How to Make a pH Indicator from Red Cabbage

Red cabbage can be a taste divider, but whether you love it or hate it, did you know it can also be a useful pH indicating tool?

In this experiment, we use red cabbage to estimate the pH of several everyday household products then test our results against a pH meter to find out just how accurate red cabbage can be as a pH indicator!

The Science Behind the Experiment

Inside of red cabbage is a type of molecule called an anthocyanin. Anthocyanins are water-soluble and vacuolar pigments that, depending on their pH, can appear purple, blue, red, or even black. The colour change in varying pH levels is due to the structure of the anthocyanin pigments, which subtly change at different pH levels.

It is these anthocyanin pigments and the subtle changes to their structure in varying pH levels, which are the underlying credentials for red cabbage as a pH indicator. We can put nature to the test by mixing a selection of known alkaline, basic and acidic substances with our homemade anthocyanin pH indicator (a.k.a cabbage juice).

A Guide to Red Cabbage pH Indicator Colours

Depending on the pH of the solution added to our cabbage pH indicator, the graphic below demonstrates the pH ranges and their expected corresponding colour. With acidic substances being red/pink, neutral substances appearing purple and alkaline products tending to be green/yellow.

Figure 1 Expected pH level and their corresponding solution colour when mixed with a cabbage pH indicator

Required Materials

• Red cabbage (Around 2 cups, chopped)
• Knife
• Water
• Pot
• Stove
• Strainer or filter paper
• Glasses for household solutions/cabbage indicator
• Various household solutions. See appendix 1 for a full list. We used;
  • Weak Bleach
  • Bi-Carb Soda
  • Hand Sanitiser
  • pH Balanced Shampoo
  • Vinegar
  • Lemon Juice
• PPE: Select based on your measured household solutions. Gloves and protective eyewear are a must.

Method

1. Cut the cabbage into small parts.
2. Place the chopped cabbage into the pot and cover with water.
3. Boil until the water is a deep purple.
4. Strain the leaves out, and the remaining liquid is your cabbage pH indicator. Let it cool down before use.
5. Pour between 50-100mL of the cabbage solution into the five glasses.
6. Add various household solutions until it starts to change colour! Compare the colour change against the colour change graphic (figure 1) to determine your approximate pH level. Then, if you have one available, test each solution with a pH meter to compare results.

Results

This experiment is a fantastic example of how naturally occurring anthocyanin pigments in cabbage can be used as a pH indicator to estimate the pH level of various household products.

Acidic solutions, such as the lemon juice, turned pink/red when added to the cabbage pH indicator. When alkaline products such as bleach were mixed, the resulting colour was green/yellow as expected. There is no or little change in our neutral substances, like the hand sanitiser and pH balanced shampoo.

Conclusion

Anthocyanins within red cabbage make it a useful (and pretty cool) pH indicating tool. When mixed with various household items, the cabbage solution provides a reasonable (but not exact) indication of pH.

So, is it time to ditch your pH meter for a natural alternative? Probably not. If you require an indication of pH only, and a sacrificial cabbage is available, then nature provides an option. However, for high volume, more precise, convenient, multi-variant, professional or dependable results, it is best to rely on a calibrated pH meter.

Want more information on pH meters? Speak with an Instrument Choice Scientist! We're here to help! Call 1300 737 871 or email [email protected].

Appendix 1

The approximate pH levels of common substances