NEET Chemistry · Equilibrium
Acid-Base Titration
Add titrant drop by drop. Watch the pH curve build and the colour change at the equivalence point. Calculate molarity from your results.
Section 1 of 5 · Foundations
Finding an unknown concentration by neutralisation
Titration is a precise way to find the unknown concentration of an acid (or base) by reacting it with a base (or acid) of known concentration until the reaction is exactly complete — the equivalence point. An indicator changes colour near this point, giving a visual signal, and the volume of titrant used lets you calculate the unknown molarity.
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Titration LabAdd base drop by drop to an acid. Watch the beaker colour change and the pH curve build live, then calculate the unknown molarity.
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IndicatorsSee why phenolphthalein works for some titrations but methyl orange is needed for others — it's about matching the colour-change range to the equivalence pH.
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Curve TypesCompare strong-strong, weak-strong, and strong-weak titration curves side by side — same volume, very different shapes.
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6 NEET trapsEquivalence point ≠ neutral point, half-equivalence = pKa, indicator range selection — the most missed exam concepts.
Acid (analyte)
Base (titrant)
pH curve
Equivalence point
Indicator colour change
Neutralisation Reaction
HA + BOH → BA + H₂O
At the molecular level, H⁺ from the acid combines with OH⁻ from the base to form water. The salt (BA) remains dissolved. The reaction is essentially complete (large K) for strong acid-base pairs.
Equivalence Point
moles of acid = moles of base
The point where the acid and base have reacted in EXACTLY stoichiometric amounts. This is NOT always pH 7 — it depends on what's in solution after the reaction (a strong/weak salt).
Indicator
HIn ⇌ H⁺ + In⁻ (different colours)
A weak acid/base itself, with the protonated and deprotonated forms having different colours. It changes colour over a narrow pH RANGE (not a single point) — must be chosen so this range includes the equivalence pH.
Molarity Calculation
M₁V₁ = M₂V₂ (1:1 reaction)
At equivalence, moles of acid = moles of base. For monoprotic acid + monobasic base: M_acid × V_acid = M_base × V_base. Adjust for stoichiometry if the reaction isn't 1:1.
Why the pH curve has that S-shape
Initial region — slow pH change: Adding base to excess acid changes pH slowly at first, because there's a large excess of H⁺ to neutralise and the relative change is small.
Near equivalence — sharp jump: As acid runs out, even a tiny additional drop of base causes a huge relative change in [H⁺] (or [OH⁻]) since there's almost nothing left to buffer the change — this is the steep vertical part of the curve.
Beyond equivalence — slow again: Once in excess base territory, adding more base again changes pH slowly, since there's now a large excess of OH⁻.
Indicator must change colour within the steep region: Because the jump is so sharp, even an indicator whose true colour-change pH doesn't exactly match the equivalence point will still appear to change "at" the equivalence point, as long as its range falls somewhere on that steep vertical segment.
NEET TIPEquivalence point and neutral point (pH=7) are NOT always the same thing. Strong acid + strong base → equivalence at pH 7. Weak acid + strong base → equivalence ABOVE pH 7 (basic salt solution). Strong acid + weak base → equivalence BELOW pH 7 (acidic salt solution).
Section 1 of 5
Section 2 of 5 · Interactive Lab
Drop-by-Drop Titration Simulator
An acid of unknown concentration sits in the beaker with a few drops of indicator. Add base from the burette drop by drop (or in bulk) and watch the colour shift and the pH curve build point by point. Find the equivalence point, then calculate the acid's molarity.
Live Titration Simulation
Beaker + Live pH Curve
Volume base added: 0.0 mL
Acid Volume (fixed)
25.0 mL
Acid Molarity (unknown — to find!)
? M
Base Molarity (known)
0.10 M
Loading…
Current pH—
StageACIDIC
Volume at equivalence—
How to find molarity from the titration
Find the equivalence point: Watch the pH curve for the steepest vertical jump — that volume of base added is V_eq. The "Reveal" button will show you the exact value once you've explored.
Apply M₁V₁ = M₂V₂: Since both acid and base here are monoprotic/monobasic, moles of acid = moles of base at equivalence: M_acid × V_acid = M_base × V_eq.
Solve for the unknown: M_acid = (M_base × V_eq) / V_acid. Plug in your observed V_eq, the known base molarity, and the fixed acid volume (25.0 mL) to find the unknown acid concentration.
NEET TIPFor weak acid + strong base, the equivalence point pH is ABOVE 7 — the resulting salt is the conjugate base of a weak acid, which itself is basic in solution (it hydrolyses to produce OH⁻). Don't assume titrating to "pH 7" finds the true equivalence point for this combination.
Section 2 of 5
Section 3 of 5 · Choosing an Indicator
Why Indicator Choice Matters
An indicator only works correctly if its colour-change pH range overlaps the steep, near-vertical part of the titration curve. Pick the wrong indicator and you'll see the colour change far from the true equivalence point — or not see a sharp change at all.
Common Indicators & Their Ranges
Titration type to test indicator against
Matching indicator to titration type
Phenolphthalein (range 8.2–10.0, colourless→pink): Best for strong acid-strong base (eq. pH 7, range still falls on the steep part) and weak acid-strong base (eq. pH > 7 — phenolphthalein's range matches perfectly).
Methyl orange (range 3.1–4.4, red→yellow): Best for strong acid-weak base (eq. pH < 7) — its range sits in the acidic region where this titration's equivalence point falls.
Why phenolphthalein fails for strong acid-weak base: The equivalence point is around pH 5.3, but phenolphthalein only starts changing colour at pH 8.2 — by then you've added far too much base, giving a falsely high reading.
