Fanelwa Ngece Ajayi
Learning about pH can be a challenge for pupils, especially in the intermediate grades.
This topic forms part of the necessary foundation required of any high school pupil considering tertiary education in any field in Natural Sciences.
It is imperative for pupils to be independent in their learning and practical skills by conducting their own experiments, mainly because a majority of primary and high schools in our country lack equipped science laboratories.
As such, pupils can test the pH of various solutions at home using red cabbages as acid-base indicators (also known as pH indicators).
pH is a measure of the acidity or basicity of solutions and it ranges from 0 to 14, with 7 being neutral. A pH value below 7 indicates acidity, while a pH above 7 indicates basicity.
pH is essentially a measure of the relative amount of free hydronium ions (H3O+) or hydrogen ions (H+) in water.
Their amount varies depending on how highly concentrated the water portion of a particular substance or solution is. Acid-base indicators are substances that change colour with pH changes. These indicators are usually weak acids or bases that, when dissolved in water, detach slightly and form ions. Therefore, pH indicators are chemical detectors of hydronium ions.
The ease in making pH indicator solutions can be of great benefit to pupils particularly when they are able to develop them in the comfort of their homes.
Red cabbage is an excellent choice for pupils to use, who may use its juice to detect the pH of various household solutions.
Red cabbage contains a pigment molecule called flavin (an anthocyanin). This water-soluble pigment is also found in the skins of grapes, apples, plums, and blueberries.
Highly acidic solutions will turn the anthocyanin-containing solution red. A neutral solution will turn purple in colour while a basic solution will appear greenish-yellow. Therefore, one can determine the pH of a solution based on the colour it imparts to the anthocyanin pigment in red cabbage juice. It merely responds to the change in the concentration of hydrogen ions.
The following is a simple experiment to illustrate the concept of pH detection using a homemade indicator. This experiment can be conducted by pupils from Grade 7 up to Grade 9 who need assistance with understanding pH in relation to acids and bases.
Required materials and household solutions: Red cabbage, knife or blender, Filter paper (coffee filter or pantyhose works as well), hot water, a large cup or tall glass container, and six small cups or glass containers. Baking soda, also known as Bicarbonate of Soda (sodium bicarbonate – NaHCO3); lemon juice (citric acid – C6H8O7); vinegar (acetic acid – CH3COOH); antacids such as ENO, Rennies or Gaviscon (calcium carbonate, calcium hydroxide or magnesium hydroxide); water (H2O); and milk.
Health and safety considerations: Safety goggles, lab coats and gloves should always be worn during experiments. Due to the less dangerous nature of this experiment, these items may be excluded if the pupils do not have them. The chemicals or solutions used in the following experiment may be safely washed down the drain with water after use.
Procedure: Chop the cabbage into small pieces until you have about two full cups. Place the cabbage into a large cup or glass container and add boiling water to cover the cabbage. Wait at least 10 minutes for the colour of the cabbage to cause a colour change in the water. Alternatively, you can put about two cups of cabbage in a blender, add enough boiling water to cover the cabbage and blend for approximately five minutes. Filter the cut or blended cabbage using any of the above-listed filters to obtain a red-purple blue liquid. This liquid has a pH of approximately 7 because it is dependent on the pH of the water. Thereafter, pour about 50 to 100ml (about a quarter of a cup) of your red cabbage indicator solution into each of the six small cups or glass containers. Then add various household solutions to each of the indicator solutions until a colour change is observed. Use a separate container for each household solution. Since red cabbage contains flavin the highly acidic solutions will turn the flavin-containing solution red, purple colour in a neutral solution and greenish-yellow colour in a basic solution.
An alternative method is the use of red cabbage pH paper strips to determine the pH of these solutions. Filter paper or coffee filters can be soaked in a concentrated red cabbage juice solution and allowed to soak for a few hours; four hours is recommended. The filter papers should then be allowed to dry by hanging each piece. After drying, they can be cut into strips and then used to test each household solution by placing a drop of each household solution onto a clean indicator strip. For both methods, all observations should be meticulously recorded and used as additional learning tools to complement any academic learning taking place in the classrooms.
Furthermore, the South African Natural Sciences Curriculum and Assessment Policy Statement (CAPS) for Grades 7 to 9 is available online at(https://wcedeportal.co.za/eresource/112876) as a tool to further guide pupils on the important aspects of this topic in their respective grades.
- Professor Fanelwa Ngece Ajayi is an Associate Professor of Physical Chemistry at the Chemistry Department, University of the Western Cape.