SOLUBILITY OF NH3 AND APPARENT PK OF NH4-SOLUTIONS AND WATER AT 37-DEGREES-C( IN HUMAN PLASMA, ISOTONIC SALT)

The solubility of ammonia, alpha NH3 (mM/mmHg), was determined at 37 d egrees C and low ammonia partial pressure (0.02-1 mmHg) in pure water (n = 24) as 46.7+/-0.40; aqueous isotonic salt solutions (n = 7) as 46 .8+/-0.81; and human plasma (n = 5) as 42.0+/-0.66. The last figure in creases to 45.3+/-0.63 if expressed in molal units (mmol/kg plasma wat er mmHg) instead of molarity with respect to the water content of the plasma (mean from four healthy and fasting donors: 0.908+/-0.005 kg H2 O/kg plasma; mean density at 37 degrees C: 1.020+/-0.002 kg/l). In pur e water, the solubility value is the mean of three different methods: (a) extrapolation of the salting-out effect of ammonia in aqueous NaOH to zero concentration; (b) slope of Henry-Dalton's law and (c) direct ly measured in pure water and 0.001 M aqueous NaOH. Based on the Hende rson-Hasselbalch equation for the system NH4+/NH3 in isotonic salt sol utions and human plasma, both constants, apparent pK and solubility, c an be derived from total ammonia concentration and pH at equilibrium w ith defined ammonia gas phase, if additionally the concentration of NH 4+ or NH3 is known. This was verified, in the first case, by determini ng the concentration of NH4+: by the experimental conditions, and in t he second, by two measurements of total ammonia concentration at two d ifferent pH values. Total ammonia concentration was measured by a spec ific enzymatic standard test and pH with the glass electrode. The mean apparent pK was 8.968+/-0.013 in isotonic salt solutions (n = 7), and in human plasma (n = 10) it was 9.014+/-0.033. (C) 1998 Elsevier Scie nce B.V. All rights reserved.