Colour chemistry 3: universal indicator and diagnostic tests

Test tubes containing solutions of pH 1–10 colored with an indicator
Figure 2.36. Test tubes containing solutions of pH 1–10 colored with an indicator. Image attribution: Alvy16, PH scale 3, CC BY 4.0

Universal indicator

We can identify the acidity levels (pH value) of many substances with colour testing.

What is pH?

pH, originally meaning “potential of Hydrogen”, is a numbering system used for measuring the acidity of substances dissolved in a water solution. A low pH value has a higher acid content. A high pH value indicates a higher alkaline or basic substance. Water has a neutral pH value of 7, which is why it’s a good solution to test the pH of other substances dissolved in it. For example, lemon juice can have a pH of around 2 – which is very acidic. Lemons have a high quantity of citric acid in them, which is why they have a low pH value. The detergent you use to wash your dishes is alkaline or basic, so it might have a pH value of around 8.

Learn more from this book Acids and Alkalis by Denise Walker.

To test the pH level of any solution, a universal indicator is used. This can be in the form of a paper tape that you dip into your solution, or it could be a liquid that is added to a solution. The universal indicator contains a set of chemicals that change colour depending on the amount of acid or base substances in the solution (Figure 2.37). They work by producing extra protons depending on how acidic or alkaline the solution is, which in turn emit photons with different wavelengths.

 

Chemistry of universal-indicator infographic
Figure 2.37. Chemistry of Universal Indicator by James Kennedy Monash. Image attribution: Chemistry of Universal Indicator by James Kennedy Monash, Creative Commons licence Attribution-Non-commercial-ShareAlike-4.0, select image to enlarge.
Indicators in NatureHydrangeas are flowering plants that can grow flowers in a range of colours, including white, pink, purple and blue.  Did you know that Hydrangea flowers can change their colour depending on the acidity of the soil?  You can turn your pink hydrangea flowers blue by adding an acidic chemical like aluminium sulphate to your plant’s water (Figure 2.38). The roots absorb this chemical, and the flower petals act as a natural indicator, turning from pink to blue if the soil pH level is more acidic than alkaline (basic).

Blue hydrangea flowers
Figure 2.38. Blue hydrangea flowers. Image attribution: photograph by Lisa Cianci, Creative Commons licence Attribution-Non-commercial-ShareAlike-4.0

You can also use red cabbage as an indicator by making a solution with chopped-up leaves and water. Red cabbage contains a water-soluble pigment called anthocyanin, which turns red if mixed with an acid substance and blue/green if mixed with an alkaline/base substance.

There are other plants that have anthocyanin and can also be used in the same way. Can you find three other examples?

Learn more about acids and bases.

You can buy universal indicator online and do your own experiments with substances around your home to see which are acids and which are bases, or you can make your own indicator with red cabbage. You can start by testing foods, drinks and cleaning products around your home. Is your tap water a pH of 7? If not, it might have other chemicals in it. See the activity below for instructions on making your own red cabbage indicator.

Activity: make a red cabbage indicator

You can make your own indicator at home with a red cabbage and a few simple materials to test the pH of different substances.

Watch this video for details and try your own experiments:

Media attribution: Test asides with home-made pH indicator, BBC Earth Lab on YouTube

 

Diagnostic medical tests

Have you ever wondered how a pregnancy test or a COVID Rapid Antigen Test (RAT) works? How do the coloured lines appear on the test strip?

A pregnancy test uses chemicals that change colour when they come into contact with certain enzymes that attach to antibodies. When you are pregnant, your body produces a hormone called human chorionic gonadotropin (hCG). This hormone is excreted by the kidneys into urine – so putting urine on the test strip will start a chemical process that tests for the hCG hormone.  The test strip contains an enzyme that will attach itself to the hCG hormone in the urine. As the urine moves along the test strip, there are two lines that can change colour – one will change colour and become visible if it comes into contact with the enzyme that is attached to the hCG – the test line. Another line will change colour and become visible if the enzymes not attached to hCG come into contact with it – the control line.

Two lines are needed to ensure the test has worked correctly because if there was only one line, it could show a false-negative result if the test didn’t work properly.

 

Diagram explaining how pregnancy tests work

Figure 2.39. How do pregnancy test work? Image attribution: Compound Interest – How do pregnancy tests work? Select image to enlarge.

Watch this video that explains how a COVID rapid antigen test works:

Media attribution: Chemist Breaks Down How At-Home Covid Tests Work | WIRED by Wired on YouTube

 

License

Icon for the Creative Commons Attribution-NonCommercial 4.0 International License

Colour Theory: Understanding and Working with Colour Copyright © 2023 by RMIT University is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License, except where otherwise noted.

Share This Book