Titratable acid or base of body fluids.

S.P.L. Sgrensen not only introduced the p H concept, but in his classical paper from 1909 “Enzymstudien 11” he also described in great detail the colorimetric as well as the potentiometric methods for p H measurement, and he pointed out that two kinds of acidity must be distinguished: Actual acidity and titratable acidity. Previous to Sgrensen the acidity of a solution was usually determined by titration. But with the dissociation theory of Arrhenius (1887) it became clear that “true” acidity was related to the concentration of free hydrogen ions. Thusan aqueous solution of 0.1 molar HC1 and an aqueous solution of 0.1 molar lactic acid show the same acidity when titrated with base using phenolphthalein as indicator (100 mmol./l), but the “true” acidity, i.e. the hydrogen ion concentration is widely different, being 100 mmol./l and 3.7 mmol./ 1, respectively. In the newer terminology the word acidity is reserved for the hydrogen ion concentration or pH, while titratable acidity is simply called titratable acid. Decreasing acidity may also be designated increasing basicity (alkalinity). In the following the acid-base definitions of Brgnsted (1923) are used. Titratable acid (T .A . ) can be defined as the excess of acid in a solution determined by the amount of strong base it is necessary to add to the solution in order to produce a given final state. Titratable base (T.B.) can be defined as the excess of base in a solution determined by the amount of strong acid it is necessary to add to the solution in order to produce a given final state. The f ina l state is defined by the hydrogen ion concentration (activity), e.g. a p H of 7.40 in the case of urine, and as the hydrogen ion concentration varies with temperature the latter must also be specified. For a given final state a positive value for T.A. corresponds exactly to a negative value for T.B. Or in other words, an addition of strong acid is completely equivalent to the removal of strong base with respect to the acid-base conditions. The water concentration and the concentration of electrolytes will of course be different . The relationship between p H and T.A. is indicated by means of the titration curve (PH as abscissa and added strong base as ordinate, see FIGURE 1). The slope of the titration curve gives the buffer capacity defined by Van Slyke (1922) : p = d Base/d p H .