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