Making your own indicator

Select from dyes to make an indicator that is able to distinguish between strong & weak acid, neutral, weak and strong alkali.

This experiment should take 70 minutes.

Equipment

• Eye protection
• Test tubes and racks or spotting tiles
• Plastic droppers
• Small measuring cylinder
• Selection of beakers
• Glass stirring rod
• Scissors
• Labels

Labelled solutions:

• Strong acid (sulfuric acid), 50 cm³
• Weak acid (ethanoic acid), 50 cm³
• Distilled water or tap water (NB check that water is neutral) weak alkali (sodium carbonate), 50 cm³
• Strong alkali (sodium hydroxide), 50 cm³
• Dye solutions A to H: Must include
• A - methyl red, 5 cm³
• B - phenolphthalein, 5 cm³
• C - thymol blue, 5 cm³
• D - bromo-thymol blue, 5 cm³
• E - litmus, 5 cm³
• F - a red food dye, 5 cm³
• G - a blue food dye, 5 cm³
• H - a green food dye, 5 cm³ 

Health, safety and technical notes

• Read our standard health and safety guidance here.
• Wear eye protection.
• Wear clothing protection if desired.
• This is an open-ended problem-solving activity, so the guidance given here is necessarily incomplete. 
• Sulfuric acid, 1M H₂SO₄ (aq) is a skin/eye irritant, see CLEAPSS Hazcard HC098a.
• For more information on ethanoic acid, see CLEAPSS Hazcard HC038a.
• Sodium hydroxide solution, 1 mol dm^–3 NaOH (aq), is CORROSIVE., see CLEAPSS Hazcard HC091a.
• Phenolphthalein is a carcinogen, mutagen and reproductive toxin. Normal laboratory solutions, however, are of low hazard. – except for flammability from the ethanol/propanol solvent.
• Thymol Blue and the other indicators are generally of low hazard but may be harmful if swallowed and/or irritant to skin/eyes. The solutions are all of low hazard. For more information on dyes and indicators, see CLEAPSS Hazcard HC032.

Possible approaches

An answer grid could be given to less able students so that they start by testing all the solutions with all the indicators. Although the food dyes are not essential to the indicator made, they are a useful distraction.

Spotting tiles would be easier to use than test tubes.

Notes

Any acid or alkaline solutions should be neutralised with weak acid or alkali (as appropriate) before being washed to waste.

Emphasise to students that only small quantities of solutions are to be used.