Oxidation Reduction Potential Calculator
Easily calculate the electrode potential (E) of a half-reaction under non-standard conditions using the Nernst equation with this Oxidation Reduction Potential Calculator.
Calculate ORP
Standard Reduction Potentials & Dynamic Chart
| Half-Reaction | E° (V) at 25°C |
|---|---|
| F₂(g) + 2e⁻ ⇌ 2F⁻ | +2.87 |
| Au³⁺ + 3e⁻ ⇌ Au(s) | +1.50 |
| Cl₂(g) + 2e⁻ ⇌ 2Cl⁻ | +1.36 |
| O₂(g) + 4H⁺ + 4e⁻ ⇌ 2H₂O | +1.23 |
| Br₂(l) + 2e⁻ ⇌ 2Br⁻ | +1.07 |
| Ag⁺ + e⁻ ⇌ Ag(s) | +0.80 |
| Fe³⁺ + e⁻ ⇌ Fe²⁺ | +0.77 |
| I₂(s) + 2e⁻ ⇌ 2I⁻ | +0.54 |
| Cu²⁺ + 2e⁻ ⇌ Cu(s) | +0.34 |
| 2H⁺ + 2e⁻ ⇌ H₂(g) | 0.00 |
| Pb²⁺ + 2e⁻ ⇌ Pb(s) | -0.13 |
| Sn²⁺ + 2e⁻ ⇌ Sn(s) | -0.14 |
| Fe²⁺ + 2e⁻ ⇌ Fe(s) | -0.44 |
| Zn²⁺ + 2e⁻ ⇌ Zn(s) | -0.76 |
| Al³⁺ + 3e⁻ ⇌ Al(s) | -1.66 |
| Li⁺ + e⁻ ⇌ Li(s) | -3.05 |
What is Oxidation Reduction Potential (ORP)?
The Oxidation Reduction Potential (ORP), also known as redox potential or electrode potential (E), is a measure of the tendency of a chemical species to acquire electrons and thereby be reduced or to lose electrons and thereby be oxidized. It is measured in volts (V) or millivolts (mV). A positive ORP indicates that a substance is an oxidizing agent (tends to gain electrons), while a negative ORP indicates it is a reducing agent (tends to lose electrons).
The Oxidation Reduction Potential Calculator is a tool used to determine this potential under non-standard conditions using the Nernst equation.
Who should use it?
Chemists, environmental scientists, biologists, and engineers often use ORP measurements and calculations. It's crucial in fields like water quality monitoring (disinfection potential), corrosion studies, electrochemistry, and understanding metabolic processes.
Common Misconceptions
A common misconception is that ORP directly measures the concentration of specific oxidants or reductants. While related to concentrations via the Nernst equation, ORP is a measure of the overall redox state or electron activity of a system, influenced by all active redox couples present.
Oxidation Reduction Potential Formula and Mathematical Explanation
The potential of an electrode under non-standard conditions (E) is related to its standard electrode potential (E°) and the activities (approximated by concentrations) of the oxidized and reduced species by the Nernst Equation:
For a half-reaction: Ox + ne⁻ ⇌ Red
E = E° – (RT/nF) * ln(Q)
Where:
- E is the electrode potential under non-standard conditions (in Volts).
- E° is the standard electrode potential (in Volts).
- R is the ideal gas constant (8.314 J/(mol·K)).
- T is the absolute temperature (in Kelvin).
- n is the number of moles of electrons transferred in the half-reaction.
- F is the Faraday constant (96485 C/mol).
- ln is the natural logarithm.
- Q is the reaction quotient, which for the given half-reaction is [Red]/[Ox], where [Red] and [Ox] are the molar concentrations of the reduced and oxidized species, respectively (assuming unit stoichiometric coefficients).
At 25°C (298.15 K), the term (RT/F) * ln(10) is approximately 0.0592 V, so the equation can also be written as:
E = E° – (0.0592/n) * log₁₀(Q) (at 25°C)
Our Oxidation Reduction Potential Calculator uses the more general form with ln(Q) and allows temperature input.
Variables Table
| Variable | Meaning | Unit | Typical Range |
|---|---|---|---|
| E° | Standard Electrode Potential | V | -3 to +3 |
| T | Temperature | °C or K | 0-100°C (273-373 K) |
| n | Number of electrons | dimensionless | 1-6 (integer) |
| [Ox] | Concentration of Oxidized Species | M (mol/L) | 10⁻⁶ to 10 |
| [Red] | Concentration of Reduced Species | M (mol/L) | 10⁻⁶ to 10 |
| R | Ideal Gas Constant | J/(mol·K) | 8.314 |
| F | Faraday Constant | C/mol | 96485 |
Practical Examples (Real-World Use Cases)
Example 1: Iron(III)/Iron(II) Couple
Consider the Fe³⁺/Fe²⁺ half-reaction: Fe³⁺ + e⁻ ⇌ Fe²⁺, with E° = +0.77 V.
