Experiment Number 8

Quantitative Inorganic Analysis Laboratory Manual

EXPERIMENT NO. 8

QUANTITATIVE DETERMINATION OF DISSOLVED OXYGEN CONTENT BY

WINKLER REDOX TITRATION

OBJECTIVES

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At the end of this experiment, the student should be able to:

 perform the water sampling and pre-treatment techniques for dissolved oxygen analysis;

 determine the amount of dissolved oxygen in a water sample from a pond in the University

using Winkler redox titration; and

 discuss the chemistry behind the Winkler method for dissolved oxygen determination

INTRODUCTION

The dissolved oxygen (DO) levels in natural water and wastewater depend on the physical,

chemical and biochemical processes involved in the water system. Dissolved oxygen determination

is a key test for water pollution control and waste water treatment process control. Table 8.1 shows

the water quality guidelines, as per American Public Health Association, Inc.

Table 8.1 Dissolved oxygen content and water quality relationship

DO Content (ppm O2), 20°C

Water Quality

8–9

6.7 – 7.9

4.5 – 6.6

Clean, good water

7 – 11 ppm: ideal for stream

fishes including cold water

Slightly polluted

fishes

Moderately polluted, can sustain life of warm water fishes

Below 4.5

Highly polluted

0–2

Cannot sustain life

Th

Winkler method, the classical determination of DO in water is based on an oxidationreduction titration process known as iodometric method. The basis of this method is the oxidizing

power or ability of the dissolved oxygen to oxidize the divalent manganese in the solution. The

oxidized Mn is precipitated to hydroxides of higher valence states (Mn2+ → Mn3+) as Mn(OH)3 with

strong alkali. Upon acidification, the oxidized Mn(III) is reduced to Mn(II) in the presence of iodide

ions with subsequent liberation of iodine equivalent to the DO content of the water sample.

Several oxidizing and reducing substances, such...