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Standartization of Sodium Thiosulphate

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pcm81:
I am testing chlorine in the pool water using a commercial kit and am interested in creating my own titration mixture. I am interested in standardizing sodium thosulphate as a titrant for Chlorine with iodine indicator.

The primary standard i have available to me is KHP.
I also have access to NaOH, KI, KMnO4, Oxalic Acid, sulfuric acid.
Per wikapedia: "Under normal conditions, acidification of solutions of this salt excess with even dilute acids results in complete decomposition to sulfur, sulfur dioxide, and water:"
Can i calibrate directly against KHP?
Any way to avoid release of SO2? I do not have a fume hood and would rather not be outside wearing a respirator... Neighbors already think i am crazy, no need to feed the rumors; not interested in earning the nickname Heisenberg either.
My target Cl in unknown sample (pool water) is 0-20ppm, so don't need super high concentration Sodium thiosulphate.

Borek:

--- Quote from: pcm81 on October 13, 2019, 09:49:22 PM ---Can i calibrate directly against KHP?
--- End quote ---

Highly doubtful. From what I remember decomposition doesn't consume the acid in the stoichiometric amount, it is catalyzed by its presence. Thiosulfate decomposes slowly even in the neutral solutions.

pcm81:

--- Quote from: Borek on October 14, 2019, 04:40:51 AM ---
--- Quote from: pcm81 on October 13, 2019, 09:49:22 PM ---Can i calibrate directly against KHP?
--- End quote ---

Highly doubtful. From what I remember decomposition doesn't consume the acid in the stoichiometric amount, it is catalyzed by its presence. Thiosulfate decomposes slowly even in the neutral solutions.

--- End quote ---
So, then would i need to calibrate via redox (indicated by color change) in KMnO4?
I have just verified, qualitatively, that there is a color change from dark red of KMnO4 with addition of Sodium Thiosulphate in acidic solution (added H2SO4) to lower pH. I have access to ORP probe. Should i expect the equivalence point being indicated by a steep slope like on pH curve in acid-base titrations or is color change alone sufficient to indicate titration end point of KMno4 vs Sodium Thiosulphate?

shchavel:
The best way to standatisation of thioslfate is too use KBrO3 as standard. But you need to buy it....

Borek:

--- Quote from: pcm81 on October 14, 2019, 08:51:49 PM ---So, then would i need to calibrate via redox (indicated by color change) in KMnO4?
--- End quote ---

I highly doubt. While thiosulphate - being a reducing agent - will definitely react with permanganate, titration requires system that is known to react stoichiometrically and predictably. For some reason all suggested methods of standardization of thiosulphate call for the reaction with I2, which suggests that's the only approach that is guaranteed to produce reproducible results.


--- Quote from: shchavel on October 19, 2019, 02:50:23 PM ---The best way to standatisation of thioslfate is too use KBrO3 as standard. But you need to buy it....

--- End quote ---

Methods I am aware off call for dichromate or KIO3. KIO3 would be definitely a bit better than KBrO3 thanks to the higher molar mass.

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