NEET TIPFor weak acid vs weak base titrations, NO indicator works well — the pH change near equivalence is too gradual (no sharp vertical jump), so no single indicator can pinpoint it accurately. These titrations are typically followed using a pH meter instead.
Section 3 of 5
Section 4 of 5 · Comparing Curve Shapes
Strong-Strong vs Weak-Strong vs Strong-Weak
The same volume of titrant produces very differently shaped curves depending on whether the acid and base are strong or weak. Compare all three side by side to see how the starting pH, the buffer region, and the equivalence pH all shift.
Three Titration Curves Overlaid
Curves to display
All 3
Strong Acid + Strong Base
Equivalence pH = 7.0
Sharpest, most symmetric S-curve. Starting pH is very low (strong acid), ending pH very high (strong base excess). No buffer region — pH rises fairly steadily except for the sharp equivalence jump.
Weak Acid + Strong Base
Equivalence pH > 7 (basic)
Starting pH is higher than strong acid (weak acid partially ionised). Shows a clear BUFFER region (flattened curve) around half-equivalence where pH = pKa. Equivalence jump is less steep than strong-strong.
Strong Acid + Weak Base
Equivalence pH < 7 (acidic)
Mirror image of weak acid-strong base. Starting pH is very low (strong acid), but the equivalence point sits below 7 because the salt formed (conjugate acid of weak base) is itself acidic.
The buffer region and Henderson-Hasselbalch
At half-equivalence, pH = pKa: When exactly half the weak acid has been converted to its conjugate base, [HA] = [A⁻], so the Henderson-Hasselbalch equation pH = pKa + log([A⁻]/[HA]) simplifies to pH = pKa exactly.
Why the curve flattens here: This is the buffer region — the solution resists pH change because both HA and A⁻ are present in significant amounts, each able to absorb added H⁺ or OH⁻.
Reading pKa off the graph: This gives an easy experimental way to find a weak acid's pKa — just titrate it and read the pH at the half-equivalence volume (half of V_eq).
NEET TIPThe strong-strong curve has NO buffer region because there's no weak conjugate pair present — both species (H⁺ from strong acid, the eventual excess OH⁻) are fully ionised, with nothing to buffer pH changes. Only weak acid/weak base titrations show the characteristic buffer plateau.
Section 4 of 5
Section 5 of 5 · Revision Sheet
Quick Reference — Formulas, Indicators & NEET Traps
Every formula, indicator range, and exam trap for acid-base titration — one page.
Key Formulas
| Quantity | Formula | Notes |
|---|---|---|
| Molarity at equivalence | M₁V₁ = M₂V₂ | For 1:1 monoprotic-monobasic reaction |
| Henderson-Hasselbalch | pH = pKa + log([A⁻]/[HA]) | Valid in the buffer region |
| pH at half-equivalence | pH = pKa | Since [A⁻]=[HA] there |
| pH of strong acid | pH = −log[H⁺] | Complete dissociation assumed |
| pH of weak acid (before titration) | pH = ½(pKa − log C) | C = initial concentration |
| pOH + pH | pOH + pH = 14 (at 25°C) | From Kw = [H⁺][OH⁻] = 10⁻¹⁴ |
Common Indicators
| Indicator | pH Range | Colour Change | Best for |
|---|---|---|---|
| Methyl orange | 3.1 – 4.4 | Red → Yellow | Strong acid + weak base |
| Methyl red | 4.4 – 6.2 | Red → Yellow | Strong acid + weak base |
| Bromothymol blue | 6.0 – 7.6 | Yellow → Blue | Strong acid + strong base |
| Phenolphthalein | 8.2 – 10.0 | Colourless → Pink | Strong/weak acid + strong base |
Equivalence pH by Titration Type
| Titration type | Equivalence pH | Why |
|---|---|---|
| Strong acid + Strong base | = 7 | Salt is neutral (e.g. NaCl) — neither ion hydrolyses |
| Weak acid + Strong base | > 7 | Salt's anion (conjugate base) hydrolyses, producing OH⁻ |
| Strong acid + Weak base | < 7 | Salt's cation (conjugate acid) hydrolyses, producing H⁺ |
| Weak acid + Weak base | ≈ 7 (depends on Ka, Kb) | Both ions hydrolyse — net effect depends on relative strengths |
NEET Traps
TRAP 1Equivalence point ≠ neutral point (pH 7). Only for strong acid + strong base are they the same. For weak-strong combinations, the equivalence pH shifts above or below 7 depending on which species is weak.
TRAP 2At half-equivalence volume, pH = pKa exactly (for weak acid titrations) — this is a favourite NEET numerical question. Don't confuse half-equivalence with half the total expected volume if the stoichiometry isn't 1:1.
TRAP 3Indicators change colour over a RANGE, not at one exact pH. Choosing an indicator means matching its range to the STEEP part of the curve, not to the exact equivalence pH value.
TRAP 4M₁V₁=M₂V₂ only works directly for 1:1 (monoprotic-monobasic) reactions. For diprotic acids (like H₂SO₄) or reactions with different stoichiometric ratios, you must account for the mole ratio explicitly.
TRAP 5Weak acid-weak base titrations have NO sharp equivalence jump — visual indicators don't work reliably; a pH meter is needed. Don't assume every titration curve has a usable steep region.
TRAP 6The buffer region (flat part of weak acid curve) is centred at pH=pKa, not at the equivalence point. Students sometimes look for the buffer plateau at the wrong volume — it's at HALF the equivalence volume, not at the equivalence volume itself.
Section 5 of 5 · Complete!