Methods & conventions

Constants and units

The core uses defining SI constants for h, c, e, N_A and k_B; R = N_Ak_B and F = N_Ae are calculated from them. Hartree energy is the CODATA 2022 measured value, not an exact defining constant.

The thermochemical calorie is 4.184 J and the watt-hour is 3600 J. Wavelengths are vacuum wavelengths. Energy conversion preserves signs; photon inputs must be positive.

Reference-potential dataset

The following limited presets reproduce the conventions of Table 2 in the cited ACS Omega paper. They are nominal values at 25 °C, not a specification for every commercial electrode. No unknown junction-potential correction is applied.

Saturated Ag/AgCl is also commonly tabulated near +0.197 V. Choose Custom for that documented convention or, preferably, for a measured calibration. The tool does not silently blend tabulations. Hg/HgO and other solution-dependent references are handled with custom offsets.

For RHE, the tool uses E_{ref vs SHE} = −ln(10)RT·pH/F at unit H₂ fugacity. SHE is the defined zero; NHE is not introduced as a second numerically exact standard. Outside 25 °C, nominal nonhydrogen offsets are blocked rather than extrapolated.

Equilibrium and direction

Use reduction-form equations throughout. For the general Nernst mode, log₁₀Q = log₁₀Q_{without H⁺} + m·pH, where m is net H⁺ consumed. If an input formal potential already includes pH, do not include that pH contribution a second time. The oxygen/water E° = 1.229 V preset is limited to 25 °C. Other temperatures require a supplied E° or Eeq.

Activity and fugacity ratios must be dimensionless. Gas fugacity is normalized to 1 bar, and positive current denotes oxidation. The residual ohmic correction is E_interface = E_measured − I R_u,remaining. The signed overpotential is η = E_interface − Eeq. The tool also reports |η| and checks the intended reaction direction.

What these tools do not infer

No activity coefficients, ionic-strength effects, liquid-junction potentials, interfacial pH, mass-transfer corrections, electrode ageing or solvent-transfer free energies are estimated. Optical input is monochromatic incident radiant power, not absorbed power. No quantum yield, catalytic rate or two-electrode cell decomposition is inferred.

Reproducibility and privacy

Calculation code and constants are bundled with the site and run in the browser. Inputs are not sent to a calculation server. Appearance preferences may be saved locally. Copying a calculation link explicitly puts the entered fields in the URL fragment; anyone receiving that link can read those values. A JSON export records inputs, output values, warnings and method version.

Invalid fields clear the previous result instead of leaving stale numbers visible. For experimental use, retain the calibration record and conditions with the export, and round results according to measurement uncertainty—not the number of digits on screen.

Validation scope

The accompanying release package contains executable numerical and browser tests plus their actual results. Local checks are not a peer-review statement, an accessibility certification or a test on every browser and physical instrument. Confirm the setup-specific conventions before using a result in a publication.

Sources and conventions

1. NIST SP 811 · Thermochemical calorie and energy conversion factors
2. NIST · CODATA 2022 fundamental constants
3. IUPAC Gold Book · Nernst equation
4. IUPAC Gold Book · Equilibrium electrode potential
5. IUPAC Gold Book · Overpotential
6. ACS Omega (2021) · Reference-potential conventions, Table 2
7. Gamry Instruments · Reference electrodes and nonaqueous calibration
8. Gamry Instruments · Understanding iR compensation
9. Tian & Anderson, J. Phys. Chem. C (2011) · O₂/H₂O standard potential

Method version 1.0.0 · 19 September 2026. Display precision is not measurement accuracy. Read the full scope and validation limits.