How to titrate sodium hydroxide !

[bluemonster]

+With the so-called Winkler method the total alkali of a sample was
determined by titration with approximately 0.50N standard HC1 to a mixed indicator
(bromocresol green-methyl red) end point. The free hydroxide was then
determined on a duplicate sample by precipitation of the carbonate with excess
barium chloride and titration to a phenolphthalein end point with the standard
acid. By subtracting the free hydroxide value from the total alkali, the amount
of carbonate was obtained. Triplicate analyses by this method indicated that the stock NaOH solution contained an insignificant amount of carbonate (>0.01%) based on the total alkali.
(A David Fary, Jr)

+Brief explanation of the so-called Warder method:
By adding HCl to a sodium hydroxide-sodium carbonate mixture you completely neutralize the NaOH molecules:
NaOH + HCl → NaCl + H2O
The carbonate ions don't react to CO2, but they are protoned to Hydrogen carbonate.
Na2CO3 + HCl → NaCl + NaHCO3

End point of this reactions is determined with Phenolphtaleine (color turns from pink to colorless).
=>Volume of HCl needed until color turn of Phenolphtaleine = VA

Afterwards you add methyl orange and titrate until the solution turns from red to orange.
In this last step the hydrogen carbonate anions are completely protonated to carbonic acid:
NaHCO3 + HCl → NaCl + H2O + CO2
=>Volume of HCl used until Methyl orange color turns = Vb


=>V(HCl)to neutralize NaOH=VA-VB
=>V(HCl)to neutralize Na2CO3= 2*VB
(alberto_Kravina)