bicarbonate (or sodium hydroxide) and severe metabolic alkalosis with hydrochloric acid.2 When acid or alkali is administered therapeutically in 50-100-mmol quantities it is rational to express their impact on the patient in the same way. There are two less familiar ways of expressing acidity within the SI. The most scientifically rigorous is to express acidity in terms of the chemical potential of H ion; the SI unit for energy, the joule, is used for this purpose so that normal arterial plasma acidity becomes-44-23 to-43-76 kJ/mol. J One can easily imagine that this unit would not be acceptable in ordinary medical use. The fourth term for acidity is the concentration of free H ions in nmol/l derived from pH in a stricter manner than indicated above. Readers seeking more information about this complex matter are referred to Professor Siggaard-Andersen's contributions cited below.' s

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