Solubility of NH 3 and apparent p K of NH 4+ in human plasma, isotonic salt solutions and water at 37°C

Abstract

The solubility of ammonia, αNH 3 (mM/mmHg), was determined at 37°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 increases to 45.3±0.63 if expressed in molal units (mmol/kg plasma water·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 H 2O/kg plasma; mean density at 37°C: 1.020±0.002 kg/l). In pure 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) directly measured in pure water and 0.001 M aqueous NaOH. Based on the Henderson–Hasselbalch equation for the system NH 4 +/NH 3 in isotonic salt solutions and human plasma, both constants, apparent p K and solubility, can be derived from total ammonia concentration and pH at equilibrium with defined ammonia gas phase, if additionally the concentration of NH 4 + or NH 3 is known. This was verified, in the first case, by determining the concentration of NH 4 + by the experimental conditions, and in the second, by two measurements of total ammonia concentration at two different pH values. Total ammonia concentration was measured by a specific enzymatic standard test and pH with the glass electrode. The mean apparent p K was 8.968±0.013 in isotonic salt solutions ( n=7), and in human plasma ( n=10) it was 9.014±0.033.