Public Lab Research note


How to measure the pH of a solution with a purple cabbage? Its relation to water quality

by alejobonifacio | December 20, 2021 13:50 20 Dec 13:50 | #28512 | #28512

The pH or hydrogen potential of a solution is a relevant variable when it comes to determining the quality of water, whether it is a drink or an aquatic environment. There are several proprietary commercial tools of varying complexity and precision to measure this parameter. However, for this activity we intend to use natural pigments that are sensitive to this parameter and thus be able to measure pH with elements that are available to anyone.
An environmental friendly method exist for pH determination of water samples. The Brasica oleracea cabage has high concentration of anthocyanins pigments that are sensitive to pH changes of a solution. This fact has been documented in several reseach papers that explore it potencial:

Purpose

Determine the pH of different samples with pigments extracted from purple cabbage (Brassica oleracea).

Necessary materials 📝

  • A cup of chopped purple cabbage
  • A cup of distilled water
  • A clean pan
  • Clean glass containers
  • Samples of liquids of known pH (vinegar, sodium bicarbonate solution, bleach, laundry soap, carbonated cola drink).
  • Sample of water that you are interested in knowing its pH. It can be from a nearby river or lake.

All steps in one

There is a YouTube link

Step 1 🔪

Cut the cabbage into small pieces with a knife to fill a cup

Step 2 🔥🔥🔥

Put the chopped cabbage with a cup of water in the saucepan and simmer for 15 minutes

Step 3 ❄️

Filter the liquid from the pot and let it rest until cool

Step 4 💧 ❓

Try different substances to find out their pH values. The image below indicates the equivalence between different colors and pH values.

Making an indicator from red cabagge 🌈 by Andy Brunning / compoundchem.com; under licence CC BY-NC-ND 2.0Making an indicator from red cabagge 🌈 by Andy Brunning / compoundchem.com; under licence CC BY-NC-ND 2.0

image description The test tubes above correspond to hydrochloric acid (dangerous), river water, sodium bicarbonate solution, carbonated cola drink and alcohol vinegar. Original picture

Step 6 📷

Share your photos with the PublicLab community and social media!!!


Wrap up

Knowing as much as we can about the aquatic environments that care us is key to being able to detect any negative effects on them in time.

Clarification: that the pH value of an environment is at normal values does not mean that this environment is not suffering some kind of disturbance, to determine that an environment is in optimal conditions requires a integrative analysis of it.


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2 Comments

First, I'd like to thank Alejo for posting this page. It is very helpful. Then, I' d like to refer you to the comments page in compound chem ( dated May 18,2017 from the author of the page),for this procedure. One of the comments was that the indicators tendeded to "go off" after a while. The author responded by saying he extended the time by refrigerating the indicator solution. Another comment mentioned freezing the solutions into Ice cubes to extend life. The first comment( about the solutions going off after a short period of time) unfortunately matches my experience. In my case, the time was a few days. In industry, pure indicator solutions usually are dissolved in alcohol. I don't know if adding some alcohol to these solutions would extend their life or not.

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Hi @Ag8n! Thanks for your reply! I had this cabbage solution in fridge almost a week and I didn't see an appreciable color change. I don't know if alcohol could be a good preservant, I'll would try it.

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