NATURAL pH INDICATORS
Introduction
pH indicators exist as liquid dyes and dye-infused paper strips. They are added to various solutions to determine the pH values of those solutions. Whereas the liquid form of pH indicators is usually added directly to solutions, the paper form is dipped into solutions and then removed for comparison against a colour/pH key. A substance with a pH of 7 (like distilled water) is neutral. A substance with a pH of less than 7 is an acid. The closer the number gets to zero, the stronger the acid is. A substance with a pH of more than 7 is a base. The closer the number is to 14, the stronger the base is.
There are many common household products and garden plants that can be used as pH indicators. Most plants contain pH-sensitive anthocyanins, making them perfect for testing acid and base levels. Many of these natural pH indicators exhibit a broad range of colours.
Since most naturally occurring organic compounds are weak electrolytes, such as carboxylic acids and amines, pH indicators find many applications in biology and analytical chemistry. Moreover, pH indicators form one of the three main types of indicator compounds used in chemical analysis. For the quantitative analysis of metal cations, the use of complexometric indicators is preferred, whereas the third compound class, the redox indicators, are used in redox titrations (titrations involving one or more redox reactions as the basis of chemical analysis).
Objectives: To learn how to prepare and use natural pH indicators for the examination of the pH of solutions.
Equipment and reagents:
- red cabbage, hibiscus tea, red onion peel and/or red beets
- a small pot for making decoctions
- 4 glass beakers 250 ml
- Pasteur pipettes (7-10 pcs)
- 9 test tubes
- test tube rack
- vinegar (aqueous 9 % acetic acid solution) or any other acidic solution
- alkaline household cleaner or aqueous NaOH solution
Experimental procedure:
1. Decoction of red cabbage, hibiscus tea, red onion peel – pieces of vegetable material cooked ≈5 minutes in a small amount of water, then allow the decoction to cool without pouring it down!
2. If blueberry juice or red beet is used, no decoction shall be made, but juice diluted in water shall be used.
3. Dilute an alkaline household cleaner in one of the glass jars!
4. Pour equal volumes of prepared decoction into three test tubes!
5. Add table vinegar to the contents of one test tube, dilute alkali to the other, and leave the third for comparison!
6. Observe the colour change!
7. Repeat the experiment with another decoction or berry juice!
8. Create a table independently and mark the observations in it!
9. Conclude the discoloration of plant materials depending on the solution environment!
Precautions:
Acids and bases are chemicals that can easily damage your skin and eyes. The severity of the hazard depends on how strong the chemical is, how long contact is maintained and what actions you take after contact is made. Use personal protective equipment (PPE) as required.
Expected results:
Participants have learned how to prepare and use natural pH indicators to examine the pH of solutions.
Resources: https://chem.libretexts.org/
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