Variability in pK'1 of human plasma.

(1) Results were presented of an investigation of the relationship between ionic strength and PK'1 (negative logarithmic form of the apparent overall first dissociation constant of carbonic acid in plasma) in separated human plasma at constant PCO2. (2) Ionic strength was varied by adding dry NaCl to diluted aliquots of plasma from six healthy people and dry NaCl plus dry NaHCO3 to diluted aliquots of plasma from six other healthy people. pK'1 was determined from simultaneous measurements of pH and PCO2, and measurements of TCO2. Values for solubility factor for CO2 (s) were corrected for differences in plasma water and in Na concentration. All plasmas were equilibrated at 37 degrees C in a tonometer with a constant gas mixture (5% CO2, 12% O2, 83% N2). Precision of pK'1 determinations averaged 0.003. pK'1 was also determined on fresh undiluted healthy plasma, similarly tonometered. (3) We report (a) considerable variability in pK'1 of fresh undiluted healthy plasma (from 6.0197 to 6.1217); and also in extrapolated values for pK'1 at a notional zero ionic strength (6.179 to 6.325); (b) that variation in plasma ionic strength alters pK'1; (c) that change in plasma bicarbonate [HCO3]p can also change pK'1; (d) that change in pK'1, changes measured pH. (4) Implications are discussed.