Question

1.) Explain why the first derivative of the titration curve can be used to identify the equivalence point of an acid-base titration (or equivalence points for polyprotic acids).

2.)  Explain the effect of pKa on endpoint detection for the titration of a weak acid with a strong base:

3.) Explain the effect of acid strength (molarity) on endpoint detecton for the titration of a weak acid with a strong base.

Answer 1

1) A first derrivative plot is a plot of delta(pH)/dV on y-axis vs volume of titrant on x-axis. The y-axis is basically slope. The plot shows a sudden change of pH at the equivalence point (1 for monoprotic acid) for the acid at every equivalent points for the polyprotic acid. That is the slope delta(pH)/dV suddenly increases or is maximum at the equivalence point and thus can easily be identified for determination of equivalence points in the acid-base titrations.

2) If the pKa is low the end point will be present at low volumes of titrant, similarly If the pKa value is large, the end point would be at higher volumes of the titrant added.

3) For a weak acid the end point will lie at basic pH > 7 value since the solution has excess of OH- ion at this point. However, in case of strong acid titration the end point would lie close to pH = 7, since equal amounts of both H+ and OH- are present in solution.