Electrochemistry

This is the landing page for the electrochemistry cluster. It covers fundamentals. These fundamentals underlie CV, EIS, LPR, and corrosion. If a term here is brief, a dedicated page covers it in depth. Follow the links.

The electrochemistry cluster is this wiki's first densely connected neighborhood. Every page carries an org-roam `:ID:`. Pages link to every other page sharing a technique or concept. The Corrosion, OCP, LPR, CV, EIS, Reference Electrodes, Potentiostat, SOP, Drying Solvents, and Conical Joints pages form a single graph. Read the hub first, then follow any link.

Why you can't measure a single electrode's potential

A voltmeter measures a potential difference. Stick a lone electrode in solution. The number is undefined until you pick a second electrode. That second electrode is the reference (see Reference Electrodes).

What you want physically is the electron energy at the metal surface. It is the Fermi level of the electrode relative to the electrolyte. You can define this absolutely. Trasatti's convention sets absolute zero at an electron at rest in vacuum outside the electrolyte. It sets the standard hydrogen electrode at about 4.44 ± 0.02 eV (Trasatti, Pure Appl. Chem. 1986, 58, 955-966, doi:10.1351/pac198658070955). Isse and Gennaro (2010) put it at 4.28 eV. The difference between 4.44 and 4.28 is still debated. Both stem from different choices about surface potential. The practical result is that every measured potential has an absolute Fermi-level energy behind it. You do not need to use the absolute scale. Just report everything against the same reference.

The Fermi-level picture explains why changing electrode material or solution shifts the reading. The metal's Fermi level sits at a different energy relative to the solution's redox levels. The potential you read is the voltage drop needed to align them. The instrument reports the two-electrode difference. The Fermi-level picture describes what is physically happening.

The reference scale

All measured potentials are relative. Electrochemists agreed on a common zero: the standard hydrogen electrode (SHE). It uses platinum and H⁺ at unit activity plus H₂ at unit fugacity. IUPAC defines $E^0_{\text{SHE}} = 0.000\ \text{V}$ at all temperatures. It is a convention, not a law.

SHE is hard to build. It needs flowing hydrogen gas. It needs a platinized Pt electrode (prone to poising). It needs careful pressure and activity control. Nobody uses SHE on the bench. Use a secondary reference instead. Its potential versus SHE is tabulated. Report numbers against that reference, or convert:

• Saturated calomel electrode (SCE): +0.241 V vs SHE
• Ag/AgCl (saturated KCl): +0.197 V vs SHE
• Ag/AgCl (3 M KCl): +0.210 V vs SHE
• Cu/CuSO₄ saturated (CSE): +0.316 V vs SHE

"-200 mV vs SCE" and "-41 mV vs SHE" describe the same potential on two different rulers. State which reference you used. A bare number without reference is nearly meaningless. See Reference Electrodes for the full table, construction, maintenance, and junction-potential physics.

The Nernst equation, stated

The Nernst equation converts a standard potential into the actual potential at given concentrations (activities):

$$ E = E^0 - \frac{RT}{nF} \ln Q $$

At 25°C:

$$ E = E^0 - \frac{0.0592}{n} \log_{10} Q $$

The 0.0592 (sometimes 0.059) comes from $2.303RT/F$ at 298.15 K. It is not a universal constant. It is specific to room temperature. Do not carry it beyond room temperature without re-deriving it. The full derivation from $\Delta G = -nFE$ is in the Corrosion notes. This page just needs you to recognize the equation. It explains why $E_{OC}$ moves with concentration, pH, or aeration (see OCP).

Potential windows

Every solvent and electrolyte combination has a potential range where it is silent. The solvent is neither oxidized nor reduced. All current comes from your analyte. Beyond this range, the solvent breaks down. Water electrolyzes into O₂ and H₂ (thermodynamic threshold 1.23 V). Practical limits depend on electrode material and overpotential. Acetonitrile reduces irreversibly past roughly -2.5 V vs SCE (on glassy carbon). THF and DME reduce earlier, especially with trace water.

The usable window depends on the system, not the solvent alone. It depends on:

• The electrode material (glassy carbon and Pt give different windows)
• The supporting electrolyte (the salt's own reduction limit, e.g. Li⁺ or tetraalkylammonium cations)
• The dryness of the solvent (see Drying Solvents). Water contamination narrows the cathodic window first.

Quick reference (approximate, mV/s, room temperature, dry solvent, fresh electrolyte):

Know your system's window before trying CV or LPR. Elgrishi et al. recommend a blank scan in supporting electrolyte alone before adding analyte (see CV). If your analyte's features are within a few hundred mV of the window edge, switch electrode or electrolyte.

The three-electrode cell

Most modern electrochemical measurements (CV, EIS, LPR, potentiodynamic polarization) use a three-electrode cell. A two-electrode setup forces current through the reference. It polarizes the reference. Then the reference loses its stable, known potential.

• Working electrode (WE) — the material you care about.
• Reference electrode (RE) — carries nearly zero current. Reports potential. (See Reference Electrodes.)
• Counter electrode (CE) — closes the current loop with the WE.

The Potentiostat holds the WE at the commanded potential relative to the RE. It sources or sinks current through the CE. Instrumentation details (iR compensation, compliance voltage) are on the potentiostat page.

OCP: the resting state and the gate before you do anything else

Before any perturbation (CV, EIS, LPR), let the cell sit. Watch the open circuit potential (OCP). It is the potential at which the WE net current is zero. Anodic and cathodic partial currents balance. It is a mixed potential from surface electrochemistry (Wagner-Traud 1938, see Corrosion page). It is not imposed by the instrument.

A drifting OCP means the system is not at steady state. Running a perturbation on a moving target measures drift, not material properties. Wait for OCP to stabilize (it can take minutes or days; see OCP). This is the cheapest quality gate in the workflow.

Where to go next

• Corrosion — full Nernst derivation, mixed-potential theory, Tafel kinetics, Evans and Pourbaix diagrams.
• OCP — what shifts the resting potential. How it is a stability gate.
• LPR — turning a small perturbation around OCP into a corrosion rate via Stern-Geary.
• Cyclic voltammetry — sweeping potential through a wide window for redox behavior.
• EIS — small-signal AC, separating charge-transfer, double-layer, and diffusion.
• Reference Electrodes — construction, conversion table, junction potentials.
• Potentiostat — instrument control, iR compensation, compliance voltage.
• Standard Operating Procedure — polishing, cell assembly, sparging.

Applied examples in the broader wiki

• The alkaline electrolyzer project is the closest engineering application. KOH compatibility, pump material selection, and conductivity monitoring.
• The DIY Fumehood and Laminar Flow Cabinet cover ventilation for electrolyte handling.

External resources worth bookmarking

• BYU PyRO ─ Electrochemistry Lessons: structured lessons with CV simulator.
• University of Oregon ─ Center for Electrochemistry: graduate-level lectures.
• Chemistry Stack Exchange ─ electrochemistry: community Q&A. Search before asking.