Environmental Analysis

1. What is the function of using KHP in the COD test?

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/(12×6 +12x 1+6×16)=5.56×10-3mole The ratio of glucose and oxygen is 1:6
The no. of mole of oxygen= 5.56×10-3 x 6
= 0.033mole
Therefore, the theoretical oxygen demands for glucose
=0.033 x (16×2)
=1.067 g oxygen/g glucose
CH3COONa + 2O2  2CO2 + H2O
The no. of mole of sodium acetate =1/(12×2 + 16×2 + 3×1 +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 (16×2)
=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)

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 shows that the amount of Cl- containing in flask 5 and 6 are the same.The different between in two flasks is that the amount of HgSO4 used in flask 6 is higher than flask 5. HgSO4 is used to eliminate Cl- in the sample. Cl- need to eliminate as it will react with dichromate which will affect the result to measure of oxygen equivalent of organic matter as the COD value will be more. In the experiment,3.0g of HgSO4 is added in flask 6 and 0.2g of HgSO4 is added in flask 5.As the more HgSO4 uesd ,the more Cl- can be eliminated.

Therefore, more Cl- can be eliminated in flask 6.As dichromate will react with Cl- ,so the amount of residual dichromate can show the amount of Cl- in sample.Therefore, when the amount of residual dichromate is less that mean more Cl- can be eliminated in the flask 6 than flask 5.So , less dichromate will used in flask 6 and the residual dichromate will be more .As the amount of residual dichromate is shown by the volume of FAS used ,the more amount of residual dichromate ,the more volume of FAS are used .The formula which used to calculate the value of COD is including the factor of the volume of FAS used ,so the more volume of FAS used ,the higher COD value of sample.

5. Compare the experimental COD values obtained for the sodium acetate samples(Flask No. 7 and 8).What is the effect of Ag2SO4 catalyst on the oxidation of sodium acetate ?

The experimental COD values of sodium acetate in flask 7and flask 8 are 334.67mg/l and -110.22mg/l respectively .The value in flask 7 is greater than flask 8.The different between flask 7 and 8 is that there is a catalyst (Ag2SO4) in flask 7 but it doesn’t have in flask 8. Ag2SO4 is used as a catalyst which will aid and increase the speed of the oxidation of sample during reflux. As the more oxidation of the sodium acetate in flask 7 than flask 8 ,the more dichromate will used and the residual dichromate will be lesser.Therefore,the volume of FAS used will be also lesser in flask 7and the COD value will be higher than the flask 8. 6. What is the reason for cooling the samples prior to titration ? (2 marks) If the sample are still in higher temperature, the oxidation of sample may still occur .Therefore the sample should be cool down which to ensure the sample are totally oxidized so that it will not affect the end point when titrate with FAS as the amount of residual dichromate in sample is affected by the oxygen equivalent in the sample.