During estimation of dissolved $\ce{CO2}$ in water, we took a water sample, added few drops of phenolphthalein and titrated it against dil $\ce{NaOH}$.
Questions
What are the reactions that happen:
- initially in the water sample?
- after addition of phenolphthalein (if any)?
- after addition of NaOH?
My speculations
Inside the sample water
I think dissolved $\ce{CO2}$ reacts with $\ce{H2O}$ to form $\ce{H2CO3}$. But then does all $\ce{CO2}$ get converted into $\ce{H2CO3}$?
After addition of phenolphthalein
In an acidic medium the equilibrium shifts to the left and the weak acid phenolphthalein ($\ce{HIn}$) mostly remains undissociated and hence colourless.
$$\ce{HIn <- H+ (excess) + In-}$$
After addition of $\ce{NaOH}$
According to my book after $\ce{NaOH}$ is added to the sample the following occurs: \begin{align} \ce{2NaOH + CO2 &= Na2CO3 + H2O}\\ \ce{Na2CO3 + H2O + CO2 &= 2NaHCO3}\\ \end{align} From my understanding \begin{align} \ce{NaHCO3 &= Na+ + HCO3-}\\ \ce{HCO3- + H2O &= H2CO3 + OH-}\\ \ce{OH- + H+ + In- &= H2O + In-}\\ \end{align} As the $\ce{H+}$ concentration decreases equilibrium shifts to the right $$\ce{HIn -> H+ + In-}$$
This $\ce{In-}$ ion imparts the pink colour which at pH 8.3 becomes susceptible to our eyes.
At the beginning some of the dissolved $\ce{CO2}$ got converted into $\ce{H2CO3}$. They remain beyond the scope of estimation, isn't it?
Are my speculations correct?