pH Measurement in Grape Juice

Distinguishing Acids and Alkalis

• Acids and alkalis are substances that have different properties.
• Natural substances can act as acid-alkali indicators, changing colour when they come into contact with acids or alkalis.
• Red cabbage extract is a natural indicator that changes colour in acidic and alkaline solutions.
• Using a mortar and pestle, prepare red cabbage extract from red cabbage leaves and warm water.
• Acidic solutions turn the red cabbage extract to a red colour.
• Alkaline solutions turn the red cabbage extract to a blue colour.

Natural Acid-Alkali Indicators

• Some natural substances change colour when exposed to acids or alkalis.
• These substances act as indicators.
• Butterfly pea flower extract is an example of a natural indicator.
• The colour of the extract changes depending on whether it is in an acidic or alkaline solution.

Practical 9.1: Comparing the Colour of Red Cabbage Extract in Acidic and Alkaline Solutions

• Aim: To study the colour change of red cabbage extract in acidic and alkaline solutions.
• Materials: A set of test tubes, test tube rack, distilled water, droppers, lemon juice, warm water, baking soda solution, mortar, pestle, beaker(250cm³), measuring cylinder (100 cm³), red cabbage leaves.

Part I: Preparing Red Cabbage Extract

• Put small pieces of red cabbage leaves in a mortar.
• Crush the leaves using a pestle.
• Pour 50cm³ of warm water into the mortar and continue crushing leaves with a pestle.
• Pour the liquid into a beaker and record the colour of the extract.

Part II: Comparing the Colour of Red Cabbage Extract in Acidic and Alkaline Solutions

• Label three test tubes A, B, and C.
• Fill each test tube about one-third full with the prepared red cabbage extract.
• Add three drops of distilled water to tube A (neutral).
• Add three drops of lemon juice to tube B (acidic).
• Add three drops of baking soda solution to tube C (alkaline).
• Shake each tube gently.
• Observe and record the colour change in each test tube.

Litmus

• Litmus is an acid-alkali indicator (a mixture of pigments from lichens).
• There are two types: blue litmus and red litmus paper.
• Blue litmus turns red in acidic substances.
• Red litmus turns blue in alkaline substances.
• In neutral substances, both litmus papers do not change colour.

Practical 9.2: Using Litmus Paper to Classify Solutions as Acidic, Alkaline or Neutral

• Aim: To classify unknown solutions as acidic, alkaline, or neutral using litmus paper.
• Materials: Blue litmus paper, red litmus paper, forceps, droppers, spot plate, unknown solutions.
• Put small pieces of blue and red litmus papers into cavities on a spot plate.
• Add a drop of a solution to a piece of litmus paper in each cavity.
• Observe and record any colour changes.
• Repeat with other unknown solutions.

pH Scale

• The pH scale measures the level of acidity or alkalinity of a substance.
• It ranges from 0 to 14.
• Substances with a pH less than 7 are acidic.
• Substances with a pH of 7 are neutral.
• Substances with a pH greater than 7 are alkaline.

Universal Indicator

• Universal indicator is a solution that gives a range of colours to indicate the pH of different solutions.
• Used to measure pH in a solution.
• A colour chart helps determine the pH value of a particular solution.

pH Meter

• A pH meter is an electronic device that measures the pH of a solution accurately.
• It must be calibrated before use.
• A probe of the pH meter is dipped into a sample solution.
• The reading is displayed directly on the screen.
• pH meters are often more precise for measuring pH levels than indicators.

Practical 9.3: Using pH Paper and Universal Indicator to Measure pH

• Aim: To measure the pH values of some common substances using pH paper and universal indicator.
• Materials: pH paper, droppers, universal indicator, pH colour chart, spot plate, distilled water, baking soda solution, lemon juice, apple juice, etc.
• Use pH paper and match the colours to a chart to find the corresponding pH values.

Practical 9.4: Studying the Advantages and Disadvantages of Using Universal Indicator and a pH Meter

• Aim: To explore the advantages and disadvantages of using universal indicator and a pH meter to measure pH values.
• Materials: Universal indicator, pH colour chart, pH meter, distilled water, vinegar, soy sauce, etc.
• Use both to test and note advantages and disadvantages.