Titration Endpoint & Indicator Calculator

Calculate titration endpoint pH and select the appropriate acid-base indicator.

Titration type

pKa of weak acid (if applicable)

Acetic acid=4.76, H2CO3=6.35

pKb of weak base (if applicable)

NH3=4.74

Concentration at equivalence C (M) (M)

Please check your inputs and try again.

Equivalence Point Versus Endpoint

These are not the same thing, and the distinction matters. The equivalence point is where stoichiometrically equal amounts have reacted — a chemical fact. The endpoint is where the indicator changes colour — what you actually observe. The difference between them is titration error.

Good indicator choice makes that error negligible by ensuring the colour change falls inside the steep vertical region of the curve, where a fraction of a drop moves the pH by several units.

Titration	pH at equivalence	Suitable indicator	Range
Strong acid + strong base	7.0	Phenolphthalein or methyl red	Either works
Weak acid + strong base	> 7	Phenolphthalein	8.2–10.0
Weak base + strong acid	< 7	Methyl red	4.4–6.2
Weak acid + weak base	Varies	None reliable	No sharp jump

The last row is important: weak-with-weak titrations have no sharp inflection, so no indicator gives a usable endpoint. Potentiometric titration with a pH meter is the only practical route.

How Indicators Work

An indicator is itself a weak acid whose protonated and deprotonated forms differ in colour. The visible transition spans roughly pK_a ± 1, because the eye needs about a tenfold excess of one form to perceive its colour cleanly.

This means the indicator's pK_a should sit close to the equivalence pH. Using methyl orange for an acetic acid titration — equivalence near 8.7 — would signal an endpoint several millilitres early, since it changes at 3.1–4.4.

Worked Examples

Example 1: Acetic acid + NaOH: pKa=4.76, C=0.05M

pH=7+0.5×(4.76+log(0.05))

Result: pH≈8.72 at equivalence

Use phenolphthalein (range 8.3-10) ✓

Example 2: HCl + NH3: pKb=4.74, C=0.1M

pH<7 at equivalence

Result: Use methyl red (4.4-6.2)

Acidic endpoint due to NH4+ hydrolysis

Example 3: Wrong indicator, wrong answer

Acetic acid titration, equivalence pH 8.7, using methyl orange

Result: Endpoint several mL early

Methyl orange changes at 3.1–4.4, well before equivalence. The result would understate the acid concentration substantially.

Example 4: Why weak-weak fails

Acetic acid titrated with ammonia

Result: No usable endpoint

Both conjugate species buffer the solution near equivalence, flattening the curve. Potentiometric detection is the only option.

Common Mistakes

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Using phenolphthalein for every titration

It changes at 8.2–10.0, which suits weak acid titrations. For a weak base titrated with strong acid, equivalence is below 7 and methyl red is correct.

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Assuming equivalence is always pH 7

Only for strong acid with strong base. Weak acid titrations end above 7 and weak base titrations below it, because the conjugate species hydrolyses.

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Attempting weak acid with weak base titrations

There is no sharp pH jump, so no indicator gives a reliable endpoint. Use a pH meter and locate the inflection point instead.

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Adding too much indicator

Indicators are themselves acids or bases and consume titrant. A few drops is sufficient; more introduces systematic error.

Frequently Asked Questions

Why does indicator choice matter?

Indicator must change color near the equivalence point pH. If it changes too early or late, systematic error results. For strong acid-base, many indicators work (large pH jump). For weak acid/base, choose carefully.

How do indicators work?

Indicators are weak acids (HIn) where HIn and In- have different colors. At pH=pKa(indicator), 50/50 mix shows intermediate color. Color change range: pKa±1. Choose indicator with pKa near equivalence point pH.

What is the difference between equivalence point and endpoint?

Equivalence is where stoichiometrically equal amounts have reacted. Endpoint is where the indicator changes colour. Good indicator choice keeps them close.

How do I choose an indicator?

Pick one whose pK_a is near the equivalence pH, so its transition range falls within the steep part of the curve.

Why can't weak acids be titrated with weak bases?

The pH change near equivalence is too gradual for any indicator to signal reliably. A pH meter detecting the inflection is required.

Why does the indicator range span about two pH units?

Because the eye needs roughly a tenfold excess of one coloured form to see it clearly, which corresponds to one pH unit either side of the indicator's pK_a.

Formula Explorer connections

Interpretation: This relationship connects hydrogen-ion activity, dissociation, conjugate ratios or titration stoichiometry to acid–base behavior. Assumption: Concentration approximates activity mainly in dilute solutions. Temperature, ionic strength, acid strength and equilibrium approximations affect accuracy.

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