CO2 solubility was measured in a wetted-wall column in 0.6-3.6 molal (in) piperazine (PZ) and 2.5-6.2 m potassium ion (K+) at 40-110 degreesC. Piperazine speciation was determined using H-1 NMR for 0.6-3.6 m piperazine (PZ) and 3.6-6.2 m potassium ion (K+) at 25-70 degreesC. The capacity of CO2 in solution increases as total solute concentration increases and compares favorably with estimates for 7 to (30 wt.%) monoethanolamine (MEA). The presence of potassium in solution increases the concentration of CO32-/HCO3- in solution, buffering the solution. The buffer reduces protonation of the free amine, but increases the amount of carbamate species. These competing effects yield a maximum fraction of reactive species at a potassium to piperazine ratio of 2 :1. A rigorous thermodynamic model was developed, based on the electrolyte nonrandom two-liquid (ENRTL) theory, to describe the equilibrium behavior of the solvent. Modeling work established that the carbamate stability of piperazine and piperazine carbamate resembles primary amines and gives approximately equal values for the heats of reaction, DeltaH(rxn) (18.3 and 16.5 kJ/mol). The pK(a) of piperazine carbamate is twice that of piperazine. but the DeltaH(rxn) values are equivalent (similar to-45 kJ/mol). Overall, the heat of CO2 absorption is lowered by the formation of significant quantities of HCO3- in the mixed solvent and strongly depends on the relative concentrations of K+ and PZ, ranging from -40 to -75 kJ/mol. (C) 2004 Elsevier B.V. All rights reserved.