Two methods are available to determine the reaction of a fluid. The first is the method of titration for acidity or alkalinity, in which a standard solution of acid or alkali is added until a certain change in the colour of a suitable indicator is detected. The second method is to determine the hydrogen-ion concentration present in the fluid. The latter is the only satisfactory method of measuring the reaction of a fluid. The hydrogen-ion concentration expresses the reaction of neutral, acid and alkaline solutions. The electrical is the standard method, but for clinical purposes is too intricate. The colorimetric method is less complicated. It is based upon the fact that each indicator has a characteristic zone of hydrogen-ion concentrations within which its colour changes occur. For details as to the theory and technique of this method, the reader may be referred to Clarke and Lubs (J. Bacteriol., 1917, ii), and Cole (Practical Physiological Chem., pp. 19–30).