Theory
Household bleach is a solution of sodium hypochlorite, NaOCl. Its strength is reported as the percentage of sodium hypochlorite (or of "available chlorine"). This lab determines that percentage by an iodometric redox titration, a method that combines oxidation-reduction chemistry, titration, and stoichiometry.
1. The redox chemistry
In acidic solution, the hypochlorite ion oxidises iodide ion to iodine:
OCl⁻ + 2 I⁻ + 2 H⁺ → Cl⁻ + I₂ + H₂O
Here the chlorine in OCl⁻ (oxidation state +1) is reduced to Cl⁻ (−1), and iodide (−1) is oxidised to iodine (0). The amount of iodine produced is exactly equal, in moles, to the amount of hypochlorite that was present.
2. Titrating the iodine
The liberated iodine is then titrated with a standard solution of sodium thiosulfate:
I₂ + 2 S₂O₃²⁻ → 2 I⁻ + S₄O₆²⁻
Each mole of iodine reacts with two moles of thiosulfate. Combining the two steps gives the key relationship: 1 mol OCl⁻ ≡ 1 mol I₂ ≡ 2 mol S₂O₃²⁻, so the moles of hypochlorite equal half the moles of thiosulfate used.
3. The starch indicator and endpoint
As thiosulfate is added, the brown iodine colour fades to pale yellow. Near the endpoint, starch indicator is added, forming a deep blue starch-iodine complex. The endpoint is the sharp point at which the last of the iodine reacts and the blue colour suddenly disappears, leaving a colourless solution.
4. The calculation
From the volume and concentration of thiosulfate, you find the moles of thiosulfate, halve it to get the moles of NaOCl, multiply by the molar mass of NaOCl (74.44 g/mol) to get the mass of NaOCl, and divide by the mass of the bleach sample to get the mass percent of sodium hypochlorite.
Apparatus and Reagents
The equipment and reagents a real bleach analysis uses. In this simulation they are modelled for you, but the observations and volumes correspond to what the titration would actually give.
- A burette, pipette, conical (Erlenmeyer) flask, and a white tile.
- A measured bleach sample (for example 5.00 mL, density about 1.05 g/mL).
- Potassium iodide (KI) solution, in excess.
- Dilute sulfuric or acetic acid to acidify the mixture.
- Standard sodium thiosulfate solution (for example 0.250 M).
- Starch indicator solution, added near the endpoint.
Instructions
This is a predict-and-check lab. In each tab you decide the answer yourself first, enter or choose it, and only then does the simulation confirm the correct result so you can compare. Record every result in your worksheet.
Simulation — The Titration Bench
Given data for this trial:
Relationship: 1 mol OCl⁻ ≡ 2 mol S₂O₃²⁻
Second trial data (a different bleach):
Team Questions
Work these out with your team, type each answer, and check it.
Example Lab Report
A worked example showing the expected format and the predict-and-check workflow.
Analysis of Bleach
Chemistry | Section: [Your Section] | Date: [Date]
Lab Members: [Names of all members present]
Objective — To determine the mass percent of sodium hypochlorite in household bleach by an iodometric redox titration, and to compare two bleach samples.
Part A — Redox Chemistry (worked example)
Hypochlorite oxidised iodide to iodine (OCl⁻ + 2 I⁻ + 2 H⁺ → Cl⁻ + I₂ + H₂O); hypochlorite was the oxidising agent. The iodine was then titrated with thiosulfate (I₂ + 2 S₂O₃²⁻ → 2 I⁻ + S₄O₆²⁻). Starch gave a blue colour that disappeared at the endpoint. Overall, 1 mol OCl⁻ ≡ 2 mol thiosulfate.
Part B — Titration (worked example)
The brown iodine faded to pale yellow; starch was added and the solution turned deep blue; thiosulfate was added dropwise until the blue just disappeared at an endpoint of 29.6 mL.
Part C — Calculation (worked example)
mol thiosulfate = 0.250 M × 0.0296 L = 0.00740 mol. mol NaOCl = 0.00740 ÷ 2 = 0.00370 mol. mass NaOCl = 0.00370 × 74.44 = 0.2754 g. percent NaOCl = 0.2754 ÷ 5.25 × 100 = 5.25%.
Part D — Second Trial (worked example)
The second bleach needed 34.0 mL of thiosulfate, giving 6.03% NaOCl. Because it required more thiosulfate, Trial 2 contained more hypochlorite and was the stronger bleach.
Discussion and Conclusion — The first bleach was 5.25% sodium hypochlorite, close to the typical household value. The method combined redox chemistry, titration to a starch endpoint, and stoichiometry, and correctly ranked the two bleaches by strength.
Practice Quiz
For extra practice on this topic, use the interactive Analysis of Bleach quiz in the Quizzes section of the site.