9. Determination of strength of acids in a mixture of acids using conductivity meter

9. DETERMINATION OF STRENGTH OF ACIDS IN A MIXTURE OF ACIDS USING CONDUCTIVITY METER

Expt. No.

Date:

AIM

To determine the strength and amount of a strong acid and a weak acid (HCl and CH₃COOH) present in one litre of the given mixture of acid solution by conductometric titration. You are provided with standard NaOH solution of ........N.

PRINCIPLE

Solution of electrolytes conducts electricity due to the presence of ions. Since specific conductance of a solution is proportional to the concentration of ions in it, conductance of the solution is measured during titration.

When the sodium hydroxide is added slowly from the burette to the solution, HCl (strong acid) gets neutralised first. Since the fast moving H⁺ ions are replaced by slow moving Na⁺ ions, decrease in conductance take place until the end point is reached.

HCl + NaOH → NaCl + H₂O (Ist neutralisation).

After the complete neutralisation of all HCl, the neutralisation of CH₃COOH starts,

CH₃COOH + NaOH → CH₃COONa + H₂O (II^nd neutralisation).

Since CH3COONa is stronger electrolyte than CH₃COOH, conductivity slowly increases until all CH₃COOH is completely neutralised. When the end point is reached, addition of NaOH will cause sudden increase in the conductance. This is due to the presence of fast moving OH^- ions.

MATERIALS REQUIRED

1. Conductivity bridge 2. Conductivity cell 3. 100 ml Beaker 4. Standard NaOH 5. Given HCI & CH3COOH mixture 6. Burette, pipette, glass rod etc., 7. Distilled water.

PROCEDURE

TITRATION

The given mixture of acids (HCl + CH₃COOH) is transfered into 100 ml standard flask and made upto zero mark using distilled water. 20 ml of the made up solution is pipetted out into a clean 100 ml beaker. The conductivity cell is placed in it and then diluted to 50 ml by adding conductivity water, so that the electrodes are well immersed in the solution. The two terminals of the cell are connected with a conductivity bridge.

The burette is washed well with water and rinsed with the given standard NaOH solution. It is then filled with NaOH solution upto the zero level.

Now 1 ml of NaOH from the burette is added to the solution, taken in the beaker, stirred for some time and then conductivity is measured. (The conductivity is going on decreasing upto the end point). This process is repeated until atleast five readings are taken after the end point (A) has been reached.

After the end point, again NaOH is gradually added, which causes increase in conductance. This increase in conductance is observed until the end point (B) is reached.

After the second end point, sudden increase in conductance is observed on further addition of NaOH. The reading (conductivity) is continuously measured for each addition of NaOH and are tabulated. Now the graph is plotted between the volume of NaOH Vs conductivity. From the graph the first end point (A) and the second end point (B) are noted. From the end points the strengths and amount of HCl and CH₃COOH present in 1 litre of the mixture of solution are calculated.

Step I

Calculation of Strength of HCI

Volume of the mixture (HCl), V₁ = 20 ml

Strength of the mixture (HCl), N₁ = ............. ?

Volume of the NaOH, V₂ = .............. (A) ml (Ist titre value)

Strength of the NaOH, N₂ = ....... N

According to the law of volumetric analysis, V₁N₁ = V₂N₂

Step II

Calculation of Strength of CH₃COOH

Volume of the mixture (CH₃COOH), V₁ = 20 ml

Strength of the mixture (CH₃COOH), N₁ = ....... ?

Volume of the NaOH, V₂ = ........... (B - A) ml (II^nd titre value)

Strength of NaOH, N₂ = ........ N.

According to the law of volumetric analysis, V₁N₁ = V₂N₂

RESULT

(i) Strength of HCl present in 1 litre of the given solutions = ……… N

(ii) The amount of HCl present in 1 litre of the given solution = …….. gms

(iii) Strength of CH₃COOH present in 1 litre of the given solution = …….. N

(iv) The amount of CH₃COOH present in 1 litre of the given solution = ……. gms