1. My sample contains a residual chlorine compound, what should I do?
First, to avoid using samples containing residual chlorine, take samples before the chlorination process. it this is not possible, you must remove the residual chlorine prior to running the BOD test.
- If the sample has no detectable chlorine present but has been chlorinated, seed as normal
Note: Do not test chlorinated/de-chlorinated samples with out first seeding the dilution water
If Chlorine is present in the sample, then proceed to de-chlorinate the sample as follows:
- Allow your sample to stand. In some samples, chlorine will dissipate in 1-2 hours by standing in the light.
- If chlorine does not dissipate, add Na2SO3 to destroy the residual chlorine. To determine how much Na2SO3 solution to use, on a 100 to 1000ml portion of neutralized sample, add 10ml of 1:1 acetic acid or 1:50 H2SO4, 10ml of potassium iodid (KI) solution (10g/100ml) per 1000ml portion, and titrate with Na2SO3 solution ot the starch-iodine end point for residual
- Then, add a relative amount of the Na2SO3 in order to neutralize the residula chlorine. Check for residual chlorine after 10-20 minutes
If there is no chlorine residual in the sample, seed as normal.
2. How can sample residue cause problems?
Organic sample residue can cause problems by acting as an additional food source for the bacteria. The bacteria utilize the sample residual as a food source. Therefore they will eat more, breathe more, and produce more, causing excessively high depletions.