If [Fe³⁺] = 0.01 M, [Fe²⁺] = 0.001 M, at 25°C (298.15 K), and n = 1:
Q = [Fe²⁺]/[Fe³⁺] = 0.001 / 0.01 = 0.1
E = 0.77 – (8.314 * 298.15 / (1 * 96485)) * ln(0.1) ≈ 0.77 – 0.02569 * (-2.3026) ≈ 0.77 + 0.0592 ≈ 0.829 V
Our Oxidation Reduction Potential Calculator would confirm this result.
Example 2: Copper(II)/Copper Couple
Consider the Cu²⁺/Cu(s) half-reaction: Cu²⁺ + 2e⁻ ⇌ Cu(s), with E° = +0.34 V.
If [Cu²⁺] = 0.005 M, at 30°C (303.15 K), and n = 2 (the activity of solid Cu is 1):
Q = 1 / [Cu²⁺] = 1 / 0.005 = 200
E = 0.34 – (8.314 * 303.15 / (2 * 96485)) * ln(200) ≈ 0.34 – 0.01306 * 5.298 ≈ 0.34 – 0.0692 ≈ 0.271 V
You can use the Oxidation Reduction Potential Calculator to find E by setting [Red]=1 and [Ox]=0.005, T=30, E0=0.34, n=2 (note: this is a simplification as [Red] isn't a concentration here, but for Q=1/[Ox] we can simulate it by setting [Red]=1).
How to Use This Oxidation Reduction Potential Calculator
- Enter Standard Potential (E°): Input the standard electrode potential for your half-reaction in Volts.
- Enter Temperature: Input the temperature in degrees Celsius.
- Enter Electrons Transferred (n): Input the number of electrons in the balanced half-reaction.
- Enter Concentrations: Input the molar concentrations of the oxidized ([Ox]) and reduced ([Red]) species involved in Q = [Red]/[Ox]. If the reaction is Ox + ne- <=> Red. If your Q involves stoichiometric coefficients other than 1, you'll need to adjust the concentrations accordingly before input or pre-calculate Q and work backward.
- Calculate: Click "Calculate" or observe the results update as you type.
- Read Results: The calculator displays the Electrode Potential (E), Reaction Quotient (Q), the RT/nF term, and temperature in Kelvin.
- Reset/Copy: Use "Reset" to go back to default values or "Copy Results" to copy the output.
The Oxidation Reduction Potential Calculator provides instant results, helping you understand how potential changes with conditions.
Key Factors That Affect Oxidation Reduction Potential Results
- Standard Electrode Potential (E°): This intrinsic property of the half-reaction is the baseline potential at standard conditions. Different reactions have different E° values.
- Temperature (T): Temperature affects the (RT/nF) term in the Nernst equation. Higher temperatures generally make the potential more sensitive to concentration changes.
- Number of Electrons (n): The number of electrons transferred influences the magnitude of the concentration-dependent term. Higher 'n' values reduce the impact of ln(Q).
- Concentration of Oxidized Species ([Ox]): Higher [Ox] (denominator in Q for Ox + ne⁻ ⇌ Red) leads to a more positive E.
- Concentration of Reduced Species ([Red]): Higher [Red] (numerator in Q for Ox + ne⁻ ⇌ Red) leads to a more negative E.
- Reaction Quotient (Q): The ratio [Red]/[Ox] directly determines the deviation from E°. When Q=1, E=E°.
- Ionic Strength: Although not directly in the simple Nernst equation, ionic strength affects activity coefficients, which are more accurate than concentrations, especially at higher concentrations. Our calculator uses concentrations, assuming activity coefficients are close to 1.
Using an electrode potential calculator helps visualize these effects.
Frequently Asked Questions (FAQ)
- What is the Nernst equation?
- The Nernst equation relates the reduction potential of an electrochemical reaction (half-cell or full cell reaction) to the standard electrode potential, temperature, and activities (often approximated by concentrations) of the chemical species undergoing reduction and oxidation. Our Oxidation Reduction Potential Calculator is based on this equation.
- What is E°?
- E° is the standard electrode potential, measured under standard conditions (25°C, 1 M concentration for solutions, 1 atm pressure for gases).
- How does temperature affect ORP?
- Temperature is directly proportional to the (RT/nF) term. Increasing temperature increases the magnitude of this term, making the potential more sensitive to changes in the reaction quotient Q.
- What if my reaction involves solids or gases?
- The activity of pure solids and liquids is taken as 1. For gases, activity is approximated by partial pressure in atmospheres. If you have solids, their "concentration" term in Q is 1. For gases, use partial pressure instead of molar concentration in the Q expression.
- What are the units of ORP?
- ORP is measured in Volts (V) or millivolts (mV).
- Can I use this calculator for a full cell?
- This calculator is designed for half-cell potentials. For a full cell, E(cell) = E(cathode) – E(anode), where you'd use the Nernst equation for each half-cell if under non-standard conditions. You can use our Nernst Equation calculator twice.
- What does a high positive ORP mean?
- A high positive ORP indicates a strong tendency for the species to be reduced (gain electrons), meaning it's a strong oxidizing agent.
- What does a negative ORP mean?
- A negative ORP indicates a tendency for the species to be oxidized (lose electrons), meaning it's a reducing agent relative to the standard hydrogen electrode (SHE).
For more details on standard potentials, see our guide on standard electrode potential.