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.