My BOD Blank is too low and/or negative. How can I fix it?
BOD blanks too low or negative
The blank must be below 0.20 mg/L, so "too low" means values that are negative. There are three possible causes for this:
- Micro-sized bubbles in the BOD bottle: Micro-sized bubbles can be difficult to see when the bottle is initially set. This is difficult to diagnose because 5 days later such bubbles may be fully dissolved. Carefully observe the neck and sloping sides of each BOD bottle after stoppering for the presence of bubbles.
- Incorrect DO measurement: Caused by an incorrect calibration, either before or after the incubation period. If you are using the Winkler Titration, use a minimum of three bottles for each dilution. If using the LDO probe, use air calibration and be sure that the calibration is recent.
- Incorrectly seated stopper: If the water temperature is too high initially (21 degrees or higher) and the stopper is simply "dropped in," oxygen exchange at the water-air interface gives negative blanks. Instead of obsessing with incubation water temperature an easier solution is to twist the stopper while seating it in each bottle.
- Be careful about bubble entrapment during setup. If you see bubbles in the bottles at setup, get them out.
- If you are using the Winkler Titration for performing the BODs, you must use a minimum of three bottles for each dilution. This means that you will be using 9 bottles * 2, or 18 bottles for each sample. For this reason the Winkler titration is not recommended.
- The Winkler titration is also highly dependent on technique. As a result it is more likely to be in error without warning than are Clark Cells or LDO probes that have been correctly air calibrated.
- If you are calibrating the LDO or Clark Cell with Winkler, then stop. Use air calibration instead. Most analysts have fewer problems with air calibration than with Winkler calibrations.
- It is absolutely essential that the stoppers be "twisted and seated" in order to guarantee good BOD results. If they are dropped in there is a high likelihood that occasional negative blanks will occur (higher DO afer 5 days than before in the bottle).