Question

A standard procedure for improving the detection of the stoichiometric point in titrations of weak bases with strong acid is to use acetic acid as a solvent. Explain the basis of this approach.

Answer 1

The strength of an acid or a base is dependent on the solvent. The amount of a kind of ion that exists in a solvent due to the presence of an acidic or basic compound determines its acidity (or basicity). To understand what I mean by this, consider water, our favourite solvent. Water dissociates into H₃O⁺ and OH^-, the strongest acid and base that can exist in water respectively. Thus a compound which completely protonates water to form H₃O⁺ is a strong acid and one which completely deprotonates water to form OH^- is a strong base. Incomplete formation of either points to a weak acid or base. However, the issue here is that many acids or bases are levelled off by the solvent. Several strong acids or bases dissociate to the same extent in water to form hydronium or hydroxyl ions. This is not really a favorable situation if the strength of the base is very low - the consequence would be that end points are not sharp enough for easy detection. This is where non-aqueous titrations come in.

If you use a solvent that is mildly acidic or basic, you can tune the acidity or basicity of your compound. Thus a weak base in water (which is a poor acid) is a stronger base in an acid (acetic acid). The advantage with an amphiprotic solvent like acetic acid is that it can both donate and accept protons. By enhancing the basicity of a weak base, it makes its titration end-point with a strong acid (generally, perchloric acid) very easy to determine. Also the slightly protophilic nature of acetic acid makes it more sensitive towards the strength of acids used as well - it can differentiate between perchloric, hydrochloric, sulfuric and nitric acids all of which behave as equally strong acids in water.

Thus the main idea of using acetic acid as solvent is improving the ease of detection of the end point by enhancing the basicity and acidity of the bases and acids, with respect to water.