Homemade pH Indicator – DIY Experiments

Abstract

Make your own homemade pH indicator! Red cabbage juice contains a pH indicator that exhibits a wide range of colours according to the acidity of the solution. This experimental contains in-depth scientific explanation of why certain compounds are coloured and how the colour change occurs. All materials and equipment can be found at home and are relatively safe to use.

Introduction

Organic compounds tend to be coloured when they contain an extensive conjugating system. A conjugated system consists of a series of connected p-orbitals with delocalised electrons. This has the effect of increasing stability and reducing the energy gap between the HOMO (highest obtained molecular orbital) and the LUMO (lowest unoccupied molecular orbital). This moves the absorption band from the UV to the visible region giving it colour.

The pigments responsible for the blues, reds and purples in many flowers, fruits and vegetables, are members of a subclass of flavonoids known as anthocyanins. In flowers they are used for attracting pollinators. In fruits they are used to attract the attention of animals, which then can disperse the plant’s seeds. They have also been shown to act as a “sunsceen” preventing damage to the cell by absorbing blue-green and UV light. It has also been proposed that they may be used by plants to provide camouflage from herbivores, who are blind to certain wavelengths. Anthocyanins can be used as pH indicators in aqueous solutions. This is because their structure undergoes a variety of molecular rearrangements as the pH changes, resulting in a colour change. This is interesting because all colour changes are a result of the rearrangement of one compound, unlike universal indicator which uses several different compounds to get the same effect.

Red cabbage contains one of these colour changing compounds and can be easily extracted using the following equipment.

Materials and Equipment

Red cabbage
knife
boiling water
Large bowl
pipet
small glasses (shot glasses are perfect)
Samples to test

The following household chemical pH’s can be determined using the homemade pH indicator; ammonia (NH3), baking soda (sodium bicarbonate, NaHCO3), washing soda (sodium carbonate, Na2CO3), lemon juice (citric acid, C6H8O7), vinegar (acetic acid, CH3COOH), antacids (calcium carbonate, calcium hydroxide, magnesium hydroxide), soda water (carbonic acid, H2CO3), lye (sodium hydroxide, NaOH)

Experimental Procedure

Chop the cabbage into small pieces and place into the large bowl.
Cover the cabbage with boiling water and allow to cool.
Decant the (now room temperature) cabbage juice. This will be your blank as it will be at pH 7.
Place samples for testing into the small glasses and add a small about of the cabbage juice.

Video

The main topic of the following videos is on the pseudoscience of homeopathy. It proves that water does not have memory using pH indicator made from red cabbage.