Neutralization & Titration
Drip base into acid until one drop flips the color — and you've counted particles you can't see, to three digits.
Builds on: 4.1 Acids & Bases: The pH Scale2.3 Stoichiometry: Recipe Math
The question titration answers
You have a bottle of acid of unknown strength — how much acid is really in there? You can’t see H⁺ or weigh it. But you can feed it exactly enough OH⁻ to eat every last H⁺, and count what you fed. That is titration: neutralization used as a measuring instrument, and it’s stoichiometry (Unit 2) with glassware.
The procedure
- Put a measured volume of the unknown acid in a flask, add a drop of indicator.
- Fill a burette (a graduated tap-tube) with base of exactly known concentration.
- Drip. Swirl. Watch. At the equivalence point, moles of OH⁻ = moles of H⁺, and the very next drop flips the indicator’s color.
- Read the volume used and compute:
The shape of the curve
Plot pH against base added and you get titration’s signature: a long lazy drift, then a cliff. Far from equivalence, added OH⁻ barely dents the surplus H⁺ (logarithms flatten it). But near equivalence the surplus is nearly gone, and one 0.1 mL drop can hurl the pH from 4 to 10. That cliff is a feature: it’s what makes the endpoint razor-sharp — a single drop of precision.
Phenolphthalein is itself a weak acid whose two forms have different colors: colorless with its H⁺ attached (acidic solution), pink without it (basic). It switches around pH 8.2–10 — right on the cliff of a strong-acid titration, which is why one drop turns the whole flask pink. Your Unit 4 capstone brews a homemade indicator that does the same trick with cabbage pigments.
⚗️ Lab — The Virtual Burette
25 mL of 0.1 M HCl in the flask, 0.1 M NaOH in the burette, phenolphthalein standing by.
- Open the burette slowly and watch the curve crawl… then find the cliff.
- Stop the instant the flask turns pink — how many mL did you use? Does c₁V₁ = c₂V₂ check out?
- Overshoot to 50 mL and see how far past neutral you land.
Work it by hand — 0 / 3
No multiple choice here: compute the value and type it. Suffixes like 2.5k, 20m or 100µ are understood; answers within ±2% count.