8/18/2019 Formal Report Experiment 8

1/4

QUANTITATIVE DETERMINATION OFDISSOLVED OXYGEN CONTENT BY WINKLERREDOX TITRATION

W. YBAÑEZNATIONAL INSTITUTE OF MOLECULAR BIOLOGY AND BIOTECHNOLOGY, COLLEGE OF SCIENCE

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

DATE SUBMITTED: 23 FEBRUARY 2016

DATE PERFORMED: 18 FEBRUARY 2016

ABSTRACT

INTRODUCTION

The dissolved oxygen (DO) content in

water is an important index in the

consideration of water suitability.

Sufficient concentration of DO is critical

for the survival of most aquatic life, aswell as in waste water treatment. It is a

key parameter for characterizing natural

and wastewaters and for assessing the

state of the environment in general.[1] 

The Winkler method is a premier and

classical method for the determination of

DO content in water sample. A sample of

water is collected in such a way that itsexposure to the atmosphere is minimized.

This is because exposure might alter the

level of dissolve O2. To fix the DO content

of the water sample, it is treated with a

solution of MnSO4, and then added with

and then with a solution of NaOH and KI

(hereafter referred to as alkaline iodide

azide).[2] 

The basis of this method is thequantitative oxidation of Mn2+ to Mn3+ by

dissolved O2 in the water sample. Then,

the trivalent Mn oxidizes I- to I2 in a

subsequent redox reaction; the amount of

I2 generated is determined by the titration

with a standard S2O32- solution. The

endpoint is determined by the color

change brought by the formation of a

starch-triiodide complex.[3] 

The objectives of the experiment was to

perform the water sampling and

pretreatment techniques accurately, to

calculate the DO content of the water

sample, and to discuss the chemistry

 behind the Winkler method for dissolved

oxygen determination.[4]

METHODOLOGY

Firstly, the solutions prepared for the

experiment were 25.0mL of 4.0M MnSO4,

25.0mL of alkaline iodide azide, 250.0mL

of 0.125M stock Na2S2O3, 50.0mL 0.5M

H2SO4, and freshly prepared starch

solution. Then, 250.0mL of 0.0125M

 Page1of 3

8/18/2019 Formal Report Experiment 8

2/4

standard Na2S2O3 solution was prepared

from the stock Na2S2O3solution.

For the standardization of the

Na2S2O3solution, 0.1500g KIO3 powderwas weighed and subsequently dissolved

into a 50mL beaker. The contents were

transferred to 100-mL volumetric flask

and bulked to mark. Three 10.00mL

aliquots were transferred each into 250-mL

Erlenmeyer flasks, diluted with 20.0mL

distilled water and added with 1.0g KI

and 10.0mL of 0.5M H2SO4. The solution

was then titrated with standard Na2CO3until pale yellow colour is observed. Until

then, 1.0mL of the starch solution was

added to the analyte. Titration was

continued until the disappearance of the

 blue colour. The procedure was repeated

for the other two flasks.

For the analysis of the water sample, a

glass bottle covered with aluminium foilwas filled to the brim with pond water. It

was added with 0.5mL MnSO4 solution

and 0.5mL alkaline iodide azide solution,

taking care to avoid inclusion of air

 bubbles. The sample was then mixed and

the formation of a brown precipitate was

observed. The sample was added with

2.0mL concentrated H3PO4, again taking

care to avoid the inclusion of air bubbles.The sample was mixed, and left to stand

for 10 minutes. 50mL aliquot of the

sample was transferred into a 250mL

Erlenmeyer flask and titrated with

standard Na2CO3 solution until pale

yellow colour was observed. 1.0mL of the

starch solution was added to the analyte

and titration was continued until the

disappearance of the blue colour. Theanalysis was performed in triplicate.

RESULTS AND DISCUSSION

The Na2S2O3 solution is not stable and is

therefore standardized against a primarystandard, IO3

- to become an effective

titrant. During the standardization, the

comproportionation redox reaction

 between IO3- and I- occurs as seen in (1).[5] 

−¿+3 H 2O(1)+¿→3  I 

3

¿

−¿+6  H ¿

−¿+8  I ¿

 IO3

¿

The reaction in (1) yields I3- which is then

titrated with S2O32-, as seen in (2).

2−¿(2)−¿+S

4O

6

¿

2−¿→3 I ¿

−¿+2S2O

3

¿

 I 3

¿

Therefore, integrating reactions (1) and

(2), we get a stoichiometric ratio of 1 mol

IO3- is to 6 moles of S2O3

2- (see Appendix C

for the dimensional analysis).

Before the standardization of the

thiosulfate solution with IO3-, excess KI

and H2SO4 was added to the solution

containing the primary standard, in that

specific order. KI was added for the

comproportionation reaction to occur.

H2SO4 is added to create an acidic

environment for the reaction, enabling it

to occur. It is important to add KI before

sulfuric acid since the H+ ions released

from H2SO4 will react with the IO3- in the

solution to form iodic acid. HIO3 is a weak

acid, and its formation from itsconstituents in solution is highly probable.

 Page2of 3

8/18/2019 Formal Report Experiment 8

3/4

8/18/2019 Formal Report Experiment 8

4/4