Chemical Oxygen Demand (COD)

Air and Water Analysis

Introduction

Chemical Oxygen Demand (COD) estimates the amount of organic matter in : Wastewater industrial effluent and water from a variety of sources.

Standard method for indirect measurement of the amount of pollution (that cannot be oxidized biologically) in a sample of water.

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 procedure is based on the chemical-decomposition of organic and inorganic contaminants, dissolved or suspended in water.

Higher the COD, higher the amount of pollution in the test sample.

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

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

Theory of COD

The COD analysis is relative fast compared with the BOD analysis.

The COD of a waste is, in general, higher than the BOD because more compounds can be chemically oxidized than biologically oxidized.

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

Organic compounds that can/can not be oxidized

The type of the organic compounds

COD BOD

Biodegradable organics (protein, sugar, etc)

√ √

Celullosa √ -

Biodegradable N-organic √ √

Non-biodegradable N-organic √ -

Free NH3 (resulted by nitrification) √(*) Chain/linear and poly aromatic hydrocarbon

√(**) -

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

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

catalyst

COD Test :

Zat organik dioksidasi oleh larutan standar K2Cr2O7 sebagai sumber oksigen dengan volume tertentu dan berlebihan

dalam asam dengan pemanasan dan adanya Ag2SO, HgSO4 dan K2SO4 proses refluks Ag2SO4 : katalisator HgSO4 : menghilangkan ion Cl- K2SO4 : meningkatkan titik didih

Larutan hasil dititrasi dengan lar.standar Fe(II) dengan indikator difenilamin

Water polluted by

organic compounds

Standard solutionK2Cr2O7

Excess volume

Ag2SO4, HgSO4

dan K2SO4

Reflux 2h, 110oC

Unreduced Standard solutionK2Cr2O7

Fe(NH4)2(SO4)2

dan ferroin

Sisa Cr2O7= (yang tidak tereduksi) dititrasi

dengan lar.standar Fe2+ (V2) Titrasi juga dilakukan terhadap larutan

blanko volume lar.standar Fe2+ untuk menitrasi lar. blanko setelah refluks (V1) VN Cr2O7

= awal

Reaksi yang terjadi

CaHbOcNd + Cr2O7= + H+ E 2j CO2 + H2O + 2Cr3+

Ag2SO4, K2SO4

( Lar.kuning ) (Larutan hijau)

Reaksi pada titrasi :

6Fe2+ + Cr2O7= + 14H+ 6 Fe3+ +7H2O + 2 Cr3+

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

O2 + 4H+ + 4e H2O 1mol O2 = 4 elektron

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

Mol Cr2O7= = 6 mol Fe2+

1 mol Cr2O7= = 6 elektron

1mol O2 = 4 elektron

mol O2 /mol Cr2O7= = 4/6

1 Mol O2 2/3 mol Cr2O7= = 2/3 x 6 mol Fe2+ = 4 mol Fe2+

Cara perhitungan

(V1– V2.) MFe x 32 x 1000

COD = O2 = ------------------------------------- (mg/L) 4V

Gangguan :

Adanya ion Cl- : dapat bereaksi dengan katalis Ag2SO4 ?

teroksidasi oleh bikromat ?penghilangan : dengan HgSO4 HgCl2

Ion nitrit :Teroksidasi menjadi nitrat ?Pencegahan : dengan asam sulfamat

Pengambilan dan pengawetan sampel :

Disarankan untuk menggunakan botol kaca atau botol plastik yang bersih dari zat organik

Jika mengandung Lumpur harus dikocok sampai merata

Sampel yang mengandung Fe(III) atau bakteri tinggi harus segera dianalisis

Pengawetan : penambahan asam sulfat pekat (pH 2)