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Re:A question for the Chemists

James Purchase wrote:

>.....My question is: how close an approximation of the B.O.D. can I get without
>using LaMotte's special test kit (which contains "seed capsules") if I
>merely use their Dissolved Oxygen Test Kit and maintain a sample of water
>(in the dark) at 20C for 5 days, i.e. - If I measure the D.O. in an aquarium
>on Day 1 and then take another sample and keep it in the dark at 20C for 5
>days and then measure the D.O. in that sample on Day 5? Is there something
>special in the "seed capsules" that are used in the B.O.D. kit or would the
>organics and bacteria naturally in the aquarium water be sufficient to
>provide at least a ballpark figure for B.O.D. in the tank?
>Last Sunday, the D.O. level in one of my tanks was measured at 7.0 ppm O2. I
>took another water sample and kept it in the dark at 20C for 5 days. Today
>(Day 5), I measured the D.O. in that sample and recorded the D.O. at 6.2 ppm
>O2. Am I correct in assuming that the B.O.D. in the aquarium is 0.8 ppm O2?
>If this is an accurate measurement of the B.O.D., how would this rate as an
>indicator of the health of the aquarium (the 0.8 ppm reading)?

The BOD determination requires that, during the five days, no oxygen get
into the sample. The difference between the initial oxygen reading and the
final one will then be an accurate measure of the oxygen used by bacteria
in breaking down the organic matter in the water.  Let's say you want to
measure the BOD of water from a nearby lake.  You get a gallon or so of
lake water and take one sample and measure the oxygen content, getting,
say, 7 ppm O2.  You take another sample and put it in a special BOD bottle
that has a glass stopper and a collar of glass around the hole where the
stopper is inserted.  You fill the bottle completely, right up to the brim
of the glass collar.  Then you insert the glass stopper tightly, and leave
the excess water in the collar.  There should be no bubbles of air in the
bottle, and the water in the collar will prevent air from being pulled past
the stopper into the bottle replacing water lost by evaporation.  The
bottle should be kept in the dark so that there is no oxygen produced by
photosynthesis.  After 5 days, the oxygen content of the water is measured
again, and has, let us say, dropped to 2.0 ppm.  The difference, between
the initial content, 7 ppm and the final content, 2 ppm, is the measure of
the BOD.  It is essential that no oxygen get into the bottle from the
atmosphere or from photosynthesis during the BOD determination.

The "seed capsules" would be sealed plastic capsules containing the
chemicals used for the oxygen determinations.

Paul Krombholz, in cool, moist, central Mississippi