Question

You have read about titrations of strong acids with strong bases, weak acids with strong bases, and weak bases with strong acids. What if you titrated a weak acid with a weak base? Sketch a pH curve and defend its shape. Label the equivalence point and discuss the possibilities for the pH value at the equivalence point.

Short answer

There is just a ‘point of inflexion’ at the equivalence point. The absence of any steep change in pH all over the titration renders titration of a weak base versus a weak acid difficult, and not much information can be extracted from such a curve.

For strong acid and strong base titration, the pH at the equivalence point is$pH=7$.

For weak acid and strong base titration, the pH at the equivalence point is$pH>7$.

For strong acid and weak base titration, the pH at the equivalence point is$pH<7$ .

Step-by-step solution

Define equivalence point

In a titration, the equivalence point and endpoint are two significant terms. Equivalence point refers to the exact point at which a chemical reaction (among titrant and analyte) ends. It is observed prior to the endpoint

Explain the answer

Let us consider a weak base such as as analyte and a weak acid such as as titrant. We start plotting the pH of the analyte against the volume of the titrant, that is, acid. The plot is as shown below.

Analyte versus titrant

If we notice, there isn’t any steep bit in this plot. There is just a ‘point of inflexion’ at the equivalence point. The absence of any steep change in pH all over the titration renders titration of a weak base versus a weak acid difficult, and not much information can be extracted from such a curve.

The pH of the solution at the equivalence point is dependent on the strength of the acid or base used in the titration.

For strong acid and strong base titration, the pH at the equivalence point is equal to 7.

For weak acid and strong base titration, the pH at the equivalence point is >7.

For strong acid and weak base titration, the pH at the equivalence point is <7.