oxalic acid and sodium hydroxide question, urgent!!!

Sodium hydroxide will react with carbon dioxide in the atmosphere to form sodium carbonate and this will decrease the number of OH- ions in the solution with time, so NaOH is not a good standard solution. Oxalic acid will not have this problem, so it is a better standard solution.

OH- + CO2 ---> HCO3-
HCO3- + OH- ---> (CO3) 2- + H2O

or
NaOH + CO2 ---> NaHCO3

NaHCO3 + NaOH ---> Na2CO3 + H2O

I hope this can help your understanding. :)