Determination of dissolved oxygen (do) in a given water sample

The equivalent weight of dissolved oxygen?

1. SHREE MALLIKARJUN COLLEGE CLASS: SYBSC
SAFETY: Refer to MSDS of manganese sulphate, sulphuric acid, potassium iodide, sodium thiosulphate,
starch
-Dr. Mithil S. Fal Desai

Aim: Determination of dissolved oxygen (DO) in a given water sample.
Chemicals: Manganese sulphate, sulphuric acid, potassium iodide, sodium thiosulphate, starch,
Apparatus: Standard volumetric flask, pipette, burette, conical flask, stop-cock, beakers, funnel and glass
rod.
Theory: Dissolved oxygen (DO) is one of the most important parameters in the analysis of water.
Molecular oxygen has poor solubility in water (8-15 mg/L). About 5-8 mg/L is considered to be good for
the aquatic life form. DO is also considered a critical parameter in boiler feed water as molecular oxygen
can enhance the rate of corrosion. In the determination of DO excess of manganese sulfate and
potassium iodide is introduced in a water sample. Addition of alkali like NaOH precipitates Mn
2+
as
Mn(OH)2 (white) which is readily oxidized by oxygen present in the solution and forms MnO(OH)2 (brown).
Re-dissolving this precipitates by addition of redox stable acid (H2SO4) liberates the oxygen which
oxidises iodide to iodine. This liberated iodine can be titrated against standardized thiosulphate solution,
thus this is an iodometric titration.
MnSO4 + 2NaOH  Mn(OH)2 + Na2SO4
2Mn(OH)2 + O2  2MnO(OH)2
MnO(OH)2 + H2SO4  MnSO4 + 2H2O + [O]
[O] + 2KI + H2SO4  K2SO4 + I2 + H2O
I2 + 2(S2O3)
2-
 (S4O6)
2-
+ I
-

From the above reaction following relation is concluded,
½ O2 ↔ MnO(OH)2 ↔ [O] ↔ I2 ↔ 2(S2O3)
2-
↔ 2e
-

Procedure: Pipette out 100 mL of water sample into a conical flask and add 0.5 g manganese sulphate,
and 0.5 g potassium iodide. Add dropwise 2N NaOH till permanent precipitates appear in the solution.
Stopper the flask and shake it vigorously for 5 min. Allow the precipitate to settle and add dropwise 2N
sulphuric acid until all of the precipitate dissolves. Titrate this solution against standardised Na2S2O3
solution using starch indicator.
Note: To observe a sharp endpoint starch needs to be added near the endpoint.
Results: i) Dissolved oxygen in a given sample of water = ____ppm.

Further reading (available in the library)
1) Experiments in Applied Chemistry, S. Rattan, S. K Kataria and Sons.
2) Vogel’s textbook of quantitative chemical analysis, J. Mendham, R. C. Denney, J. D. Barns, M.
Thomas, B. Sivasankar, Pearson India Education, 6
th
Edition.
Questions.
1) What is the significance of dissolved oxygen?
2) Why starch is added near end point?
3) Why KI and Mn salt is added in excess?
4) How quality of water is evident from the amount dissolved oxygen?
5) Can we use nitrate of chloride salts of manganese in the determination of dissolved oxygen?