Blank is too High
Almost every analyst will say at some point or other that "there is nothing wrong with my BODs except the blank!" This is certainly an easy statement to make since it is easy to have working GGA, good repeatability, and repeatable samples even with a bad blank. The problem with a bad blank is that despite consistency it is still possible to get an incorrect measurement for our samples because the blank is high.
The blank is a very important indicator of whether or not the BOD test is working as intended and it should not be considered an irrelevant problem. Despite what many think it is entirely possible to ALWAYS have good blanks. In fact, doing the following will practically GAURANTEE good blanks.
- Use recyclable PET BOD bottles (gains about 0.04 mg/L lower blanks on average).
- Use air calibration instead of winkler titration (gains about 0.09 mg/L lower blanks on average).
- Use a water polisher instead of deionized or distilled water (gains about 0.03 mg/L blanks on average).
- Add BOD buffering reagents as late as possible and rinse thoroughly a carboy having contained them.
- "Seat" the stoppers carefully. Do not just drop them in the BOD bottles without twisting to seat them.
- Carefully maintain your DO probe and replace approximately once per year if using a polarographic probe.
- The new LBOD probe requires hardly any maintenence and is a good alternative. Using this probe is approved in many regions. Some regions require a 4 bottle check of the users proficiency that takes about 20 minutes.
Consider this...CBOD blanks (which should never be done unless a regulator insists on it!) that average 0.26 mg/L will likely average 0.10 mg/L when following all of our recommendations on blanks. Blanks should read less than 0.10 mg/L routinely. Occasionally getting over 0.10 mg/L is okay but that should be the exception not the rule.