do we need to know how to test for BOD???? (1 Viewer)

[poodoorfan]

do we need to know the equations and how to test for BOD and dissolved oxygen or just the name and general idea of how it's tested for?

 

[kerry]

it depends on whether you want to enter your exam feeling under prepared or over prepared.


i always like the latter option, however it is a personal thing.


there are a number of ways to test water quality, so if the question asks you to state two or three then you will be laughing.


but they could ask a question along the lines of...

sally took two water samples from her local catchment. the first sample she tested ASAP and the amount of dissolved oxygen was x.
the second sample ske kept on the window ledge, and tested five days later. the amount of DO was y.

what was the amount of oxygen used by bacteria and other assorted micro organisms in sallys' water sample?


and then they may ask you how to improve the validity and reliablity of the experiment... and if you don't know that it is imperative to keep the sample in a dark place, you are in trouble.

so here is something i just copied straight from CSU website. merry christmas!!


Biochemical oxygen demand (BOD)
BOD measures the amount of oxygen used by bacteria and other micro-organisms during a five-day period. The sample bottles are held below the water surface and away from the bank. One sample is measured for DO as soon as possible while the other sample is kept in a dark place for 5 days and then tested for DO. The BOD is calculated by subtracting the DO value after 5 days from the initial DO value. The reading is given as milligrams per litre (mg L-1).

 

[kerry]

ps. here is a summarised method of measuring dissolved oxygen levels!!

Sampling Method
Rinse the sampling bottle (0688-DO) with the sample to be tested.
Replace the top on the empty sampling bottle and submerge it in water sample to be tested.
After submerging the sampling bottle, remove the top under the water and fill the bottle while tapping the sides to release the air bubbles.
Replace top while the bottle is submerged.
Remove the full, capped bottle from the water and remove the top.

Preserving (fixing) Sample for Testing
Add 8 drops of Manganous Sulfate Solution (4167) to the bottle of sample water.
Add 8 drops of Alkaline Potassium Iodide Azide (7166) to the bottle of sample water. Put the top on the bottle and invert 2 or 3 times. A precipitate will form and should be allowed to settle below the shoulder of the bottle before continuing with the next step.
Use the 1.0 gram measuring spoon (0697) provided with the kit to add 1 gram of Sulfamic Acid Powder (6286).
Put the top on the bottle and invert to mix until the precipitate is dissolved. A clear yellow to orange brown color will develop depending on the oxygen content of the sample.
The sample is now "fixed" and ready to be tested.

NOTE: Once the sample is fixed, contact between the sample and the atmosphere will not affect test results.
Testing the Water Sample
If the color of the fixed sample is already a faint yellow, skip steps #3 and #4 and continue with the second part of step #5 which is the addition of Starch Indicator Solution.

Fill the titration tube (0299) to the 20 ml mark with the fixed sample.
Fill the Direct Reading titrator (0377) with Sodium Thiosulfate (4169).
Place the titrator in the top of the titration tube and add one drop of the Sodium Thiosulfate and swirl.
Continue to add Sodium Thiosulfate dropwise until the sample is a faint yellow.
Remove the top of the titration tube and the titrator. Add 8 drops of Starch Indicator Solution (4170WT). This will cause the sample to turn blue.
Replace the top of the titration tube and the titrator. Continue to add Sodium Thiosulfate dropwise until the blue color disappears.
Read the titrator scale in parts per million (ppm).
NOTE:If the titrator is empty (10 ppm) and a color change has not been observed, refill the titrator with Sodium Thiosulfate and continue the titration until the color change occurs.

If more than one dissolved oxygen reading is taken, take average of all the readings and record that number.

Most natural water systems require 5-6 parts per million of dissolved oxygen to support adiverse population.

 

[poodoorfan]

ok thanks for that.

 

[Dreamerish*~]

I'm quite sure Kerry is talking about Winkler's method, for which we need to know nothing else than the name.

Look here and scroll down until you see the bit about the oxygen-sensitive electrode. This would be the method to learn if you choose to be over-prepared. It's a lot easier to remember than a heap of equations.

This is basically the same thing as DO - essentially, it's just DO performed twice - once at the beginning and once again five days later to measure the difference.

The syllabus dot point for this section says:

• Identify that water quality can be determined by considering ... dissolved oxygen and biochemical oxygen demand.​

It doesn't look like we need to go into that much detail.

 

[Casmira]

biological oxygen demand can be tested by leaving a sample of water in a dark cupboard for 5 days and testing oxygen levels before/after ?

 

[Dreamerish*~]

biological oxygen demand can be tested by leaving a sample of water in a dark cupboard for 5 days and testing oxygen levels before/after ?

Are you asking a question or making a statement?

If it's the latter, you're right. Except it's at 20ºC.