Introduction

Acid–base homeostasis is essential to allow normal tissue and organ system function. Specifically, intracellular enzyme systems require an appropriate pH for maintenance of activity. Extracellular fluid pH is normally maintained around a level of 7.35–7.45. This is achieved through reversible chemical buffer systems and through homeostatic responses mediated by the lungs and the kidneys.

The pH scale

This scale for measurement was introduced by Sorenson in 1909 to facilitate dealing with hydrogen ion concentrations. This avoids the need to deal with negative indices and to accommodate the very wide range of H+ and OH- solutions that are encountered in acid–base reactions, when measured in nanomoles per litre.

The pH is the negative logarithm to base 10 of H ion activity. Acidic solutions always have a pH less than 7.0. The pH scale is an exponential scale, where a 0.3 unit fall in pH reflects a doubling of hydrogen ion concentration.

Bronsted–Lowry definitions

An acid is a proton donor to a base.

A base is a proton acceptor from an acid.

Acid–base reactions are proton transfer reactions.

An acid losing a proton to a base forms a base itself.

A base accepting a proton forms an acid.

Every acid has its conjugate base, and every base its conjugate acid.

An acid–base reaction is a dynamic equilibrium between two conjugate acid–base pairs.

A substance that can act as both an acid and as a base is called an amphoteric substance. Water has amphoteric properties.

The strength of an acid is specified by its dissociation constant.

A strong acid has a dissociation constant greater than that of H3O+ and is almost completely ionised in aqueous solutions. A weak acid has a dissociation constant less than that of H3O+ (1 in aqueous solutions) and is only partially ionised in aqueous solutions.

Factors affecting body acid–base balance

1. • Body acid production creates a hydrogen ion load, mainly from oxidation of proteins, carbohydrates and other organic molecules. This amounts to 50–100 mmol of hydrogen ions per day in the adult, and includes volatile acids (such as carbonic acid) and fixed acids (such as phosphoric and sulphuric acids). The body is a net producer of acid.