3 | CALCULATION OF DISSOCIATION CONSTANTS
(pKa)
As mentioned, the potentiometric titration method was used in this study
for determining the dissociation constants of MDEA and the other amines.
The suggested protonated mechanism of MDEA is reported in Equation 1.
Equation 2 is used for calculating the dissociation constants before any
thermodynamic correction.9
MDEAH+ + H2O MDEA +
H3O+ (1)
\begin{equation}
pK_{a}^{M}=pH+log\left(\frac{\text{MDEA}^{+}}{\text{MDEA}}\right)\text{\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ }\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ (2)\nonumber \\
\end{equation}The details of calculating the dissociation constants as well as the
thermodynamic corrections are in the Supporting Information section with
the values of A and B taken from the literature.17 In
Equation 3, the ionic strength (I) is a function of the molecular
concentration of ionization species (Ci) in the solution
and the valency of species (zi). In Equation 4, the
ionic strength is used to determine the extended Debye-Huckel activity
coefficient (γ) and ki is an adjustable parameter. This
parameter depends on the ion size and its value was reported in the
literature18 while Equation 5 is used to determine the
true protonated amine concentration.
\begin{equation}
I=0.5\sum{C_{i}z_{i}^{2}}\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ (3)\nonumber \\
\end{equation}\begin{equation}
\gamma_{i}=\ 10^{-\frac{Az_{i}^{2}\sqrt{I}}{1+Bk_{i}\sqrt{I}}\text{\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ }}\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ (4)\nonumber \\
\end{equation}\begin{equation}
\left[\text{MDEA}H^{+}\right]=\ \frac{\{MDEAH^{+}\}}{\gamma_{i}}\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ (5)\nonumber \\
\end{equation}For the first dissociation constant values, the value of ionic strength
(I) can be simply determined as in the example of MDEA reported in the
SI section. However, the ionic strength (I) for the second dissociation
constant can be calculated by the method of Albert et
al.13 as reported in detail in the SI section of the
literature.9