Taste: The Sour and Bitter Story

Taste Acid / Base Chemistry is a large factor in one of our most complex and enjoyable senses. Acids are sour, Bases are bitter, and Salts are, well, salty --- and we can tell this without any instruments other than our bodies. Consider the reaction of a strong acid with a strong base:

The equilibrium of this reaction is so far to the right that we may write the reaction equation with just a forward arrow. The left hand side of the above reaction is vague intensionally because of the hidden process of solvation. The dissolution of the acid and base in water may be though of as occurring before the aqueous reaction takes place.

for the base

and for the acid

The salt is 'produced' immediately upon mixture of these species since the dissolution of the salt gives two of the product ions of the Acid Base reaction

Thus the 'actual' reaction of the acid with the base is the reaction that produces water from the remaining reacting ionic components

This affinity for these two special ions for each other (Keq >>1) is one of the most important driving forces in acid-base chemistry.

Arrhenius, of kinetics fame, gives us his definition of the species we taste as:

According to the simplest of Acid Base reactivity models, any acid base reaction can be identified as the following reaction

Consider the reaction of the weak acid (acetic acid, CH3COOH = HAc; Ka = 1.8 x 10-5) with a weak base (ammonia, NH3, Kb = 1.8 x 10-5)

It is clear that Acetic acid does indeed produce H+ in by dissociation in solution, and although this dissociation equilibrium leaves most of the acetic acid in solution undissociated, Arrhenius still would define HAc as an acid. But NH3 does not contain a hydroxyl group. The production of OH- by the dissolution of ammonia involves the reaction with water itself

So, indirectly, ammonia is an Arrhenius base because of its production of OH- as an aqueous species upon its dissolution.

Bronsted and Lowry have a different definition of what an acid and a base are. They see the above reaction as the transfer of a proton (H+) from the acid to the base. Thus

Note that this definition of acids and bases is relative, and the acid and base are defined in pairs. Moreover, upon examination of the reaction proceeding from right to left, on also finds a proton donor and an acceptor and thus an acid and a base. So according to B&L, all acid-base reactions are of the form