AIM:

To find the pH of the following samples by using pH paper/universal indicator:

(a) Dilute hydrochloric acid (HCl)

(b) Dilute sodium hydroxide (NaOH) solution

(c) Dilute ethanoic acid (CH₃COOH) solution

(d) Lemon juice

(e) Water

(f) Dilute hydrogen carbonate (NaHCO₃) solution

MATERIALS REQUIRED:

Apparatus: Test tubes, test tube stand, droppers or glass rod, pH paper/universal indicator, standard colour chart, glazed white tile

Chemical compounds: HCl, ethanoic acid (acetic acid/vinegar), lemon juice, distilled water, and sodium bicarbonate solution.

THEORY

1. pH is the measure of the concentration of hydrogen ion in the solution.

2. pH is defined as the negative logarithm of the concentration of hydrogen ion in a solution at room temperature (298 K).

3. The hydrogen ion concentration (H⁺) for an acidic solution is always less than 7 on the pH scale.

4. The hydrogen ion concentration of a basic solution is always greater than 7 on pH scale.

5. The hydrogen ion concentration of a neutral solution is 10 mol/L. Therefore, its pH is 7.

6. The pH of a solution can be measured by using a pH paper, universal indicator or pH meter.

PROCEDURE

1. Pour the given solution in separate clean and dry test tubes marked as A, B, C, D, E, and F and place them in the test tube stand, as shown in the figure below:

2. Take six strips of pH paper and place them on a tile.

3. Place a drop of each test solution on the six strips of the pH paper using a dropper.

4. Note the colour of the pH paper and compare it with the colour chart of the pH paper.

5. Record the pH value corresponding to the colour.

6. Similarly, using a new dropper each time perform the same experiment with the remaining test samples using a new strip of pH paper.

7. Record your observations in the observation table below.

OBSERVATION TABLE

RESULT

1. The pH of dilute solutions of hydrochloric acid, ethanoic acid and lemon juice is less than 7, and therefore, they are acids.

2. The pH of dilute solutions of sodium hydroxide and, sodium bicarbonate is more than 7, and therefore, these solutions are bases.

3. The pH of water is 7 and therefore, it is neutral.

PRECAUTIONS

1. The acids and bases solution should be used with care.

2. Use distilled water for preparing solutions.

3. Use only standard colour charts for finding the pH value.

4. Do not touch the pH paper with dirty and wet hands.

5. Keep the pH paper away from fumes produced during a chemical reaction.