Determining pH During Titration Process

What factors determine the pH at various stages of the titration process?

How does the presence of different species and equilibrium reactions affect the pH during titration?

The pH at different stages of the titration process:

The pH at the various stages of the titration process depends on the species present and the relevant equilibrium reactions involved.

Explanation:

To calculate the pH of this titration process at the various stages, we need to both identify the species present at the stage and use the appropriate equilibrium.

(a) Before the addition of any HCl: At this step, no HCl has been added, so we look at trimethylamine in water, which is a basic reaction with water as an acid. Start by finding the initial concentration of the base using the formula Kb = [HA][OH-]/[A-]. Use the Kb and the initial base concentration to solve.

(b) After the addition of 13.4 mL of HCl: The starting moles of trimethylamine is greater than the moles of HCl added, forming a buffer solution. Write the equilibrium and calculate concentrations.

(c) At the titration midpoint: At this point, [HA]=[A-] and the solution is acting as an ideal buffer. Use the Henderson-Hasselbalch equation to determine pH.

(d) At the equivalence point: The moles of trimethylamine will be equal to the moles of HCl added. Calculate the excess HCl concentration and pH.

(e) After adding 56.2 mL of HCl: Calculate the excess HCl and determine the pH via the concentration.