Which bleach is the best buy?

This month they are investigating chlorine containing bleaches.

This session should take 90 minutes.

Equipment

Apparatus

• Burettes
• Funnels
• Volumetric flasks (available but not visible)
• Conical flasks and white tiles
• Pipettes
• Safety fillers
• Indicator paper (as a distractor).

Chemicals

• Sodium thiosulphate solution, 0.1 mol dm^–3
• Potassium iodide solution, 10 %, 80 cm³
• Dilute sulfuric acid, 80 cm³ below 1.52 mol dm^–3
• Starch solution as indicator. 
• Solutions of four different brands of chlorine-containing bleach (diluted 50 times by water). NB Environmental friendly bleaches do not contain chlorine. 

Health, safety and technical notes 

• Read our standard health and safety guidance here.
• Wear eye protection.
• Wear clothing protection.
• This is an open-ended problem-solving activity, so the guidance given here is necessarily incomplete.
• Household bleach solutions (containing sodium chlorate(I) / sodium hypochlorite) sold for the domestic market are probably corrosive, typically they contain about 5% sodium chlorate(I). 
• Even quite dilute bleach is an irritant if more than 0.15 Mol dm^–3 NaOCl.
• Bleaches can release toxic chlorine gas. Undiluted solutions should be used in a well-ventilated laboratory, and pupils with asthma should keep away from them. 
• Some bleaches also contain detergents and thickening agents, which may cause excessive frothing in this experiment.  
• Sulfuric acid, dilute H₂SO₄(aq), is a skin/eye irritant between 0.5 and 1.52 mol dm^–3. Above that, it is corrosive. Below, it is of low hazard. See CLEAPSS Hazcard HC098a.
• Sodium thiosulfate, 0.1 M Na₂S₂O₃ is of low hazard. See CLEAPSS Hazcard HC095a.
• Potassium iodide is an eye irritant. A 10% solution is also an eye irritant, but any concentration lower than that is of no significant hazard. See CLEAPSS Hazcard HC047b.
• Diluted bleach solutions are of low hazard, but for anything more than very small quantities of bleach, ‘neutralise’ with iron II salts or sodium thiosulfate and then wash to waste.

Possible approaches

To find the amount of chlorine in each bleach

The bleach solutions are mixed with potassium iodide and then acidified. The chlorine present in the bleach oxidises iodide to iodine (ie chlorine is above iodine in the reactivity series):

Cl₂ + 2I^– → 2Cl^– + I₂

The iodine produced is then reduced quantitatively by sodium thiosulfate

2S₂O₃2- + I₂ → S₄O₆2- + 2I^–

Thus, by adding iodide ions to the bleach and titrating it against sodium thiosulfate, it is possible to determine the amount of available chlorine present in the bleach.

The molar ratio Cl₂ : I₂ : S₂O₃2- = 1 : 1 : 2

TITRATION

25 cm³ of bleach in conical flask. Add (i) 10 cm³ of KI solution, (ii) 20 cm³ dilute sulfuric acid (both in excess).

Titrate with sodium thiosulphate solution (0.1 mol dm^–3) using starch as the indicator (added near the endpoint which is colourless). This procedure uses approximately 10 cm³ of sodium thiosulfate – the accuracy of the burette readings can be increased by using more dilute solutions of sodium thiosulfate.

You should not give students cost/size of bottle (see data table) – unless asked for. Experiment could be used initially as an assessed practical (ie for design purposes where students have to suggest own methods). A couple of lessons later, students could carry out their experiment and then present their results for assessment. Credit could be given for accuracy, consistency and treatment of results. 

Possible extension

Test viscosity (‘staying power’), test ‘bleaching power’ on, eg ink. Discuss/investigate/report on environmental problems due to bleaches. Compare the bleaching power of chlorine and non-chlorine containing bleaches (NB Environmental friendly bleaches contain hydrogen peroxide as the oxidising agent that still liberates iodine).