Theory: In this experiment, the pH of a solution is utilized as an indicator for determining the end-point of a strong acid-strong base titration. The base solution is standard while the concentration of acid is unknown. A fixed quantity of solution of strong acid is taken in a beaker and its’ initial pH is recorded. To this, if we start adding a strong base solution, we find that the pH of the reaction mixture follows a graph of the following type:
Volume of base added
Procedure: Preparation of Solutions: Preparation of 0.1 N NaOH solution Pipette out 10 ml of the given1 N NaOH solution and transfer it to a 100 mL measuring cylinder. Dilute it up to the mark with distilled water.
Performing the Titration: 1. Wash the burette with distilled water and mount it on the stand. 2. Fill the NaOH solution (prepared earlier) into the burette. Open the stopper and allow the solution to flow back into the beaker to remove the air from the burette. Once a continuous solution column is achieved, close the knob and put the solution back into the burette. 3. Note the initial volume level in the burette. 4. Using a pipette, take out 30 mL of given HCl solution into a 100 mL beaker. 5. Dip the pH electrode into the beaker and turn the pH-meter on. Measure the initial pH of the acid solution.
6. From the burette, start adding the NaOH solution in 1.0 mL increments. Note down the pH after each increment. 7. Continue with the previous step till about 45 mL and then empty the burette into a waste container. 8. Plot a graph between the observed pH value and the volume of NaOH solution added. Locate the end point as the point of maximum slope on the graph (see figure in theory). 9. Calculate the strength using the data obtained.
Observation Table: Tabulate the volume added and the observed pH.
Calculations: The relation applicable for calculating the Normality of HCl solution is: N1V1 = N2V2 Here, N1= Concentration of acid , N2= Concentration of NaOH (0.1) V1= volume of acid taken = 10 mL, and V2= volume of base required for complete neutralisation (read from the graph) Once the normality of HCl is calculated, its strength in g/L can be calculated by multiplying the Normality with the molecular weight: Strength (in g/L) = N1 X Molecular weight of HCl
Result: Report the strength of HCl solution obtained as the final outcome.
Precautions: 1. In general, be very careful when handling expensive glassware. 2. Concentrated reagents are very corrosive to human skin; exercise extreme caution while handling them.
