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Chemical Oxygen

Demand(COD)

Air and Water Analysis

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Introduction

Chemical Oxygen Demand (COD) estimates the

amount of organic matter in :

Wastewater industrial effluent

and water from a variety of sources.

Standardmethodfor indirect measurementof

the amountof pollution(that cannot be oxidizedbiologically) in a sampleof water.

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Theory of COD

Chemical oxygen demand (mg/L COD) is

defined as the amount of oxygen consumed per

liter under the condition of the experiment COD test procedureis based on the chemical-

decomposition of organicand inorganic

contaminants, dissolved or suspended in water.

Higher the COD, higher the amount of pollution

in the test sample.

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The standard test for COD involves :

Digesting the sample with strong sulphuric acid

solution in the presence of chromium and silver salts. During the oxidation of organic materials by

dichromate in sulfuric acid with a silver compound as

a catalyst, carbon is oxidized to form carbon dioxide

and hydrogen in the organic compound is converted

to water.

All organic compounds are oxidized completely

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Theory of COD

A mercuric compound is normally added to

reduce interference from the oxidation of

chloride ions by the dichromate. In this experiment, the dichromate is reduced to

green chromic ion (Cr+3) by oxidizable organic

compounds

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Theory of COD

The COD analysis is relative fast compared with

the BOD analysis.

The COD of a waste is, in general, higher thanthe BOD because more compounds can be

chemically oxidized than biologically oxidized.

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BOD/COD ratio in several waters

The type of Water BOD5/COD

Domestic waste water 0.4-0.6

Domestic municipal after primer

deposition

0.6

Air buangan domestik (penduduk) 0.2

River water 0.1

The higher BOD/COD more biodegradable organic compounds

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Organic compounds that can/can not

be oxidizedThe type of the organic

compounds

COD BOD

Biodegradableorganics (protein,sugar, etc)

Celullosa -

BiodegradableN-organic

Non-biodegradableN-organic -

Free NH3(resulted by nitrification) (*)

Chain/linear and poly aromatic

hydrocarbon

(**) -

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(*) : after 4 days can be prevented by adding

inhibitors

(**) : can be oxidized by addition of Ag2SO4

catalyst

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COD Test :

Zat organik dioksidasi oleh larutan standarK2Cr2O7sebagai sumber oksigen denganvolume tertentu dan berlebihan

dalam asam dengan pemanasan dan adanyaAg2SO, HgSO4 dan K2SO4 proses refluksAg2SO4: katalisator

HgSO4 : menghilangkan ion Cl-

K2SO4 : meningkatkan titik didih Larutan hasil dititrasi dengan lar.standar Fe(II)

dengan indikator difenilamin

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Water polluted by

organic compounds

Standardsolution

K2Cr2O7Excess

volume

Ag2SO4, HgSO4dan K2SO4

Reflux 2h, 110oC

Unreduced

Standard

solution

K2Cr2O7

Fe(NH4)2(SO4)2dan ferroin

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Sisa Cr2O7= (yang tidak tereduksi)

dititrasi dengan lar.standar Fe2+(V2)

Titrasi juga dilakukan terhadap larutanblankovolume lar.standar Fe2+untuk

menitrasi lar. blanko setelah refluks (V1)

VN Cr2O7=awal

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Reaksi yang terjadi

CaHbOcNd + Cr2O7= + H+ E 2j CO2+ H2O + 2Cr

3+

Ag2SO4, K2SO4

( Lar.kuning ) (Larutan hijau)

Reaksi pada titrasi :

6Fe2++ Cr2O7=+ 14H+ 6 Fe3+ +7H2O + 2 Cr

3+

1 mol Cr2O7= = 6 elektron 1 mol Fe2+ = 1 elektron

O2+ 4H++ 4eH2O 1mol O2 = 4 elektron

1 mol Cr2O7== 1,5 mol O21mol O2 =

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MolCr2O7== 6 mol Fe2+

1 mol Cr2O7== 6 elektron

1mol O2 = 4 elektron mol O2 /mol Cr2O7= =4/6

1 Mol O2 2/3 molCr2O7= =2/3 x 6mol Fe2+ = 4 mol Fe2+

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Cara perhitungan

(V1V2.) MFe x 32 x 1000

COD = O2 = ------------------------------------- (mg/L)

4V

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Gangguan :

Adanya ion Cl-:

dapat bereaksi dengan katalis Ag2SO4?

teroksidasi oleh bikromat ?

penghilangan : dengan HgSO4HgCl2

Ion nitrit :

Teroksidasi menjadi nitrat ?

Pencegahan : dengan asam sulfamat

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Pengambilan dan pengawetan sampel :

Disarankan untuk menggunakan botol kaca atau

botol plastik yang bersih dari zat organik

Jika mengandung Lumpur harus dikocok sampaimerata

Sampel yang mengandung Fe(III) atau bakteri

tinggi harus segera dianalisis

Pengawetan : penambahan asam sulfat pekat

(pH 2)

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