I test BOD of inlet sample of sewage water, in which there is no problem in inital DO but when i incubate it for 5 days, after incubation when i go for estimation of final DO, on addition of MnSo4 &Azide respectively the colour of bottle turns white insted of yellow & finally on addtion of H2So4 it totally yruns transparent. Whereas the other two samples of blank and outlet does not show such change.
Dear Parvaiz Rather
Most likely this white precipitate is calcium carbonate and/or sulfate depending on the pH and mineral composition of the sewage at time of sampling. When H2SO4 (acid) is added, then the pH drops and the carbonates and sulfates get fully dissolved. As result the sample becomes transparent.
Dear Parvaiz,
Here is some additional information that may also be of interest. The brown precipitate is MnO(OH)2, containing Mn(IV), which forms when dissolved oxygen (DO) oxidizes the Mn(II) added in the form Mn(OH)2. If there is no DO in the sample after incubation, then the brown color will not form. Were the inlet samples sufficiently diluted so that some DO remained after the 5-day incubation? A dilution factor of 20-60 or more may be needed, depending on the sewage strength.
By the way, the azide is used to prevent interference caused by any nitrite present. If the lab sink has a U-trap made of lead, the unreacted azide going down the drain when you pour out the BOD bottles can react with the lead to form explosive lead azide (PbN6), posing a real risk to any plumber or other person who services or otherwise contacts the trap.
Best of luck with your work!
- Peter
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