Redox Indicator Color-Change Endpoint

Titrant Added

Volume V 10.0 mL

Redox Couples (E°, 1 e⁻)

Analyte E°₁ (reductant titrated) 0.77 V

Titrant E°₂ (oxidant added) 1.44 V

Redox Indicator

reduced form oxidized form

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Electrode potential E

0.00 V

Indicator oxidized

0 %

True equivalence V_eq

25.0 mL

Predicted color-change V

— mL

Endpoint error (indicator − true)

— mL

How it works

During a redox titration the solution's electrode potential E climbs along a Nernstian S-curve as oxidant is added, jumping sharply near the equivalence volume V_eq. For 1-electron couples:

V < Veq:  E = E1° + 0.05916·log10(V / (Veq−V))
V > Veq:  E = E2° + 0.05916·log10((V−Veq) / Veq)
V = Veq:  E = (E1° + E2°) / 2

A redox indicator is itself a reversible couple, In_ox + e⁻ ⇌ In_red, with its own standard potential E°_ind and its own Nernst equation:

E = E°ind + 0.05916·log10([Inox]/[Inred])
fraction oxidized = 1 / (1 + 10^((E°ind − E)/0.05916))

The indicator visibly changes color when this fraction crosses ~50%, i.e. when the solution potential E passes through E°_ind. That happens at whatever titrant volume makes E = E°_ind on the curve above — not necessarily at V_eq. The steeper the potential jump and the closer E°_ind sits to (E1°+E2°)/2, the smaller the resulting titration error. This is exactly why real analytical methods pair each redox couple with a specific indicator (ferroin for Ce⁴⁺ titrations, diphenylamine for dichromate) rather than using one indicator for everything.

Volume slider / Auto-titrate — adds titrant; the beaker's dissolved indicator molecules flip color individually as the local potential crosses their personal threshold, so the whole liquid changes color over a narrow volume window rather than instantly.
E°₁ / E°₂ sliders — move the two half-reaction potentials, changing how sharp the equivalence-point jump is and where it sits.
Indicator dropdown — swap indicators to see how a poorly matched E°_ind produces a large color-change error, even though the chemistry at the true equivalence point never changed.

Titrant stream Indicator molecules

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