1.0 OBJECTIVE

To measure level of pollutant in waste water by assess the strength of discharge waste

through oxidation and reduction agent

2.0 THEORY

Chemical Oxygen Demand is used as a measure of oxygen requirement of the sample

that is susceptible to oxidation by strong chemical oxidant. The dichromate reflux

method is preferred over procedures using other oxidants like potassium

permanganate because of its superior oxidizing ability, applicability to a wide variety

of samples and ease of manipulation. Oxidation of most organic compounds is 95-

100% of the theoretical value. 

Most applications of COD determine the amount of organic pollutants found in

surface water, making COD a useful measure of water quality. It is expressed in

milligrams per liter (mg/L), which indicates the mass of oxygen consumed per liter of

solution. Older references may express the units as parts per million (ppm).

To determine compute the Normality of Standard Ferrous Ammonium Sulfate ( FAS),

we use formula:

To determine the Chemical Oxygen Demand (COD) concentration in mg/L for the

sample, we use formula:

Volume 0.0416M K2Cr2O7 solution titrated, mL x 0.25

Volume FAS used in titration, mL

COD as mg O2 / L = [(A – B) x NFAS x 8000]

[mL sample]

3.0 PROCEDURE

1. Add substances to the round bottom flask (labeled as Blank (B) and Sample (S):

2. Reflux / boil the mixture of 2 hours chemical digestion

C (Organic matter ) + Cr2O72- H+ CO2 + H2O + Cr3+

BLANK

SAMPLE

BLANK (B)

20 ml distilled water

2.10ml potassium dichromate K2C2O7

3.30ml Sulfuric acid reagent (H2SO4 + Ag2SO4

SAMPLE (S)

20 ml water sample

2.10ml potassium dichromate K2C2O7

3.30ml Sulfuric acid reagent (H2SO4 + Ag2SO4

* Oxidizing agent ,Cr6+

is reduced to Cr3+

during digestion

* Organic matter are oxidized by oxidizing agent.

3. At a meantime prepare for standard ferrous ammonium sulfate (FAS) titrant.i. Add substances into the conical flask

ii. Cool the solution to room temperature.iii. Titrate solution with ferrous ammonium sulfate (FAS)

4. Titration

5. After 2 hours, disconnect reflux condenser.6. Transfer the solution to the cone flask and dilute the mixture up to 150 mL with

distilled water.7. Cool the solution to room temperature.8. Add with 3 drops of ferroin indicator. 9. Place the magnetic bars and stir it with magnetic stirrer.10. Titrate K2Cr2O7 with ferrous ammonium sulfate (FAS)

1. 90 ml distilled water

2. 10ml potassium dichromate K2C2O7

3. 30ml Concentrated Sulfuric acid reagent (H2SO4 )

1. Jot down first reading of burette.2. Titrate FAS until the color change from blue – green to reddish brown3. Stop titration.4. Jot down last reading.

4.0 RESULT

Table 1: FAS standard Titration Data:

Reading of burette / pipette Cone flask (standard)

First reading 0.0

Last reading 14.0

Volume of FAS standard (mL) 14.0

Calculation to determine the volume of FAS standard (ml)

Last reading - First reading = Volume of FAS standard ( ml)

Cone flask 2 (standard)

14.0 ml – 0.0 ml = 14.0 ml

5.0 DATA ANALYSIS

5.1 Calculation the molarity for the FAS standard:

Normality of FAS solution

Volume 0.0416M K2Cr2O7 solution titrated, mL x 0.25

Volume FAS used in titration, mL

Normality of FAS solution = 10 mL x 0.25 N

14.0 mL

= 0.179 N

Notes: The 0.25 is the Standard potassium dichromate solution, 0.0416M K2Cr2O

undergoes a six-electron reaction; the equivalent concentration is 6 x 0.0416M

or 0.2500N.

Table 2: Sample and Blank Titration Data:

Normality of FAS

(N)

Sample volume

(ml)

Volume of FAS used

In the original sample (S)

(mL)

In the blank sample (B)

(mL)

0.179 150 9.5 10.0

5.2 Calculation the COD concentration in mg/L for the samples:

COD as mg O2 / L = [(b – s) x NFAS x 8000]

[mL sample]

Where:

A = mL FAS used for blank

B = mL FAS used for sample

NFAS = Normality of FAS, and

8000 = milliequivalent weight of oxygen x 1000 mL/L

Notes: The 8000 milliequivalent is to express the results in units or milligrams per

liter of oxygen since 1 liter contains 1000mL and the equivalent weight of

oxygen is 6.

COD as mg O2 / L = ( 10.0 – 9.5 ) x 0.179 x 8000

10

= 71.6 mg/L

6.0 DISCUSSION

Chemical Oxygen Demand (COD) is commonly used to indirectly measure the

amount of organic compounds in water. Most applications of COD determine the amount

of organic pollutants found in surface water.

In this experiment, we use standard potassium dichromate (K2C2O7) solution

(0.04167 M) because it is a strong oxidizing agent under acidic conditions. The reaction

of potassium dichromate with organic compounds is given by:

In oxidizing process of the organic, potassium dichromate is reduced (since in all

radix reactions, one reagent is oxidized and the other is reduced), forming Cr3+. The

amount of Cr3+ is determined after oxidization is complete, and is used as an indirect

measure of the organic contents of the water sample.

