Lab, Redox Titration · AP Chemistry

% Hydrogen peroxide in a drugstore disinfectant. You will titrate a commercial H₂O₂ solution against acidified potassium permanganate of known concentration, then use a balanced redox equation to report the peroxide content in both molarity and percent weight-by-weight.

CED: Topic 4.9 (oxidation–reduction, redox titration) · Science Practice 5: analyze data with appropriate calculations · Ties forward: Unit 9 electrochemistry (the same half-reaction bookkeeping).

Purpose & Learning Goal CED 4.9 · SP 5

Determine the concentration of hydrogen peroxide in a commercial solution, reported as molarity and % w/w, by oxidation–reduction titration with standardized 0.0200 M KMnO₄. This is College Board Recommended Experiment 8, Determination of concentration by oxidation-reduction titration (FRQ history: 1993, 2000, 2003B).

Background read before lab

Permanganate (MnO₄⁻) is a strong oxidizing agent: in acid, manganese is reduced from +7 to +2, going from deep purple MnO₄⁻ to nearly colourless Mn²⁺. Hydrogen peroxide acts here as the reducing agent, its oxygen is oxidized from −1 to 0 as it leaves as O₂ gas.

Half-reactions (acidic):

MnO₄⁻ + 8 H⁺ + 5 e⁻ → Mn²⁺ + 4 H₂O (reduction)

H₂O₂ → O₂ + 2 H⁺ + 2 e⁻ (oxidation)

Balanced overall (electrons cross-multiplied, LCM = 10):

2 MnO₄⁻ + 5 H₂O₂ + 6 H⁺ → 2 Mn²⁺ + 5 O₂ + 8 H₂O

Key mole ratio: 5 mol H₂O₂ react with 2 mol MnO₄⁻ → n(H₂O₂) = (5/2) × n(MnO₄⁻).

Self-indicating endpoint: permanganate is its own indicator. While H₂O₂ remains, each drop of purple MnO₄⁻ is decolourized instantly. The first drop with no peroxide left to react leaves a faint, persistent pink: that is the endpoint.

Safety PPE required entire lab

Splash goggles and an apron/lab coat are worn the whole period. No contact lenses without goggles over them.
3.0 M H₂SO₄ is corrosive, it burns skin and eyes and damages clothing. Add acid to the flask carefully; if any contacts skin, rinse 15 min and tell Dr. Mulinta.
0.0200 M KMnO₄ stains skin brown and stains clothing permanently. It is a strong oxidizer, keep it away from paper towels and organics.
H₂O₂ at drugstore strength is a mild irritant; the acidified peroxide + permanganate mixture releases O₂ gas: no flames, work in a ventilated area.
Glass buret: handle with two hands, clamp securely, read at eye level. Report chips/cracks before filling.
Waste: nothing acidic or coloured goes down the drain (see Clean-Up). Acidified oxidizers corrode the metal piping.

Materials & Equipment

Kathy (lab tech) will pre-set: the standardized 0.0200 M KMnO₄ in labelled dispensing bottles, 3.0 M H₂SO₄ in a dropper/dispensing bottle behind a splash guard, the drugstore H₂O₂ "unknown," distilled-water wash bottles, and the labelled waste bucket at each bench.

Per pair:

50 mL burette + burette clamp + ring stand · small funnel · 125 mL Erlenmeyer flask · 10 mL graduated cylinder · two small beakers (one for ~5 mL H₂O₂, one for ~50 mL KMnO₄)
electronic balance (shared, ±0.01 g) · distilled-water wash bottle · white paper or white tile to read the endpoint against · disposable gloves

No heavy gear needed: permanganate is self-indicating, so there is no pH meter, no indicator solution, and no heating. Everything except the balance is standard bench glass.

Procedure aim for 2–3 good trials

Obtain about 5 mL of the H₂O₂ unknown in a dry, clean beaker.
Take a clean 125 mL Erlenmeyer flask (it need not be dry if the last rinse was distilled water). Weigh the empty flask and record the mass.
Using the 10 mL graduated cylinder, measure 1.0–1.5 mL of the H₂O₂ and pour it into the flask. Record this volume of H₂O₂ solution.
Weigh the flask + H₂O₂ and record. (Empty subtracted from this = mass of H₂O₂ solution titrated.)
Add about 10 mL of 3.0 M H₂SO₄ and about 30 mL of distilled water to the flask. Swirl.
Obtain about 50 mL of 0.0200 M KMnO₄ in your second beaker.
Prepare the burette: rinse with distilled water, then with a small volume of KMnO₄ (discard to waste). Fill, run liquid through to fill the tip (no air bubble), and record the initial reading at eye level (read the top of the dark meniscus for KMnO₄).
Titrate: add KMnO₄ to the swirling flask. Early drops decolourize fast; slow to dropwise near the end. Stop at the first faint pink that persists ~30 s while swirling. Record the final reading.
Empty and rinse the flask; repeat for additional trials. Leave 15 minutes for clean-up.

Clean-Up: all excess H₂O₂, KMnO₄, and H₂SO₄ go in the waste bucket, never down the sink (acidified oxidizers corrode pipes). Rinse the burette several times with water to remove all permanganate; only very dilute pink rinse water may go down the drain. Wash all glassware thoroughly.

Data Tables record in pen, sig figs + units

Concentration of standardized KMnO₄: __________ M (from the bottle label)

Measurement	Trial 1	Trial 2	Trial 3
Volume of H₂O₂ solution (mL)
Mass of empty flask (g)
Mass of flask + H₂O₂ (g)
Mass of H₂O₂ solution (g)
Burette: initial reading (mL)
Burette: final reading (mL)
Volume KMnO₄ used (mL)

Qualitative: colour at the start ___________________ · colour change you watched for ___________________ · what the endpoint looked like ___________________

Analysis Questions show every step

Use one good trial (or your trial average) and show full work with units and significant figures.

Assign the oxidation number of Mn in MnO₄⁻ and of O in H₂O₂. State what is oxidized and what is reduced, and identify the oxidizing agent and the reducing agent.
Write the two balanced half-reactions in acidic solution and combine them into the balanced overall equation. Confirm both atoms and charge balance.
Calculate the moles of MnO₄⁻ delivered from the burette (use the volume used and the 0.0200 M concentration).
Using the mole ratio from the balanced equation, calculate the moles of H₂O₂ in your titrated sample.
Calculate the mass of H₂O₂ (M = 34.02 g/mol) and the percent by mass (% w/w) of H₂O₂ in the solution you titrated.
Calculate the molarity of H₂O₂ in the original solution (use the volume of H₂O₂ you measured in step 3). Note: a ~1.2 mL reading on a 10 mL graduated cylinder carries large relative error (±5–10%), so this volume (not the burette) is the limiting precision here; report the molarity to 2 significant figures.
The label on a drugstore bottle reads "3% hydrogen peroxide." Compare your result. Give one realistic source of error and state whether it would make your % w/w come out too high or too low, with reasoning.
Forward link: in this titration manganese gains electrons (is reduced). In Unit 9 we build galvanic cells from exactly this bookkeeping. In one or two sentences, predict whether MnO₄⁻/Mn²⁺ would be the cathode or the anode when paired with a reaction that wants to give up electrons, and why.