Analysis of an Acid Base Titration Curve the Gran Plot Experiment # 9 Analysis of an Acid Base Titration Curve the Gran Plot

Procedure: We started with drying about 5.0g of Potassium Hydrogen Phthalate (KHP) in 105°C for about an hour. Then, we led it to cool down in the desiccator for 20min. Using the analytical balance in the balance room, we weighed out approximately 1.5g sample of KHP and dissolved it in 250-mL of distilled water. Then we prepared our standard 0.1M NaOH solution by adding about 5.3mL of 50wt% NaOH solution in 1L of boiling water. Once we made the solution we had to standardize it to obtain the exact concentration. To standardize our NaOH solution we weighted .51g samples of KHP which we dissolved in 25mL of distilled water, and titrated them with our NaOH solution. From the volume added we calculated the exact concentration of our standard NaOH solution.

Then, by using the pipette we transfer 100ml of KHP solution into 250-mL beaker, placed stirring bar in the beaker and added the phenolphthalein indicator. We placed the pH electrode in the beaker and measured the pH of the solution. We started with adding 1.5mL aliquots of the NaOH solution. Once we were within 4mL of the calculated end point we added base in 0.4mL aliquots, when within 1mL we added base by one drop at a time. After each addition we waited 30 sec and measured the pH of the solution. The end point was indicated by the appearance of the pink color. After end point, we added five more 1mL aliquots and measured the pH. The final step in this experiment was to plot the pH measured vs. the volume of the base added.

Question: Why is it important to standardize the NaOH solution and what is the proper way of doing it? We prepared the NaOH solution with approximately the desired concentration. If we want to obtain valid analytical results we need to know the exact concentration of our titrant that is why we need to standardize it. We prepared solutions of known concentration by dissolving measured masses of KHP (0.51g) in 25 mL of distilled water. Then we titrate them with our unknown solution of NaOH. From the volume added we calcualted the axact concentration of our standard NaOH solution.