Regarding Analysis for CO2 Absorption in NaOH
Phenolphthalein becomes colorless again (from pink) in strongly basic medium with time. It is a known phenomenon called the alkaline fading of phenolphthalein. https://scholar.google.com/scholar?hl=en&amp;as_sdt=0%2C44&amp;q=fading+of+phenolphthalein+in+alkaline+solution&amp;btnG=

It is not a good choice to use phenolphthalein in the presence of sodium carbonate or bicarbonate. Methyl orange is typically used when NaOH and Na2CO3 are present together. Please consult the classic Vogel's "Quantitative Inorganic Analysis" for more details. Alternatively, you may try a potentiometeric titration (pH vs. volume of titrant). Measure the pH after every 0.5 mL addition of titrant and construct a titration curve. 

The reaction of CO2 with NaOH is very fast.