How to titrate sodium hydroxide !

[bluemonster]

Recently, Mr Cu Thanh Long have a question for 04-05 students. It's content:
"How to titrate sodium hydroxide if it is mixed sodium carbonate as impurification???"
This is a good question, but, I have ever looked at this question is asked in our 4rum, hix, why???
Everybody join to discuss this problem, please !!!
And whereby, we will discuss to Warder and Winkler method !!! hehehe !

PS: you can discuss by Vietnamese !

[bluemonster]

Nguyên văn bởi votuantu20

ủa, sao bạn BM lại nói tiếng Anh vậy???

well, don't spam Tu, please !!!
Since we are discussing about the problem in class, and this is a question that anyone will have a plus mark !!! So, I do not discuss about this problem by vietnamese (when I also can discuss by English) as make easy to plus mark for everybody ! hahaha ! Can you answer this problem !?
Best regard !

[aqhl]

Hi all, let me try to answer this question. Can we use alkaline earth metals ions such as Ca2+, Ba2+, Sr2+ to precipitate carbonate ions. The solubility products of these substances are relatively small: CaCO3 = 4.96 x 10-9, BaCO3 = 2.58 x 10-9, SrCO3 = 1.6 x 10–9. Thereafter the solution of NaOH is commonly titrated. Is there something wrong in this question ? It looks quite easy !

Anyway, I have downloaded some papers about this problem. Due to the fact that analytical chemistry is not my favorite, I am too lazy to read them carefully. Let's read them for more information if you disagree with my answer.

Kind regards.

http://rapidshare.de/files/35755471/Na2CO3.zip

[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)