Determine Approximate pH of Acidic Solutions by Indicator Method

THEORY:

All aqueous solutions contain certain amounts of hydrogen ion concentrations. pH of a solution is defined as the negative common logarithm of the hydrogen ion concentration of that solution.

Mathematically.

pH = -log₁₀ [H⁺]

Suppose, the hydrogen ion concentration of a solution is [H⁺] = 10^-3 mol/1

Then, its pH = -log₁₀ (10^-3)

        = 3

A buffer solution is the solution whose pH value remains almost constant even if kept for a long time. Standard buffer solutions of known pH values can be prepared in the acidic range by mixing definite volumes of acetic acid and sodium acetate. Now, if the same number of drops of a universal indicator are added to equal volumes of the buffer solution, they will exhibit different shades of colour. Then, the pH of the given unknown solution can be determined by matching the colour against the standards.

APPARATUS :

1) Test tubes 

2) Test tube stand 

3) Droppers

CHEMICALS:

1) 0.2 M acetic acid 

2) 0.2 M sodium acetate

3) Universal Indicator 

4) Methyl orange solution

5) Lemon juice

PROCEDURE:

i) Prepare standard buffer solutions of pH 3, 4, 5 and 6 by mixing 0.2 M acetic acid and 0.2 M sodium acetate as follows.

Test tube No.	Vol. of aceticacid (0.2M) in ml	Vol. of sod. acetate(0.2M) in ml	[H+]mol/1	pH
1	9.8	0.2	10-3	3
2	8.0	2.0	10-4	4
3	3.0	7.0	10-5	5
4	0.5	9.5	10-6	6

ii) Arrange the test tubes in a test tube stand and add 3 drops of universal indicator (or methyl orange only) into each of them, and observe the change in colour against the white paper behind them.

iii) Now prepare the following solutions

a) 10 ml water + 2 drops of dil HCI

b) 10 ml water + 2 drops of dil H₂SO4

c) 10 ml water + 2 drops of acetic acid

d) 9 ml water + 1 ml lemon juice

e) 10 ml water saturated with CO₂ gas by blowing

Put 3 drops of the same indicator into each of them and determine their approximate pH by matching the colour against the standards.

Universal Indicator and pH Paper

A single acid-base indicator has a narrow range of pH values over which it shows its colour change. (refer to table 9.1 of chapter IX)

Universal indicators are mixtures of several acid base indicators. They can show several colour changes over a wide range of pH values.

pH papers are also nothing but the strips of paper impregnated with solutions of universal indicators.

Both the universal indicator and the pH paper can be used easily to find the approximate pH of any solution without preparing the buffer solution as was done in experiment no. 3 above.