Quantitative Determination of Dissolved Oxygen Content by Winkler Redox Titration

QUANTITATIVE DETERMINATION OF DISSOLVED OXYGEN CONTENT BY WINKLER REDOX TITRATION

D. UAYAN

INSTITUTE OF BIOLOGY, COLLEGE OF SCIENCE

UNIVERSITY OF THE PHILIPPINES, DILIMAN, QUEZON CITY 1101, PHILIPPINES

DATE SUBMITTED: 11 FEBRUARY 2014

DATE PERFORMED: 30 JANUARY 2014

ANSWERS TO QUESTIONS

1. Give the pertinent chemical equations and stoichiometry in the standardization of Na2S203.

Na2S203 was first standardized using the primary standard KIO3 with starch as the indicator. This reaction yielded a chemical equation:

IO3- + 5I- + 6H+ ↔ 3I2 + 3H2O (1)[1]

Since iodine is a halogen, it reacted with the water produced in the first reaction:

I2 + H2O ↔ H++ I-+ HIO (2)[1]

A pale yellow solution would then be obtained after the first titration. Afterwards, the presence of iodine was then implicated by the addition of starch[3].

Another titration was made after the addition of starch, and slowly the blue color faded:

I3- + 2S2O32- ↔ 3I- + S4O62- (3)[1]

The moles produced in every equation would be used to determine the moles of O2 in the sample analysis titration.

2. Explain the purpose of the addition of H2SO4 and excess KI during standardization and why the acid was added before KI.

To produce more I2, the application of the principle of the common-ion effect was observed[2]. The addition of H2SO4 before the titration occurred would cause the equilibrium to shift towards the left, producing more iodine. Similarly, the addition of KI would yield a sharper titration endpoint as a result[2].

3. Give the pertinent chemical equations and stoichiometry in the sample analysis. Explain stepwise how I3 was produced from the dissolved O2 in the water sample. Explain briefly why the reagents are added in a definite sequence.

The addition of MnSO4 to the water sample yielded this reaction:

Mn2++2OH-↔MnOH2 (4)[1]

The dissolved oxygen was then chemically bound to MnOH2, which then resulted into a brown “floc”, also known as a manganic...