Stoichiometry to find the concentration of O2?

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Discussion Overview

The discussion revolves around the stoichiometric calculations to determine the concentration of dissolved oxygen (O2) in water using a Winkler titration method. Participants explore the chemical reactions involved and the procedural details of their experiments, including variations in conditions and reagents used.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Experimental/applied

Main Points Raised

  • One participant presents a stoichiometric calculation involving the reaction of Mn2+, O2, and OH- to find the concentration of O2, detailing the volumes and concentrations used in their experiment.
  • Another participant requests clarification on the procedure and the specific reactions involved, questioning whether sodium persulfate or sodium thiosulfate was used.
  • A later reply confirms the use of sodium persulfate in a biological oxygen demand experiment and outlines two trials with different conditions and volumes of Na2S2O8 used to reach equivalence points.
  • Another participant challenges the use of sodium persulfate, asserting that the Winkler method traditionally employs thiosulfate for titration and asks for the chemical equations of the reactions that occur during the experiment.

Areas of Agreement / Disagreement

Participants express differing views on the reagents used in the Winkler titration method, with some asserting the use of sodium persulfate while others argue that thiosulfate is the correct reagent. The discussion remains unresolved regarding the appropriate methodology and the reactions involved.

Contextual Notes

Participants have not fully detailed the assumptions behind their calculations or the specific conditions of their experiments, leading to potential ambiguities in the stoichiometric relationships and the reactions described.

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2 Mn2+ + O2 + 4 OH- -> 2 MnO2 + 2 H2O
***** Use the stoichiometry of the chemical reaction to calculate the concentration (mg/L) of O2(aq) and concentration of O2 (aq) (Molarity).
DI Water (22 degrees C)
Initial Buret Reading: 20.0ml
Final Buret Reading: 28.7 ml
Volume Na2S2O8 to reach the equivalence point: 8.7ml

I would appreciate any help, thanks!

This is what I have done:
1 mol O2 (2mol MnO2/1 mol O2) * (1 mol I2/1 mol MnO2) * (2 mol S2O3^-2/1 mol I2)= 4 mol S2O3^-2

0.00500 Na2S2O3 (0.0087L)= 4.35*10^-5/4mol= 1.0875 * 10^-5

---------------0.00500 M Na2S2O3 standard solution
---------------II. MnO2 (s) + 2I¯ + 4H+---> Mn2+ + I2 (aq) + 2H2O
III. 2S2O32- + I2 (aq) ---> S4O62-+ 2I
 
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It is a little bit chaotic, can you explain in more details what the procedure and reactions involved are? Have you used sodium persulfate, or sodium thiosulfate (I guess the latter, unless it was a back titration)?
 
Borek said:
It is a little bit chaotic, can you explain in more details what the procedure and reactions involved are? Have you used sodium persulfate, or sodium thiosulfate (I guess the latter, unless it was a back titration)?

It's a biological oxygen demand experiment-winkler titration. We used sodium persulfate.



We did two trials:
FOR TRIAL 1:
DI WATER (22 C)
VOLUME Na2S2O8 TO REACH EQUIVALENCE PT = 8.7ml
Find the concentration of O2 (aq) (mg/L)
Find the concentration of O2(aq) (Molarity)

TRIAL 2:
DI WATER (0 C)
VOLUME Na2S2O8 TO REACH EQUIVALENCE PT = 2.55ml
Find the concentration of O2 (aq) (mg/L)
Find the concentration of O2(aq) (Molarity)
 
Strange, as far as I was able to check Winkler method calls for thiosulfate to titrate iodine. One starts with Mn2+, it gets oxidized by the dissolved oxygen producing brown precipitate (containing Mn3+ and/or Mn4+), this precipitate after adding sulfuric acid oxidizes iodides to iodine, iodine is titrated with thiosulfate. No place for persulfate.

Can you write equations of reactions that take place during the experiment?
 

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