Blank is too Low
First of all, what is meant by "my Blank is too low"? The blank must be below 0.20 mg/L, and it would seem that any low value would be good. But what happens if the blank is -0.05 mg/L? What does this mean? There are two reasons that this might occur.
MicroBubbles in the BOD Bottle
This can happen when the blank has small bubbles that are difficult to see when the bottle is initially set. This is a difficult problem to diagnose 5 days later because small microbubbles that might have been in the bottle when it was set might be fully dissolved by the time that bottle has been read. As a result one might not see anything out of the ordinary, but when the measurement is performed there is a higher DO at the end of the 5 days than there is at the beginning. This results in a low BOD blank.
Incorrect DO measurement
This can be caused by an incorrect calibration, either before (when setup), or after the incubation period. This can lead to a slight off calibration (on the low side), initially resulting in a negative depletion. If this happens routinely, and if it is severe (less than -0.20 mg/L), then it probably indicates a severe problem with the measurement system.
Incorrectly Seated Stopper
The BOD bottle stopper must be "seated" or low blanks will be a possibility. Specifically if the water temperature is too high to begin with (even 21 degrees is high) and the stopper is simply "dropped in" oxygen exchange at the interface can lead to negative blanks. Some analysts think that the initial water temperature is the issue and so become obsessed with having exactly 20 degree water for the incubation water. The simpler and easier solution is to simply twist and seat the stopper in each water bottle and insuring the complete lack of bubbles in the bottle after sealing.
- 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 hve 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).
- The best available method for DO measurement is to use LDO with air calibration. The least problematic method is generally to use LDO and to calibrate it using air calibration at least weekly, and preferably daily.