8. Determination of strength of given hci using ph meter

8. DETERMINATION OF STRENGTH OF GIVEN HCI USING ph METER

Expt. No.

Date:

AIM

To determine the strength of given HCl using pH meter, a standard solution of NaOH of .......... N is provided.

PRINCIPLE

Since the pH of the solution is related to the H+ ion concentration by the following formula,

pH = - log [H]

measurement of pH of the solution gives the concentration of H⁺ ions in the solution. When NaOH is added slowly from the burette to the solution of HCI, the fast moving H⁺ ions are progressively replaced by slow moving Na⁺ ions. As a result pH of the solution increases.

HCI + NaOH → NaCl + H₂O

The increase in pH takes place until all the H⁺ ions are completely neutralised (upto the end point). After the end point, further addition of NaOH increases the pH sharply as there is an excess of fast moving OH^- ions.

MATERIALS REQUIRED

(i) pH meter, (ii) Glass electrode, (iii) 100 Beaker, (iv) Standard NaOH, (v) Given HCI, (vi) Burette, pipette, Glass rod etc., (vii) Distilled water.

Calculation

Step I: Calculation of Strength of HCI

Volume of HCl,  V₁ = 20 ml

Strength of the HCl, N₁ = ........... ?

Volume of the NaOH, V₂ = ……. (titre value)

Strength of the NaOH, N₂ =.......... N

According to the law of volumetric analysis, V₁N₁ = V₂N₂

Step II: Calculation of amount of HCl

The amount of HCl present in 1000 ml of the given solution = …..... N × Eq. wt. of HCl (36.45)

= ……….. 36.45

= ………..  gms

PROCEDURE

TITRATION - I

The given hydrochloric acid solution is transfered into 100 ml standard flask and made up to the zero mark using distilled water. 20 ml of this made up solution is pipetted out into a clean 100 ml beaker and diluted by adding 20 ml of distilled water The glass electrode is dipped in it and connected with a pH meter.

The burette is washed well with water and rinsed with a small amount of given NaOH solution. It is then filled with the same upto zero mark.

Titration is carried out by adding std. NaOH solution in portions of 1 ml from the burette to the HCl solution taken in the beaker and pH of the solution is noted for each addition. This process is continued until atleast 5 readings are taken after the end point, and the range at which the end point lies is found out by plotting volume of NaOH added against pH (graph I).

TITRATION - II

Another titration is carried out by adding std. NaOH solution in portions of 0.1 ml near the end point and pH of the solution is noted after each addition. The addition of NaOH is continued even after the end point for further 1 ml. The accurate end point is found out by plotting ΔpH /ΔV against volume of NaOH added (graph II). From the end point, the strength of HCl solution and hence the amount of HCl is calculated.

RESULT

1. Strength of the given HCl solution = …………. N

2. Amount of HCl present in 1 litre of the solution = …… gms.