrosion: Corrosion is the unintended oxidation of metals. Iron rusting is an electrochemical process — iron is oxidised (Fe → Fe²⁺ + 2e⁻) while oxygen is reduced (O₂ + 4H⁺ + 4e⁻ → 2H₂O). Water and electrolytes (salt) accelerate corrosion by improving conductivity. Prevention methods: Sacrificial anodes: attach a more reactive metal (zinc, magnesium) that oxidises preferentially, protecting the iron. Used on ships, pipelines, and offshore structures. Galvanising: coating iron with zinc. The zinc acts as a sacrificial anode even if the coating is scratched. Electroplating: depositing a protective metal (chromium, gold) onto a surface using electrolysis. Painting, oiling: physical barrier preventing access of water and O₂. Biological redox systems: The electron transport chain (ETC) in mitochondria is a series of redox reactions. Electrons from NADH and FADH₂ are passed through protein complexes (I, II, III, IV) to oxygen. The free energy released pumps H⁺ across the inner mitochondrial membrane, creating the proton gradient that drives ATP synthase. Complex IV (cytochrome c oxidase) is the final electron acceptor, reducing O₂ to H₂O. Reactive oxygen species (ROS): Incomplete reduction of O₂ produces reactive oxygen species (superoxide O₂⁻•, hydrogen peroxide H₂O₂, hydroxyl radical •OH). These damage DNA, proteins, and lipids. Antioxidants (vitamin C, vitamin E, glutathione) act as reducing agents, scavenging ROS by donating electrons harmlessly.

Medical devices rely heavily on electrochemical principles: Blood glucose monitoring: The glucose meter uses an electrochemical biosensor. Glucose oxidase enzyme catalyses oxidation of glucose, transferring electrons to an electrode. The resulting current is proportional to glucose concentration. This is an amperometric (current-measuring) device. Reference range: 4.0–7.8 mmol/L (fasting: 3.9–5.5 mmol/L). Cardiac pacemakers and defibrillators: Battery-powered implantable devices. Pacemakers use lithium-iodine batteries (very reliable, predictable discharge curve, 5–15 year lifespan). ICDs (implantable cardioverter-defibrillators) use lithium-silver vanadium oxide batteries capable of delivering high-energy shocks. pH electrodes: The glass pH electrode is an electrochemical sensor — the potential difference across a thin glass membrane is proportional to pH. Used in blood gas analysers to measure arterial pH (normal 7.35–7.45), PCO₂, and PO₂. Nerve conduction: Action potentials are electrical signals generated by ion flows across membranes — an electrochemical process. Electromyography (EMG) and electroencephalography (EEG) measure the electrical activity of muscles and brain respectively. Iontophoresis: Drug delivery technique using electrical current to drive charged drug molecules across the skin. Used for local anaesthetic delivery and treatment of hyperhidrosis.