Blank in this experiment is to make sure that no outside organic material be

accidentally added to the sample to be measured. A blank sample is created by adding all

reagents such as acid and oxidizing agent to a volume of distilled water. The oxygen demand

in the blank sample is subtracted from the COD for the original sample to ensure a true

measurement of organic matter.

For all organic matter to be completely oxidized, an excess amount of potassium

dichromate must be present. Once oxidation is complete, the amount of excess potassium

dichromate must be measured to ensure that the amount of Cr3+ can be determined with

accuracy. To do so, the excess potassium dichromate is titrated with ferrous ammonium

sulfate (FAS) until the entire excess oxidizing agent has been reduced to Cr3+. Typically, the

oxidation-reduction indicator Ferrous is added about 3 drops during this titration step as well.

Once all the excess dichromate has been reduced, the Ferroin indicator changes from blue-

green to reddish-brown. The amount of ferrous ammonium sulfate added is equivalent to the

amount of excess potassium dichromate added to the original sample.

In this experiment, COD is determined by using the formula;

Where b is the volume of FAS used in the blank sample, s is the volume of FAS in the

original sample, and n is the normality of FAS. If milliliters are used consistently for volume

measurements, the result of the COD calculation is given in mg/L.

In our experiment, the result that we got is not accurate because of the error while

experiment. The largest error is caused by using a non-homogeneous sample. Every effort

should be made to blend and mix the sample so that solids are never excluded from any

aliquot. Another like the equipments were not in good condition before starting the

experiment and the equipment is not really clean before used. Then, reading of COD may be

not accurate because less that 2 hour, the volume may be decrease during burning, loss of

material during testing and potassium dichromate (K2Cr2O7) was not mix and swirl well with

distilled water.

So, according to all of the errors, I suggest some method to improved and avoid the

error so that the error can not be repeated. The first thing is we should always use the largest

sample practical and use the largest glassware that is in keeping with good laboratory

practice. Then, we should use volumetric flasks and volumetric pipettes with a large bore. To

make sure the oxidizing agent (K2Cr2O7) can be measured precisely, we must use a

volumetric pipette and use the same one each time if possible. When titrating, be certain that

the burette is clean and free of air bubbles and we should always read the bottom of the

meniscus and position the meniscus at eye level.

7.0 CONCLUSION

According to the experiment, I can make conclusion that the Chemical Oxygen

Demand is to measure the level of effectiveness of waste water sample in water or waste

water.

After I do the experiment, I made conclusion that measurement of Chemical Oxygen

Demand ( COD ) is not only depend on the ability of microorganism to degrade the waste

but it is also about the knowledge of the particular substance. This experiment is not easy

like we assume because every mixture of substance should be doing carefully and if a

little mistake happen on our mixture, for sure it will affect to our result.

As a conclusion, I can also say that this COD test is including the measurement of the

ability of hot chromic acid solution to oxidize organic matter and pollutants in natural and

waste waters and to assess the strength of discharged waste such as sewage and industrial

effluent waters. This analyzes both biodegradable and non-biodegradable (refractory)

organic matter expressed as O2.

So for the last result of the experiment, we got the COD is 136.4 mg/L. Actually, we

should get our COD within range 60m/L to 80mg/L. This error maybe was affected by

several factors that we have discussed before.

8.0 QUESTION

i. What is the purpose of using blank in this experiment?

We use blank in this experiment is to compensate for any error that may result

because of the presence of extraneous organic matter in the reagents. It is to

make sure that no outside organic material will be accidentally added into the

sample to be measured.

ii. Name the oxidation and reduction agent used in COD test.

The oxidation agent that used in COD test:

i. Potassium Dichromate (K2C2O7)

ii. Oxygen (sulfuric acid, (H2SO4).

The reduction agent that used in COD test:

i. Ferrous ion ferrous ammonium sulfate, Fe (NH4)2(SO4)2.

iii. Give the chemical formula of the following:

a) Sulfuric acid = H2SO4

b) Ferrous Ammonium Sulfate = Fe (NH4)2(SO4)2

c) Potassium dichromate = (K2C2O7)

d) Argentums Sulfate = (Ag2SO4)

e) Mercuric Sulfate = (HgSO4)

iv. Calculate the molecular mass of the following:

a) Sulfuric acid = 2(1) + 32 + 4(16) = 98

b) Ferrous Ammonium Sulfate = 56 + 2(14 + 2) +2(32 + 4(16)) = 280

c) Potassium dichromate = 2(39) + 2(52) + 4(16) = 246

d) Argentums Sulfate = 108 + 32 + 4(16) = 204

e) Mercuric Sulfate = 201 + 32 + 4(16) = 297

v. Explain the precaution that must be taken during COD test?

To make sure the result of the experiment is accurate, we must know and

conscious about the precaution that must be taken during COD test is doing. First,

we must make sure all the equipment such as pipette, glass beads, flask and beaker

that we use along the experiment should be cleaned by using distilled water.

However, burette that we use in titration process should be cleaned by using FAS

because the mole of FAS may change easily when it mixed to any substance or

liquid. Then, we must make sure the volume of substance for each bottle must be

same. We should use glove to transfer the flask after heated from condenser to

fume hood. We must transfer the acid sulfuric reagent into substance in fume hood

because the reaction between acid sulfuric reagents with asid asetic will produce

poison gas that may effect our healthy. To avoid parallax error, we must be careful

during recording the read on the pipette.