2 | CHEMICALS AND APPARATUS
All the nine amines of Methyldiethanolamine (MDEA) (≥99% in mass purity), N-(2-aminoethyl)-1,3-Propanediamine (n-2AOE13PDA) (97% in mass purity), 2-Methyl-pentamethylene diamine (2-MPMDA) (99% in mass purity), N,n-dimethyldipropylenetriamine (DMAPAPA) (99% in mass purity), 3,3’-Diamino-n-methyldipropylamine (DAOMDPA) (96% in mass purity), Bis[2-(n,n-dimethylamino)ethyl]ether (2DMAOEE) (97% mass purity), 2-[2-Dimethylamino)ethoxy]ethanol (DMAOEOE) (98% mass purity), 2-(Dibutylamino)ethanol (DBEA) (99% mass purity) and N-propylethanolamine (PEA) (98% mass purity) were purchased from Sigma Aldrich Canada.
A Denver Instrument pH meter model 270 was used to measure the pH values of the amine solutions for the different temperatures at each titration step. The pH meter was calibrated with three pH buffer solutions purchased from VWR International. Buffer solutions at pH of 10.00, 7.00 and 4.00 were used. The uncertainties reported by the manufacturer for the buffer solutions were 0.02, 0.01 and 0.01 for pHs of 10.00, 7.00 and 4.00, respectively. The calibration information of the meter is reported in Table S2 in the SI section. The equipment and experimental procedures were validated by determining the pKa of Methyldiethanolamine (MDEA) and compared with the works in the literature.14-16 Table S3 is the validation comparison of MDEA in this study with data published in the literature. Note that the pH meter memory could only store three values around 10.00, 7.00 and 4.00, and therefore the reading would not very accurate if the pH values were much lower than 4.00 or higher than 10.0. pH values were therefore reported within 1.5 pH unit around the calibration values would be considered accurate enough for the study. Dissociation constants with pH values over 11.5 or under 2.5 were not be reported in the study.
Hydrochloric acid (HCl) was purchased from VWR International with a concentration of 0.100 M (±0.002 M). The cylinder gas of nitrogen (N2) with 99.99% purity was purchased from Praxair for flushing the space on the top of the amine solution to replace CO2 and O2 gases in the atmosphere. A water bath was used to control the experimental temperature by pumping water through a glass beaker that stores the amine. The uncertainty of the temperature was 0.01K. The aqueous amine solutions were mixed with the concentration of 0.01000 M (±0.00002 M) for at least 10 minutes before starting the experiments.
Initially, 50 mL of the solutions were filled into the storing beaker with a stirring magnetic bar (30 rpm) until vapour-liquid equilibrium was reached. The beaker was always closed to avoid oxidization and vaporization. The initial pH values of each solution were recorded. For each step in the titration process, 0.5 mL of hydrochloric acid was added to the aqueous solutions while stirred at a moderate speed. After 20 to 30 seconds, the pH values were recorded when values became stable.