The aim of this experiment is to measure the BOD and DO of water.

Authors Avatar

Experiment #14

December 17/2002

DO and BOD Winkler titration

AIM:

The aim of this experiment is to measure the BOD and DO of water.

INTRODUCTION:

The quality of water depends on several factors including oxygen-demanding wastes, disease-causing pathogens, microorganisms affecting health, plant nutrients, suspended solids and dissolved minerals. Other pollutants may be excess acidity due to acid rain, thermal pollution and substances such as benzene, chromium and mercury that are all toxic to aquatic life.

As humans and land animals obtain oxygen from respiration from the air, for plants and animals to survive in aquatic systems, water must contain a minimum amount of dissolved oxygen. The dissolved oxygen (DO) content of a body of water is an important indicator of its quality. At 200C, DO content of 8 to 9 ppm O2 at sea level is considered to be water of good quality.

The Biochemical Oxygen Demand (BOD) is a measure of the amount of oxygen consumed by the biodegradable organic wastes and ammonia in a given amount of water over a time period, normally 5 days at 200C. The greater the oxygen demanding wastes, the higher the BOD.  

The BOD of a sample of water can be determined by the Winkler method. The sample of the water is saturated with oxygen so the initial concentration of dissolved oxygen is known. A measured volume of the sample is then incubated as a fixed temperature for 5 days while microorganisms in the water oxidize the organic material. An excess of a manganese (II) salt is then added to the sample. Under alkaline conditions manganese (II) ions oxidise to manganese (IV) oxide by the remaining oxygen according to the following reaction:

Potassium iodide is then added which is oxidised by the manganese (IV) oxide in acidic solution to form iodine.

Join now!

The iodine released is then titrated with standard sodium thiosulfate solution.

By knowing the number of moles of iodine produced, the amount of oxygen present in the sample of water can be calculated and hence its concentration.

MATERIAL AND EQUIPMENT:

BOD bottle

250ml conical flask

50ml burette

H2SO4

NaOH

MnSO4

0.005M KI

0.05M Na2S2O3

Sample water

METHOD:

We fill a BOD bottle to overflowing with the sample and replace the bottle stopper so that no air is trapped in the sample. It is important to minimize sample mixing and exposure to air. ...

This is a preview of the whole essay