The pIi’ of Carbonic Acid in Cerebrospinal Fluid*

In the course of experiments concerned with respiratory control, it was desired to assess the effects of hyperthermia upon the pCOZ and [HCOa-] of cerebrospinal fluid. To use the HendersonHasselbalch equation (1) for this purpose required not only measurements of pH and total carbon dioxide concentration, but knowledge at various temperatures of the pK’ of carbonic acid1 and the solubility coefficient, X, of carbon dioxide in cerebrospinal fluid. The solubility of carbon dioxide in cerebrospinal fluid at different temperatures should be closely predictable from extensive measurements on solutions of sodium chloride of comparable ionic strength (2,3). However, the pK’ for carbonic acid in cerebrospinal fluid has been derived only at one temperature, 38” (4). The value of 6.13 obtained is considerably higher than that found for blood plasma (5). Since the necessary pH measurements in that study were performed with a calorimetric technique of limited accuracy, and because the data used in determining pK’ were not presented, it was felt necessary to carry out additional studies of the pK’ of carbonic acid in cerebrospinal fluid. In the present investigation, attempts were made to obtain further information regarding the absolute value of pK’ and the possible influences upon it of changes in pH and temperature.