Biological oxygen demand (BOD) of water sample analysis.

Then we convert this to milligrams per dm³ first by multiplying by 20 to arrive with the number of mols present in 1dm³ (because the pond water sample used was 50cm³). Then we multiply that number by 32 (molecular mass of oxygen) to attain the number of grams of oxygen. Then finally, the number is multiplied by 1000 to be converted into milligrams.

The biological oxygen demand (BOD) would therefore be the difference between the DO of sample A and sample B.

Processed data

Graph

Uncertainties

The overall uncertainty for sample A is thus:

Similarly, the overall uncertainty for sample B is thus:

Conclusion

The presence of organic waste present in the water source is evident in that the amount of oxygen present in sample B (9.20) is clearly fewer than sample A (11.8). The biological oxygen demand calculated (2.60) from the titration results indicate that the water quality is in between very good and moderately clean. However, compared to my classmates, my sample had the second greatest BOD. This is not surprising as the source I collected from is next to a car park and road with frequent cars passing by; therefore it is likely there will be organic pollutants in the water.

Similarly, amongst my classmates’ water samples, Ian’s water from the valley had the greatest BOD – 3.20. This suggests that the valley is exposed to some form of pollution. Because the valley is behind our school, the most likely source is from the exhaust of the private cars that congest around the area. Similarly, Don also collected from the same valley, and his sample also had a relatively high BOD of 2.60. The water sample with the smallest BOD is Kenneth Ho’s tap water (0.32) – a good indication that our local tap water is safe and clean.

Evaluation

1. The overall uncertainty for sample A and B were very large, both exceeding 100%. This is because it is extremely difficult to measuring a small number such as 0.33cm3 precisely. Not only is the number small, but the hundredth decimal (0.03) is also required for an accurate measurement. However, there is little to avoid this problem as we were already using the smallest pipette available (1cm3 graduated pipette). The best way to avoid this problem is to increase the measurement to perhaps 1cm3. However, the rest of the measurements made in the experiment must also follow suit, so to have the mol ratios remain the same.
2. When collecting the sample, the bottle could have been improperly filled, and thus have a small pocket of air that could have dissolved into the sample water, thus making the data inaccurate. The bottle should have been closed inside the water source to avoid any air left in the bottle.
3. Oxygen could have dissolved into the water sample when fixing it. To avoid this, the adding solutions should be added inside the water sample, and not above it. Speed is also necessary when adding the solutions, because dissolved escape may also potentially escape out. To avoid this, the process can be carried out in an air tight location.
4. Sample A was not ‘fixed’ for oxygen immediately after collection. The recorded results can thus be lower than the actual. In addition, in the period of time in which oxygen has not been fixed yet, the sample could have been handled carelessly and the water could have been agitated, thus likely to affect the dissolved oxygen content. Both situations can be avoided if the sample was to be fixed immediately after collection.
5. The location in which my water sample was collected was next to a busy road side; therefore the indication that the water is moderately clean may not be accurate, because the sample was left unfixed for only three days. The period of time could be extended (perhaps to five days) in order to see the BOD more clearly.
6. It was quite difficult to determine when the solution became a “straw” colour. Thus, if starch indicator was added before it was a “straw colour”, an excess of sodium thiosulphate could have been added and thus alter the results. A standard solution with the “straw” colour could have been made prior the experiment for reference to ensure the precision.