Indicator puzzle

Extract colours from given plants and use them to determine the pH of three given solutions. No indicator paper is allowed! 

This experiment should take 70 minutes.

Equipment

• Eye protection
• Test tubes and racks (or spotting tiles – if possible, 2 per group)
• Small beakers
• Pestles and mortars
• Glass droppers
• Filter funnels and papers
• Scissors
• Labels
• Students may need access to hot water (eg a kettle, NOT Bunsen burners)
• Sand

Solvents

• Ethanol
• Propanone (acetone)

Samples of coloured plants

• Red cabbage
• Beetroot
• Geranium petals (strongly recommended because they show two colour changes, at low and high pH)
• Rose petals
• Violets
• Delphiniums
• Lupins
• Elderberries
• Blackberries etc
• Dark-coloured flowers will work best. (Suitable materials could be collected in season and stored in a freezer.)
• Access to buffer solutions with clearly marked pH values, eg pH 2.0, 4.0, 6.0, 8.0, 10.0.
• Solutions X, Y, and Z of unknown pH (you could also provide ‘buffers’ here)

Health, safety and technical notes

• Read our standard health and safety guidance here.
• Wear eye protection.
• Natural dyes will stain clothes, wear lab coats or aprons if desired. 
• This is an open-ended problem-solving activity, so the guidance given here is necessarily incomplete.
• Propanone (acetone) is highly flammable and an eye/respiratory irritant. See CLEAPSS Hazcard HC085a.
• Ethanol is highly FLAMMABLE (if methylated spirits are used, they are also harmful if ingested and cause damage to organs). See CLEAPSS Hazcard HC040a.
• Because of the use of flammable solvents, Bunsen burners should not be used. If a hot solution is required, make hot water available from a kettle. 

Possible approaches

To extract colours from given plants (“A local florist very helpfully gave us some “newly dead” flowers for free and they were excellent!”) and use them to determine the pH values of three given solutions.

Initially students could tackle the problem individually to prepare one plant extract, then in groups to solve the X, Y, Z puzzle.

Red cabbage was found to be the best indicator with a sensitive enough response to allow accurate pH estimations (changes through red(cerise) – mauve – blue – turquoise).

Other materials (blackberry, beetroot) are interesting to try but very much the same in their response (red up to about pH 6, blue-red above this value).

Red rose and carnation were also tried – results seemed to vary, some students finding these more useful than others.

NB “Especially interesting are the extracts from red cabbage, radish skin, rhubarb skin, and turnip skin, which act as universal indicators” (see “Edible Acid-Base Indicators” Robert C. Mebane and Thomas R. Rybolt. J.Chem.Educ., April 1985, Volume 62, Number 4, p285).

Difficulties with the sequence of events

Some students will not see the relevance of testing the ‘indicator’ against known pHs, then using their results to test X, Y and Z.

The concepts may need to be explained to them. Also, they tend to know universal indicator quite well and assume that all indicators are red in acid and blue in alkali. Teachers could cut the buffers down to only four and label them:- strong acid (pH 3), weak acid (pH 6), weak alkali (pH 8), strong alkali (pH 10).

Likely areas for giving guidance

How to prepare a plant extract; how to tackle the testing of the extract systematically.

How to record systematically.

How to interpret results.

Experiments could also be re-stated for lower ability range – maybe as a series of structured steps, to make it easier to understand.

“Indicator puzzle”

Can you make an indicator solution and use it to find out the pH of unknown solutions X, Y and Z? You do have some solutions whose pHs you know – pH 2, pH 4, pH 6, pH 8 & pH 10.

Stage 1

• Each person’s job is to extract the colour from the plant sample they have been given.

Stage 2

• Is your coloured extract an indicator? Try to find out, using the solutions whose pHs we know.

Stage 3

• Working in a group, decide which is the most useful indicator to help solve this problem: What are the pH’s of X, Y & Z?
• Write down your answers. 

Possible extension

Test a variety of household substances with your natural indicator. Make natural indicator ‘papers’ by putting the plant pigment onto a piece of blotting paper.