Chemistry project

Chemical Oxygen Demand

Aim: To determine the chemical oxygen demand of sample water

Definition

The total amount of oxygen required to chemically oxidize the bio

degradable and non biodegradable organic matter.

PrincipleChemical oxygen demand (COD) is a measure of the capacity of water to consume oxygen during the decomposition of organic matter. COD measurements are commonly made on samples of waste waters or of natural waters contaminated by domestic or industrial wastes. Chemical oxygen demand is measured as a standardized laboratory assay in which a closed water sample is incubated with a strong chemical oxidant under specific conditions of temperature and for a particular period of time. A commonly used oxidant in COD assays is potassium dichromate (K2Cr2O7) which is used in combination with boiling sulphuric acid (H2SO4). Because this chemical oxidant is not specific to oxygen-consuming chemicals that are organic or inorganic, both of these sources of oxygen demand are measured in a COD assay.

• Potassium dichromate • Sulphuric acid • Ferrous ammonium sulphate • Silver sulphate • Mercury sulphate • Ferroin indicator • Organic free distilled water

Chemicals Required

• COD digester• COD vials with stand • 250 ml conical flask (Erlenmeyer flask)• Pipettes• Burette & burette stand • Pipette bulb • Tissue papers • Wash bottle

Apparatus Required

➢Cod values are particularly important in the surveys designed to determine and control the losses to sewer systems.

➢The ratio of BOD to COD is useful to assess the amenability of waste for biological treatment . Ratio of BOD to COD greater than or equal to 0.8 indicates that wastewater highly polluted and amenable to the biological treatment.

➢It is useful to assess strength of wastes ,which contain toxins and biologically resistant organic substances.

Environmental significance

Preparation

Preparation Of AgSO4:Take 0.5gm of AgSO4 powder in a test tube and add 100 ml of dil. Sulphuric acid and stir for 10 min.

Preparation of Ferrous Ammonium Sulphate Solution:Take 33 gm of FAS crystals in test tub and add 1000 ml of distilled waterSolution is found to be yellow.Add dilute sulphuric acid to remove the yellow colour.Now the solution turns nearly transparent.

ProcedurePipette out 10 ml of water sample into clean sample tube. Add 20 ml of Potassium dichromate solution, 30ml of silver sulphate solution and a pinch(0.5 gm.) of mercuric sulphate to it. Add one porcelain bit to the reaction vessel. Insert reaction vessel in the holes of the heating block, which has attained 150 degree Celsius, fix the air condenser on reaction vessel. Reflux the content for 30 min. ,cool to room temperature and transfer the contents in 200 ml conical flask. Transfer the contents by washing the tubes with 10 ml distilled water. Titrate the excess potassium dichromate with FAS solution using 4 to 5 drops of ferroin indicator. The end point is change of color from yellow to reddish brown.Repeat the same procedure with blank, Pond, using distilled water instead of sample.

Review 2 Readings..Types Of water

Volume OF sample taken(ml)

Burette Reading(Initial)

Burette Reading(Final)

Volume of FAS

Sewage Water

10 0 ml 27.9 ml 27.9 ml

Blank Water(Tap Water)

10 0 ml 28.4 ml 28.4 ml

Review 3 Readings..

Sample Water

Volume of Sample Taken(ml)

Burette Reading(initial)

Burette Reading (Final)

Volume OF FAS used.

Blank 10 0 ml 21.7 ml 21.7 ml

Pond 10 0 ml 16.9 ml 16.9 ml

Sewage 10 0 ml 12.4 ml 12.4 ml

Calculations

For The Pond water: Pond ppm= [0.1*1000*8*(21.7-16.9)]/10 = 304 ppm

For The Sewage water:Sewage ppm=[0.1*1000*8*(21.7-12.4)]/10

= 744 ppm