# Redox Titration Curve Calculator - Equivalence Potential, Jump Range, Indicators & Argentometry

> Free redox titration curve calculator: equivalence potential, jump range and indicator choice for Ce⁴⁺, KMnO₄, dichromate and iodine, plus pAg curves.

- URL: https://qilingtools.com/en/chemistry/redox-titration
- Category: Chemistry Tools (https://qilingtools.com/en/chemistry)
- Tags: [Chemistry](https://qilingtools.com/en/tags/chemistry)
- Last updated: 2026-10-29
- Price: Free, no sign-up
- Privacy: Runs locally in the browser; files are never uploaded

In a redox titration the potential before equivalence is set by the Nernst equation of the analyte couple, and after equivalence by the titrant couple. For symmetrical couples the equivalence potential is E_eq = (n₁E₁°′ + n₂E₂°′)/(n₁ + n₂), and the jump from −0.1% to +0.1% runs from E₂°′ + 3×0.059/n₂ to E₁°′ − 3×0.059/n₁. For Ce⁴⁺ titrating Fe²⁺ in 1 M H₂SO₄ (formal potentials 1.44 and 0.68 V), E_eq = 1.06 V and the jump is 0.86–1.26 V, so ferroin (1.06 V) is an ideal indicator. For Fe³⁺ titrating Sn²⁺ in 1 M HCl, E_eq = (0.68 + 2×0.14)/3 = 0.32 V with a 0.23–0.50 V jump. Using standard potentials at [H⁺] = 1, permanganate titrating Fe²⁺ gives E_eq = (5×1.507 + 0.771)/6 ≈ 1.38 V. This calculator solves the mass balances and the electron balance at every point instead of using region formulas. That makes it exact for unsymmetrical couples such as Cr₂O₇²⁻/Cr³⁺, I₂/I⁻ and S₄O₆²⁻/S₂O₃²⁻, and for couples that involve H⁺. It also lists six redox indicators with formal potentials and transition ranges and flags those inside the jump. Precipitation mode plots pAg against volume for AgNO₃ titrating Cl⁻, Br⁻, I⁻ and SCN⁻, using CRC Ksp values. For 0.1000 M AgNO₃ titrating 0.1000 M Cl⁻, pAg at equivalence is ½pKsp = 4.88. Halide mixtures precipitate in order of increasing Ksp, so the curve shows two or three breaks. The Mohr titration error is calculated from the chromate concentration, and the Mohr, Volhard and Fajans methods are compared.

## How to use

1. Choose Redox titration or Precipitation (argentometry), or click an example (Ce⁴⁺–Fe²⁺, permanganate, dichromate, iodometry, Fe³⁺–Sn²⁺, halide mixtures)
2. Pick the oxidant and reductant couples, standard or formal potentials (or custom values), which one is the titrant, and the concentrations and volume; for argentometry tick the halides and enter their concentrations
3. Read the balanced reaction, equivalence volume and potential, jump range, suitable indicators, the interactive curve and the working; export the curve as CSV

## Features

- E vs % titrated curve solved exactly from mass and electron balances
- Common couples: Ce⁴⁺, MnO₄⁻, Cr₂O₇²⁻, I₂, Fe³⁺ with Fe²⁺, Sn²⁺, S₂O₃²⁻, with the balanced titration reaction
- Standard potentials with [H⁺] correction, or formal potentials in several media, or custom values
- Oxidant or reductant as titrant (e.g. thiosulfate titrating iodine)
- Six redox indicators with formal potentials and transition ranges; self-indicating permanganate and starch notes
- Argentometric pAg and pX curves with stepwise precipitation of halide mixtures
- Mohr method titration error and a comparison of the Mohr, Volhard and Fajans methods
- Reaction log K′ as a check that the reaction goes to completion

## FAQ

### How do you calculate the equivalence point potential of a redox titration?

For two symmetrical couples (the oxidised and reduced forms have the same coefficient, as in Ce⁴⁺/Ce³⁺ and Fe³⁺/Fe²⁺), E_eq = (n₁E₁°′ + n₂E₂°′)/(n₁ + n₂), which does not depend on concentration. Ce⁴⁺ titrating Fe²⁺ in 1 M H₂SO₄ gives (1.44 + 0.68)/2 = 1.06 V. Fe³⁺ titrating Sn²⁺ in 1 M HCl gives (0.68 + 2×0.14)/3 = 0.32 V. When H⁺ takes part (MnO₄⁻, Cr₂O₇²⁻), E_eq also depends on pH. For unsymmetrical couples such as Cr₂O₇²⁻/Cr³⁺ it also depends on concentration. This tool solves the balances numerically, so it gives the exact value.

### What determines the size of the potential jump?

The jump is the change in potential between 99.9% and 100.1% titrated. For symmetrical couples it runs from E₂°′ + 3×0.059/n₂ to E₁°′ − 3×0.059/n₁. So it depends on the difference between the formal potentials and is usually independent of concentration. A difference above about 0.4 V allows a redox indicator to be used. The medium can shift the jump. Adding H₃PO₄ in a dichromate titration of iron complexes Fe³⁺ and lowers the Fe³⁺/Fe²⁺ formal potential, which moves the start of the jump down so diphenylamine sulfonate (0.84 V) falls inside it.

### How do I choose a redox indicator?

The indicator's formal potential E°′In should lie inside the jump and as close as possible to the equivalence potential. Its colour changes over roughly E°′In ± 0.059/n. Common choices are sodium diphenylamine sulfonate (0.84 V), N-phenylanthranilic acid (0.89 V) and ferroin (1.06 V). Permanganate is self-indicating because its own purple colour marks the end point. Iodine titrations use starch, which forms a deep blue complex with I₂.

### What is pAg at the equivalence point, and how do the Mohr, Volhard and Fajans methods differ?

When AgNO₃ titrates a single halide X⁻, [Ag⁺] = [X⁻] = √Ksp at equivalence, so pAg = ½pKsp. For AgCl (Ksp = 1.77×10⁻¹⁰) that is 4.88. The Mohr method titrates Cl⁻ or Br⁻ directly with chromate as indicator: red-brown Ag₂CrO₄ forms at pH 6.5–10.5. The Volhard method works in nitric acid. Excess Ag⁺ is back-titrated with thiocyanate, and iron(III) alum gives red FeSCN²⁺ at the end point. The Fajans method uses adsorption indicators such as fluorescein or eosin, which change colour on the surface of the AgX colloid at the equivalence point.

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