Another way of measuring pH is to use a digital pH meter. A probe is placed in the solution and the pH is displayed on the meter. The advantage of using a pH meter is that it indicates very small differences in pH. pH meters are used in hospitals to measure the pH of a patient’s bodily fluids.
Investigating the pH of a variety of substances
Plan, design, carry out and write up an investigation to examine the pH of a variety of substances.
Making Indicators
Many plants contain substances that can be used as pH indicators because they change colour in acidic or alkaline solutions. These dyes must first of all be removed from the plant cells.
Extracting dyes from red cabbage
The following steps for extracting dyes from red cabbage to produce an acid–base indicator have been mixed up. Rewrite the steps in the correct order.
1. Collect the cabbage indicator in a beaker. 2. Filter the red cabbage and dye mixture. 3. Leave it to stand for 5–10 minutes. 4. Cut the red cabbage into small pieces.
5. Dip some strips of filter paper into the dye and allow them to dry.
6. Add warm water.
7. Grind up the red cabbage using a mortar and pestle.
8. Test the indicator using a variety of acids and bases.
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Fig. 13.2.4 Lichens growing on stone
Testing the effectiveness of a variety of plant indicators
Plan, design, carry out and write up an investigation to find out if some of the following substances produce an effective acid–base indicator: beetroot, curry powder and carrots. You will need to carry out some research online to help with planning this investigation.
1. What precautions will you take to ensure the research you carry out is reliable? 2. How will you make sure this a fair test? 3. How will you know if the indicator is effective? 4. Draw a diagram of the set-up for this investigation.
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Litmus indicator is a solution of dyes taken from lichens. Lichens are organisms that are found growing on leaves and branches, on bare rock (including walls) and on exposed soil surfaces. The roofs of many buildings have lichens growing on them.
