pK a Values of Some Piperazines at (298, 303, 313, and 323) K Farhad Khalili, † Amr Henni,* ,§ and Allan L. L. East ‡ Department of Industrial Systems Engineering and Department of Chemistry and Biochemistry, University of Regina, Regina, Sask. S4S 0A2 Canada, and Carbon Management, R & D Centre Saudi Aramco, Dhahran, 31311 Saudi Arabia The dissociation constants of the conjugate acids of six cyclic diamines [piperazine, 1-methylpiperazine, 2-methylpiperazine, 1-ethylpiperazine, 1-(2-hydroxyethyl)piperazine, and 1,4-dimethylpiperazine] were cal- culated using the potentiometric titration method at (298, 303, 313, and 323) K. The pK a values of piperazine were compared with published data to validate the procedure used. The thermodynamic quantities (ΔH° and ΔS°) for the dissociation processes were determined using the van’t Hoff equation. A trend is proposed related to the variation of the pK a with the addition of different radical groups to the base piperazine molecule. Introduction The amount of carbon dioxide in the atmosphere has increased dramatically during the last 50 years due to the combustion of fossil fuels and other hydrocarbons. Today, there is an inter- national inclination to moderate the release of CO 2 into the atmosphere. Carbon dioxide capture and sequestration can play a crucial role in achieving emission cuts required to control greenhouse gas levels. 1 Analytical scientists and separation engineers require an understanding of pK a because it impacts the choice of techniques used to identify and isolate the compounds of interest. pK a is the core property of any electrolyte and defines its biological and chemical behavior. In biological terms, the pK a value will give an idea about where the molecule will be found with a polar phase or nonpolar phase (partition). From a computational chemistry point of view, pK a calcula- tions are a benchmark for quantum mechanical and free solvation energy calculations. Aqueous solutions of alkanolamines such as monoetha- nolamine (MEA) are the most commercially used chemical solvents in postcombustion CO 2 capture technologies, and their applications in this process have been studied exten- sively. Recently, other aqueous solutions of diamines such as piperazine became of interest to scientists due to their fast reaction rate with CO 2 . Studies show that piperazine has a much faster reaction rate compared to MEA (the most commonly used solvent in CO 2 capture technology). 2 Di- amines (for example, piperazine) are also known to have a higher capacity (solubility) for CO 2 absorption than monoam- ines and can reach very high loading (higher than 3 mol CO 2 / mol piperazine) at very high CO 2 partial pressures. However, the absence of a hydroxyl group in piperazine makes it less soluble in water at high concentrations. The solvent is therefore usually proposed, for now, as an additive to other slower reacting alkanolamines such as methyldiethanolamine (MDEA). 3-7 The basicity of the solvent, quantified by the pK a of its conjugate acid, is a key factor for the reaction rate and absorption capacity of the solvent in the process. A linear relationship between the pK a of an acid or base with its catalytic effect on reaction rate was reported by Brønsted et al. 8 In previous work, a Brønsted relationship between the rate constant of the reaction of amines with CO 2 and the basicity of such amines was investigated. 2,9-14 The pK a values of many alkanolamines (mostly conven- tional amines) at different temperatures are available in the literature. 15-18 However, the pK a values of less common amines, especially piperazines, have not been reported yet. In this work, the pK a values of a series of six piperazines (diamines) were determined at different temperatures. Chemicals and Apparatus Piperazine, 2-methylpiperazine, 1-methylpiperazine, 1-eth- ylpiperazine, 1-(2-hydroxyethyl)piperazine, and 1,4-dimeth- * To whom correspondence should be addressed. Tel.: 966 3 872 5304. E-mail: amr.henni@aramco.com. † Department of Industrial Systems Engineering. ‡ Department of Chemistry and Biochemistry. § R & D Centre Saudi Aramco. Table 1. pH Values of the Calibration Buffers pH T/K buffer 1 buffer 2 buffer 3 298 4.00 ( 0.00 7.00 ( 0.03 10.00 ( 0.00 303 4.01 ( 0.02 6.99 ( 0.03 9.95 ( 0.05 313 4.03 ( 0.03 6.97 ( 0.00 9.87 ( 0.03 323 4.06 ( 0.02 6.98 ( 0.05 9.81 ( 0.03 Table 2. Comparison for First pK a of Piperazine with Literature Values T/K piperazine 298 303 313 323 Hetzler et al. 22 5.33 5.24 5.06 4.89 Hamborg and Versteeg 25 5.41 5.31 5.14 4.96 Pagano et al. 23 - 5.54 5.37 - Enea et al. 24 4.63 5.67 5.47 4.18 this work 5.35 5.27 5.02 4.93 Table 3. Comparison for the Second pK a of Piperazine with Literature Values T/K piperazine 298 303 313 323 Hetzler et al. 22 9.73 9.61 9.37 9.14 Hamborg and Versteeg 25 9.71 9.59 9.35 9.12 Pagano et al. 23 - 9.68 9.48 - Enea et al. 24 9.76 9.69 9.48 - this work 9.73 9.66 9.37 9.14 J. Chem. Eng. Data 2009, 54, 2914–2917 2914 10.1021/je900005c CCC: $40.75  2009 American Chemical Society Published on Web 07/15/2009