Question

What is the COD (chemical oxygen demand) of ethanol, expressed as g COD/g ethanol?

(a) 1.30 g COD/g ethanol

(b) 32 g COD/g ethanol

(c) 2.24 g COD/g ethanol

(d) 2.09 g COD/g ethanol

The question was asked in quiz.

My question is based upon Stoichiometry of Growth and Product Formation topic in division Material Balance of Bioprocess Engineering

Answer 1

Right choice is (d) 2.09 g COD/g ethanol

The best I can explain: Ethanol (C2H5OH) has a degree of reduction of 12 (mol electrons/mol ethanol). Oxygen has a degree of reduction of -4 (mol electrons/mol oxygen)and a molecular weight of 32 g/mol, which corresponds to 8 g oxygen (=COD)/mol electrons. Multiplying 12 by 8, this results in 96 g COD/mol ethanol, or, 96/46 = 2.09 g COD/g ethanol.