We recently had to change the pH probe that we use to check the pH of our BOD dilution water. Ever since doing this, the pH of the BOD dilution water that we make up according to standard methods has been low (about pH = 6.4). When we use the new probe to measure a known standard, it reads right on the mark. We can't figure out why our dilution water is not falling in the pH 6.5 - 7.5 range when we are making it up exactly as standard methods calls for. We suspect that our old probe was incorrect, since our new probe is consistent with pH paper results whereas our old probe was not.
What could be going on here? We have been adding over twice as much buffer to get the dilution water pH value in range.
Thank you!

Where is your water coming from?  If you have slightly acidic water to begin with (sometimes what comes out of DI systems has a pretty low pH) that buffer won't do what it is supposed to do.  Make sure your buffer is at the right pH to begin with.  Was it made with the old probe or the new probe checking the pH?  The 21st edition of SM does not have a pH requirement spelled out for the dilution water, at least not one that I can find.

We had a very similar situation at a lab where I used to work.  Our DI water was often around 4 and when we added all the nutrients and buffer the dilution water pH would typically be in the mid 6 range.  We were making 19 liters at a time so our solution was to add 1 drop of 10 N sodium hydroxide to the water.  This got us to 7.2 ± 0.1 every time.  I would definitely check for acid residual from your washing procedure by checking the pH of DI water added to the empty bottle.  If it falls below the pH of the DI water in some other container you know you have something going on in the bottle.  I always did my routine cleaning of the dilution water containers with an isopropyl alcohol solution.  Any little bit of that left over from rinsing wouldn't cause any uncorrectable problems.  It isn't in high enough concentrations to be toxic and the minimal contribution to depletion would be corrected for in the seed bottles. I would do acid washing once a week (we usually made 10-12 batches of dilution water per week) and get very excessive with the rinsing after acid washing.

I would keep the addition of anything beyond the stipulated buffers out of your DW.  Check the pH of your DI water prior to adding buffers, you may have to change your source of DI water.  Yes, there could be an acidic residue inside your glass jars so check to make sure that is not the case.  Use an organic cleaner that does not leave a residue and once per month use a cleaner that can remove everything (like Chem-Solv).  Rinse, rinse, rinse, rinse!  

Don't increase the amount of buffer in order to increase the pH!  That violates the conditions of the test and invalidates your results.  Adding NaOH effectively does the same thing.  Find out why the pH of the water is so low to begin with and solve that problem in order to straighten out the entire test.

What is the source of your set up water?

~Eric

Perhaps that old unit is causing your low pH in the DI water to begin with and you need to correct that problem.  

~Eric

Well, Standard Methods Edition 19 states that distilled water can contain ammonia or volatile organics.  If your unit deionizes the water fed to it then there could be soluble organics leaching into it from the resin bed.  

Type I reagent-grade water ought to have 10 megohms resistivity as a minimum, so you are right at the lower limit.  Looking at 9020B (microbiological examination quality control guidelines) in Standard Methods, it recommends keeping a close check on the lifetime of deionization system cartridges and activated carbon units.  When these items approach the exhaustion point the quality of the water produced abruptly becomes unacceptable.

If you are using a still to produce the feed water for the unit above, then do not store the water since it will absorb additional carbon dioxide and ammonia from the air.  Proper maintenance is important here as well.  

Since the BOD test is a bioassay test, using microorganisms to attain the results, it then follows logically that the water used in the test ought to meet the standards used in microbiolgical test methods.  

Table 9020:I in Standard Methods shows that there are several montoring tests that ought to be done on an ongoing basis each year for reagent-grade water used in bioassays.  I would apply these limits in an overlay fashion to those given in the BOD methodology in order to eliminate any problems with your water.  I would double check my history of changing out cartridges and carbon.  I would do some pH checks on cleaned glassware, etc with the use of bromthymol blue and observe the color reaction.  If you are using buffer solutions, and not ampules, it could be a matter of allowing the dilution water time to"age" and the pH will then rise into the acceptable range.  

I wish you the best in your efforts to identify the source of your problems.

~Eric

Yes! The problem was our probe. We should have thought of this since we started experiencing problems when we had to go to a different probe. We had no way of knowing which probe was correct. Obviously, we need to come up with a better QC program for the samples that we measure. Also, we are searching for a new probe that is suitable for measuring low ionic strength samples.
Thank you!

For pure water it is recommended that you sprinkle a few grains of KCl (same stuff in the pH probe internal solution) into the sample to allow it to come to equilibrium much faster.  Otherwise, your reading will drift.