As you have already seen, water is a very active molecule, forming and breaking hydrogen bonds between its molecules (see Figure 3.12, p. 19). This constant juggling of hydrogen bonds gives the water molecule the capability to break apart or disassociate.

Acid–base reactions occur in the presence of water as a result of this quality of disassociation of water into hydrogen ions (protons) and hydroxyl ions:

The environment of living tissues is largely water and when certain compounds enter this environment they can be encouraged to disassociate. The degree to which they dissociate depends on the number of hydrogen ions present. It is this balance of hydrogen ions that forms the basis for acid–base mechanisms.

Acids disassociate in water to form a hydrogen ion and the ion of the acid (Figure 8.1). In other words, in the right conditions, acids tend to give away their hydrogen ions, donating them to the aqueous solution.

Figure 8.1 The action of acids and bases with regards to the hydrogen ion. Note that each reaction is reversible; this is a very important feature of these reactions in pharmacology.

The strength of an acid depends on the number of hydrogen ions in solution. The more hydrogen ions there are, the stronger the acid. Weaker acids such as ethanoic acid, otherwise known as vinegar or acetic acid, are weak acids because they are not very good at letting go of their hydrogens, compared to a strong acid like sulphuric acid, which is prepared to give away a very large amount. The difference in activity can be appreciated by the fact that putting sulphuric acid on your chips would result in them being disintegrated.

Bases accept hydrogen ions from aqueous solutions in the right conditions (Figure 8.1). A strong base is one that is very keen to grasp free hydrogen ions, whereas a weak base is not good at grasping free hydrogen ions. The result, regardless of strength, is a compound that has a positively charge ion.

Acids are thought of as proton donors (give away protons) whereas bases are considered to be proton acceptors (accept protons).

The pH range extends from 0 to 14. Although academics will argue that the situation is more complex than this, this range is more than adequate. The middle of this range – pH 7 – is considered neutral. Any number greater than 7 means it is a base and any number lower then 7 means it is an acid:

So, the higher the number, the stronger the base; the lower the number, the stronger the acid.

pH is a measure of the hydrogen ion concentration. It is usually calculated by measuring the concentration of hydrogen ions in a solution using something called logarithms. This is why it might seem a little strange that a solution with a low pH has a high concentration of hydrogen ions and one with a high pH has a low concentration of hydrogen ions. Discussion of how this logarithmic calculation is worked out is beyond the scope of the book, but the point is worth mentioning as this apparent contradiction can be confusing.