SAS4113 Environmental Analysis COD ASS

Module: SAS 4113 Environment Analysis

Name: Lee Ka Man (English) 李嘉雯 (Chinese)

Group no.: 1 Class: AS114103/A1 Date: 24/5/2013

Assignment 1 –Tutorial Questions on Chemical Oxygen Demand (COD) Experiment

1. What is the function of using KHP in the COD test? (2 marks)

KHP is used as a control standard because KHP is stable pure organic compounds which have the theoretical value of COD(500ppm), so it can used to gain an idea of how accurate the lab was.

2. Calculate the theoretical oxygen demands(g oxygen/g compound) for glucose and sodium acetate. (5 marks)
C6H12O6 + 6O2  6CO2 + 6H2O
The no. of mole of glucose = 1/(12x6 +12x 1+6x16)=5.56x10-3mole
The ratio of glucose and oxygen is 1:6
The no. of mole of oxygen= 5.56x10-3 x 6 = 0.033mole
Therefore, the theoretical oxygen demands for glucose
=0.033 x (16x2)
=1.067 g oxygen/g glucose
CH3COONa + 2O2  2CO2 + H2O
The no. of mole of sodium acetate =1/(12x2 + 16x2 + 3x1 +23)=0.012mole
The ratio of sodium acetate and oxygen is 1:2
The no. of mole of oxygen=0.012 x2 = 0.024mole
Therefore, the theoretical oxygen demands for sodium acetate
=0.024 x (16x2)
=0.768 g oxygen/g sodium acetate

3. What percent of oxidation of glucose is obtained in the COD test ? (3marks)
The experimental COD (mg/l)value for glucose
=[(24.85-19.90)x0.1002 x8000]/20.00
= 198.396mg/l
Oxygen demand of glucose =(198.396/1000)/(300/1000) = 0.661g oxygen/g glucose
Percent of oxidation of glucose
=[(0.661 g oxygen/g glucose)/(1.067g oxygen/g glucose)]x100%
=61.95%

4. Explain, qualitatively, the COD values obtained from the two NaCl-samples( Flask No. 5 and 6) (4 marks)

The COD value in flask 5 is 118.24mg/l and The COD value in flask 6 is24.05mg/l.The COD value in flask 5 is larger than in flak 6.The sample using in flask 5 and flask 6 are the same which is 1% NaCl solution. It