Chemistry - How does pH of soda drink vary with time
Solution 1:
I will make few simplifications:
I will neglect presence of other acids like citric or phosphoric acid, considering just water and carbon dioxide.
I will use the simplified exquation of $\mathrm{pH}$ of a weak acid ( :
$$\mathrm{pH}=\frac 12 (\mathrm{p}K_\mathrm{a} - \log {[\ce{CO2}]}) \tag{1}$$
- I will consider $\ce{CO2}$ escaping as the exponential process of the 1st order kinetics.
$$[\ce{CO2}]=A + B \cdot \exp{(-Ct)} \tag{1a}$$
- I will neglect the final equilibrium $\ce{CO2}$ concentration, compared to initial one:
$$[\ce{CO2}]=B \cdot \exp{(-Ct)} \tag{1b}$$
We can realize that the exponential function for $\ce{CO2}$ escaping kind of nullifies the logarithm function of $\mathrm{pH}$ definition:
$$\mathrm{pH}=\frac 12 \left(\mathrm{p}K_\mathrm{a} - \log {\left(B \cdot \exp{(-Ct)}\right)}\right) \tag{2}$$
$$\mathrm{pH} \simeq \frac 12 \left(\mathrm{p}K_\mathrm{a} - \log {B} + \frac1{2.303}Ct\right) \tag{3}$$
Therefore, $\mathrm{pH}=f(t)$ is approximately linear.
It is not linear if we consider all factors, especially if other acids are present.
Additional requested clarification:
The equation (1) is quite notoriously known simplified equation for $\mathrm{pH}$ of weak acid. It can be directly derived from definition of a dissociation constant of an acid:
$$K_\mathrm{a}=\frac{\ce{[H+][A-]}}{\ce{[HA]}} \tag{4}$$
involving 2 simplifications:
- solution is acidic enough to ignore water auto-dissociation:
$$K_\mathrm{a} \simeq \frac{\ce{[H+]}^2}{\ce{[HA]}} \tag{5}$$
- solution contains enough of a weak acid, so it's majority is not dissociated:
$$K_\mathrm{a} \simeq \frac{\ce{[H+]}^2}{c} \tag{6}$$
where
$$c=\ce{[HA]}+\ce{[A-]} \tag{7}$$
Then
$$\ce{[H+]}=\sqrt{ K_\mathrm{a} \cdot c} \tag{8}$$
After logaritmization of (8), we get (1).
Solution 2:
Pure water has a pH that goes form 7.00 at 25°C, to 6.92 at 30°C and to 6.13 at 100°C. As you obtain an effect which is contrary to this one, it means that your solution is loosing dissolved $\ce{CO2}$, so that the solution becomes more and more alcaline (or basic). After some time, you may obtain a precipitate of $\ce{CaCO3}$, depending on the composition of your soda.