WEF Discussion Forums
Laboratory Management and Technical Issues
Copying and pasting from a posting on a different thread:
I am glad to be part of this forum. This is regarding CBOD analyses. Our lab generally has trouble with the unseeded water blank depleting too much.
0.2 mg/L or less is the requirement. Many times re-cleaning all the glassware in a highly alkaline cleaner will fi the problem, but not always.
Does anyone else have a similar problem/ And how are you addressing it?
Here is the reply I posted to this same query that showed up in your initial posting, Victor.
the anonymous person asking about CBOD blanks. Are you adding the
pyridine inhibitor to the blank? If so, discontinue doing
that...pyridine is an organic compound and it has an oxygen demand."
I wrote to the WEF webmaster and offered my thoughts on the new forum protocol. Sometimes it is just "change" that causes a lot of consternation until we learn how to use it. But in this case, it appears they have really cut the legs off a very useful tool by allowing only members to originate postings. Maybe if enough of us complain they will rethink their "original posting" protocol which appears to me to be nothing more than a membership drive.
I think I might have finally made this forum work. As for the CBOD blank, we set up a blank with and without the inhibitor. We use without the inhibitor to evaluate the dilution water of less than 0.2 mg/L DO depletion. We look at the blank with inhibitor to evaluate the inhibitor. Too much DO depletion with the inhibitor indicates that the inhibitor is contaminated or that the didpensor cap is deteriorating. These conditions will effect the BOD results and should be addressed.
A few years ago I switched to disposable bottles from Environmental Express and haven't failed a blank since. Cost isn't too bad, about $0.50 a bottle. When you figure the time spent and chemicals used to clean BOD bottles it may be a wash.
We wash our BOD bottles with alkaline cleaner about every two weeks (in addition to the regular washing). This seems to help quite a bit.
We wash our BOD bottles with an alkaline cleaner after each use. This is to ensure that any grease or the nitrification inhibitor is removed and will not affect the BOD test on the next run. We then wash them in a glassware washer with a deionized water rinse. We also acid wash them on a monthly basis to ensure all organic buildup is removed that the alkaline cleaner does not. Our blanks are always good.