Chemical Oxygen Demand (COD)Air and Water Analysis

Introduction

Chemical Oxygen Demand (COD) estimates the amount of organic matter in :Wastewaterindustrial 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 CODChemical oxygen demand (mg/L COD) is defined as the amount of oxygen consumed per liter under the condition of the experimentCOD 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 CODA 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 CODThe 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 watersThe higher BOD/COD more biodegradable organic compounds

The type of WaterBOD5/CODDomestic waste water 0.4-0.6 Domestic municipal after primer deposition 0.6 Air buangan domestik (penduduk) 0.2 River water0.1

Organic compounds that can/can not be oxidized

The type of the organic compounds COD BOD Biodegradable organics (protein, sugar, etc)Celullosa -Biodegradable N-organicNon-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 refluksAg2SO4 : katalisatorHgSO4 : menghilangkan ion Cl-K2SO4 : meningkatkan titik didihLarutan hasil dititrasi dengan lar.standar Fe(II) dengan indikator difenilamin

Water polluted by organic compoundsStandard solutionK2Cr2O7Excess volume

Ag2SO4, HgSO4 dan K2SO4Reflux 2h, 110oCUnreduced Standard solutionK2Cr2O7Fe(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 terjadiCaHbOcNd + 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 elektron1 mol Cr2O7= = 1,5 mol O2 1mol O2 =

Mol Cr2O7= = 6 mol Fe2+1 mol Cr2O7= = 6 elektron1mol O2 = 4 elektronmol 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 HgCl2Ion nitrit :Teroksidasi menjadi nitrat ?Pencegahan : dengan asam sulfamat

Pengambilan dan pengawetan sampel :Disarankan untuk menggunakan botol kaca atau botol plastik yang bersih dari zat organikJika mengandung Lumpur harus dikocok sampai merataSampel yang mengandung Fe(III) atau bakteri tinggi harus segera dianalisisPengawetan : penambahan asam sulfat pekat (pH 2)