🎓 Prepared by students from Boğaziçi University

What is Acid-Base Titration?

Titration is a laboratory technique used to find the concentration of an unknown solution by reacting it with a known solution. In acid-base titration, we add a standard acid (or base) dropwise until the equivalence point is reached — detected by an indicator color change.

Short answer

At the equivalence point, moles of acid = moles of base: n(acid) = n(base). Using M₁V₁ = M₂V₂, where M₁ and V₁ are the molarity and volume of one solution, and M₂ and V₂ are for the other.

Titration procedure

1
Setup
Fill burette with standard solution (known concentration).
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2
Measure
Pipette a fixed volume of unknown solution into a flask.
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3
Add indicator
Add phenolphthalein (or methyl orange) to the flask.
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4
Titrate
Add standard solution dropwise, swirl flask continuously.
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5
Equivalence point
Stop when indicator color just changes (sharp color change).
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6
Calculate
Use M₁V₁ = M₂V₂ to find unknown concentration.

Try it: interactive calculator

M₂ (unknown molarity)

0.08mol/L

= (0.1*20)/25

M₁ (known molarity)0.1 mol/LV₁ (volume used to titrate)20 mLV₂ (volume of unknown)25 mL

Step-by-step worked examples

A burette contains 0.2 M HCl. If 25 mL of HCl is used to titrate 30 mL of NaOH, find [NaOH].

HCl + NaOH → NaCl + H₂O (1:1 ratio)
M₁V₁ = M₂V₂
(0.2 M)(25 mL) = M₂(30 mL)
M₂ = 5 / 30 = 0.167 M

0.15 M H₂SO₄ is titrated with 0.3 M NaOH. 20 mL of acid needed. Volume of base used?

H₂SO₄ + 2NaOH → Na₂SO₄ + 2H₂O (1:2 ratio)
(0.15)(20) × 2 = (0.3) × V_NaOH
6 = 0.3 × V_NaOH
V_NaOH = 20 mL

15 mL of unknown HCl is titrated with 0.1 M NaOH (25 mL). Find [HCl].

M₁V₁ = M₂V₂
(0.1 M)(25 mL) = M_HCl(15 mL)
M_HCl = 2.5 / 15 = 0.167 M

Flashcards

Quick quiz

Q1.0.1 M HCl (30 mL) titrates 40 mL NaOH. [NaOH] = ?

Correct answer: A. M₁V₁ = M₂V₂ → (0.1)(30) = M₂(40) → M₂ = 3/40 = 0.075 M.

Q2.At the equivalence point…

Correct answer: C. Equivalence point = moles of acid exactly equals moles of base.

Q3.Which indicator is best for strong acid–strong base titration?

Correct answer: B. Methyl orange (red→yellow) is better for strong acid–strong base. Phenolphthalein works too but has a broader range.

Q4.In a titration, 15 mL of 0.2 M acid neutralizes 10 mL of unknown base. Base molarity?

Correct answer: A. (0.2)(15) = M₂(10) → M₂ = 3/10 = 0.3 M (assuming 1:1 ratio).

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Common mistakes

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Forgetting the stoichiometry (1:1 vs 1:2 ratio). — Correct: H₂SO₄ reacts with 2 NaOH (1:2), so multiply by 2 if applicable.

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Confusing equivalence point with endpoint. — Correct: Equivalence = theoretical point (moles equal); endpoint = indicator color change (observable).

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Not swirling the flask, causing poor mixing. — Correct: Constant swirling ensures accurate indicator detection.

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Using the burette initial reading as volume used. — Correct: Volume used = final reading − initial reading.

FAQ

What is the formula for titration?

M₁V₁ = M₂V₂ — the product of molarity and volume is equal for both solutions.

What happens at the equivalence point?

Moles of acid exactly equal moles of base, causing a sharp color change in the indicator.

How do you know titration is complete?

When the indicator changes color (endpoint), which should coincide with the equivalence point if the right indicator is used.

What is back titration?

Adding excess known solution, then titrating the excess with another known solution — used when direct titration isn't practical.