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Biological oxygen demand (BOD) of water sample analysis.

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Introduction

Biological oxygen demand (BOD) of water sample analysis Data collection Pipette solution Sample A: Fish pond water fixed on the day of collection (50cm�) Sample B: Fish pond water fixed three days after collection (50cm�) Burette solution Sodium thiosulphate (0.0250mol/dm�) My results: Sample A Sample B Trial 1 Trial 2 Trial 1 Trial 2 Burette readings (cm�) �0.10cm� Initial 0.00 4.00 8.00 10.30 Final 2.90 7.00 10.30 12.60 Total 2.90 3.00 2.30 2.30 Mean 2.95 2.30 Qualitative data: - Collected sample was very murky - The first end point colour was of a yellow colour, perhaps not light enough - Addition of starch indicator produced a grey-black solution; perhaps did not add enough indicator, however this did not pose as a problem - There was one occasion where I suspected to have added too much sodium thiosulphate, however the second trial indicated that I did not go past the end point too much Calculations To calculate the dissolved oxygen content (mg/dm� or ppm), we must first determine the number of mols of thiosulphate from the mean titrated volume of both samples. ...read more.

Middle

�0.10 0.33 33.0 �0.19 0.33 33.0 �0.10 0.33 33.0 �1.00 50.0 2.00 �0.10 2.30 4.35 Overall uncertainty (%) 105 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. ...read more.

Conclusion

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. ?? ?? ?? ?? 4 Stephanie Chan 13HT Chemistry HL - Mr. Fryer 1 ...read more.

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