2.6 Determination of chemical oxygen demand (codMn) in water

The COD value indicates the amount of oxygen which is needed for the oxidation of all organic substances in water in mg/l or g/m3.

The COD (Chemical Oxygen Demand) is closely related to the laboratory standard method named Dichromate-Method. With this method the chemical oxygen demand is determined during chromic acid digestion of organic loads in waste water. Based on this method the COD became a commonly used sum parameter in waste water analysis. It is used for planning of waste water treatment plants, for controlling the cleaning efficiency and for the calculation of waste water taxes.

There is two ways of determination: oxidation using dichromate ion or using permanganate ion. Dichromate oxidation is used for waste water samples and permanganate oxidation for drinking and natural waters.

For determination of chemical oxygen demand in water our group employed: 9 test tubes, byrette, magnetic stirrer, magnetic bars, sulpuric acid (H₂SO₄, 4 mol/l), potassium iodide (KI, 0,1 mol/l), solution of KMnO₄ (0,002 mol/l), starch 1%. sodium thiosulphate (0,01mol/l) and two more solutions, which we prepared during lab: salicylic acid solution for quality control (2 mg/l) and solution of KMnO₄ of 0,02 concentration.

At first, we prepared two needed reagents for lab work. For making KMnO₄ of 0,002 mol/l we diluted 10 ml of KMnO₄ (0,02 mol/l) in 90 ml of distilled water in volumetric flask. Salicylic acid solution (2 mg/l) was prepared by diluting 1ml of salicylic acid of 1000mg/l in 499 ml of distilled water. After this we started to determinate. Our group had to prepare three different kind of samples: 3 blanks, 3 quality control samples, 3 water samples. For the blanks 10 ml of distilled water was pipetted into three tubes. For preparing the quality control samples we pipetted 10 ml of salicylic acid solution into three test tubes. For the water samples our group filled three test tubes by 10 ml of water. Then, 0,5 ml of 4 mol/l H₂SO₄ and 2,0 ml of diluted KMnO₄ (0,002 mol/l) was added into each tube. We put all 9 tubes in boiling water for 20 minutes and let them cool down in cold water bath. After cooling we added 1 ml of potassium iodide (KI) and 5 drops of starch solution in each tube. For titrating all three tubes we dropped 0,01 mol/l sodium thiosulphate to the point where the blue color disappears. To determine COD we needed to measure the expansion of sodium thiosulphate

Samples	№ of tube	Consumption of sodium thiosulphate,ml	Average consumption,ml
Distilled water	1	2,5	2,5
	2	2,5
	3	2,5
Treated water	1	1,1	1,33
	2	1,4
	3	1,5
Acid salicylic acid	1	2,2	1,9
	2	2
	3	1,4

.

For calculating results we needed to use the following formula:

COD_Mn = (V₂ – V₁) _* C_{1 *} 800 _* f , where

V₁ = consumption of sodium thiosulphate (ml) in titration

V₂ = consumption of sodium thiosulphate (ml) in titration of blank

C₁ = concentration of sodium thiosulphate (0,01 mol/l)

f = dilution factor

For treated water: COD_Mn = (V₂ – V₁) _* C_{1 *} 800 _* f

COD_Mn = (2,5 – 1,83) _* 0,01 _* 800 _* 1= 9,36mg/l

For salicylic acid: COD_Mn = (V₂ – V₁) _* C_{1 *} 800 _* f

COD_Mn = (2,5 – 1,9) _* 0,01 _* 800 _* 1= 4,8mg/l

COD_Mn for quality control sample should be 2,94 mg/l, but we get 4,8mg/l. There is quite long difference. It can be caused by uncertainness in measuring of volumes and concentration or by different impurities in solutions.