What Is a Titration? Finding Concentration

You're handed a beaker of acid and told to work out how concentrated it is. You can't weigh it — it's already dissolved. You can't see it. What you can do is find out exactly how much base it takes to cancel it out, and work backwards. That's a titration.

The short answer: a titration is a lab technique for finding the unknown concentration of a solution by reacting it with a solution of known concentration, added drop by drop until the reaction is exactly complete. An indicator signals that moment, and the volume used lets you calculate the answer.

What a titration actually is

The setup is always the same three pieces of glassware:

• The burette holds the solution you know — the titrant — and lets you add it in measured drops.
• The conical flask holds the solution you don't know — the analyte — measured out precisely with a pipette.
• A few drops of indicator go in the flask to signal the finish.

You run titrant in slowly, swirling. Near the end you go drop by drop. The instant the indicator changes colour and stays changed, you stop and read the burette.

The logic is simple: at that point you know the exact volume and concentration of the titrant, so you know how many moles of it reacted. The balanced equation tells you how many moles of the unknown that corresponds to. Divide by the volume in the flask and you have its concentration.

Endpoint vs equivalence point

These two get used interchangeably in conversation, but they're not the same thing and exam questions do ask.

• The equivalence point is the theoretical moment when exactly enough titrant has been added to react with all of the analyte — a stoichiometric fact.
• The endpoint is the moment you observe the indicator change colour.

A well-chosen indicator makes the endpoint fall within a drop of the equivalence point. A badly chosen one makes them diverge, and that gap is systematic error. This is exactly why indicator choice matters:

• Strong acid + strong base → equivalence at pH 7; most indicators work.
• Weak acid + strong base → equivalence above pH 7; use phenolphthalein (changes around pH 8.3–10).
• Strong acid + weak base → equivalence below pH 7; use methyl orange (changes around pH 3.1–4.4).

Universal indicator is no use here — its colour drifts gradually instead of flipping.

The calculation

For a reaction where acid and base combine in a 1:1 ratio, the shortcut is:

M₁V₁ = M₂V₂

where M is concentration and V is volume. Rearrange for whichever value is missing.

When the ratio isn't 1:1 — sulfuric acid gives two protons, so it reacts with NaOH in a 1:2 ratio — the shortcut breaks. The reliable method always works:

1. Moles of titrant = concentration × volume (in litres)
2. Moles of analyte = moles of titrant × the mole ratio from the balanced equation
3. Concentration of analyte = moles ÷ volume of analyte (in litres)

Worked examples

1. The straightforward case.
25.0 mL of HCl is titrated with 0.100 mol/L NaOH. The endpoint comes at 20.0 mL. Find the HCl concentration.

HCl + NaOH → NaCl + H₂O, a 1:1 ratio, so M₁V₁ = M₂V₂:
M₁ × 25.0 = 0.100 × 20.0
M₁ = 2.00 / 25.0 = 0.0800 mol/L

2. Where the shortcut fails.
25.0 mL of H₂SO₄ needs 30.0 mL of 0.100 mol/L NaOH. Find the acid concentration.

H₂SO₄ + 2NaOH → Na₂SO₄ + 2H₂O — a 1:2 ratio.
Moles NaOH = 0.100 × 0.0300 = 3.00 × 10⁻³
Moles H₂SO₄ = 3.00 × 10⁻³ ÷ 2 = 1.50 × 10⁻³
Concentration = 1.50 × 10⁻³ ÷ 0.0250 = 0.0600 mol/L

Use M₁V₁ = M₂V₂ blindly here and you'd get 0.120 — exactly double, and exactly wrong.

3. Reading the burette.
Initial reading 1.20 mL, final reading 26.45 mL. Titre = 26.45 − 1.20 = 25.25 mL. The burette starts wherever it starts; only the difference counts.

Getting a reliable result

• Do a rough run first, fast, to find the approximate endpoint. Then do accurate runs, slowing to drops near that volume.
• Repeat until concordant — titres within about 0.10 mL of each other — and average only the concordant ones.
• Rinse the burette with the titrant and the pipette with the solution it will deliver — but rinse the conical flask with distilled water only. Water left in a burette or pipette dilutes the solution, whereas rinsing the flask with the analyte adds extra moles and pushes every titre too high.
• Swirl constantly and wash the flask walls down with distilled water; adding water to the flask doesn't change the moles present, so it costs you nothing.
• Read the bottom of the meniscus at eye level.

Common mistakes to avoid

• Using M₁V₁ = M₂V₂ when the ratio isn't 1:1. Always check the balanced equation first. This is the single most common lost mark in the topic.
• Forgetting to convert mL to litres. Concentration is mol per litre; 25.0 mL is 0.0250 L.
• Averaging every run including the rough one. The rough titre is an overshoot by design — leave it out.

FAQ

What is a titration used for?
To find the unknown concentration of a solution. You react it with a solution of known concentration until the reaction is just complete, then calculate backwards from the volume used.

What is the difference between endpoint and equivalence point?
The equivalence point is when stoichiometrically exact amounts have reacted; the endpoint is when the indicator visibly changes. A good indicator makes them nearly identical.

Which indicator should I use for a titration?
It depends on the pH at the equivalence point — phenolphthalein for a weak acid with a strong base, methyl orange for a strong acid with a weak base. Universal indicator is unsuitable because its change is too gradual.

What is a concordant titre?
Repeat results that agree closely, usually within 0.10 mL. Only concordant titres are averaged for the final calculation.

The takeaway

A titration turns a volume you can measure into a concentration you can't. Add a known solution until the indicator flips, read the burette, convert to moles, apply the mole ratio from the balanced equation, divide by volume. The technique is easy; the marks live in the mole ratio and the unit conversion.

Background → [What Is Neutralization?] (sibling) is the reaction underneath, and [Litmus vs Universal Indicator] (sibling) explains indicator choice. For the maths → What Is the Mole? Avogadro's Number Made Simple and What Is a Chemical Equation? Balancing Made Simple.

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