Can you explain what you mean by this comment? You don't believe that it is possible that there is a ~0.14 difference in the pH of 2 different buffer solutions for the same pH from different manufacturers?
No, I don't or at least I think it very unlikely. I did change from impossible to very unlikely because with the cheap stuff that comes in from China it is indeed possible though one would think if this particular brand of buffer was so far off someone would have flagged it (which is of course what you are doing if your hypothesis is correct).
I wouldn't have thought so, but that is certainly the case.
Well it isn't certainly the case yet but what we hope to do is either determine that it is certain or that something else (the meter) is involved.
It seems less likely than the alternative explanation that they are actually the same pH but that my meter measures them differently.
Given what I knew at the time I posted it is actually much more likely that the meter was involved. Lot's and lot's of people who post here have a $15 Chinese pH meter and wonder why they get fishy readings because they know they followed the calibration procedure precisely when they got the thing 6 mos. ago.
My meter was rock-solid on both measurements when it locked in and only fluctuated up and down by 0.01. If this is indeed what you are suggesting - is there something that could cause a pH meter to measure that the pH of 2 solutions, which are actually nearly identical in pH, to be so far apart? I wouldn't think that relative measurements of buffer solutions would be a difficult task for a $100 meter.
Many meters in that price range and below are unstable meaning that they don't hold a calibration for more than a few minutes. If you calibrate your meter and then read the 4 buffer every couple of minutes and that series of readings is stable over the period of time it takes to make all the measurements then the comparison (difference) measurements are reasonable even if the calibrating buffers are off. It stability we need.
Method-wise, I'm not sure what could possibly go wrong here. The samples were at the same temperature, tested in glass containers that had been rinsed well with DI water, same volume used for each sample, and tested back-to-back (rinsing well with DI water in between).
All sounds good but you hadn't mentioned that in the previous post. Bad technique is responsible for lots of bad pH readings. I recently posted in another thread that a new user of a pH meter will find that his pH readings become mysteriously better (more consistent) as he gains experience with it.
QUOTE=cheesebach;7871124]It seems I'm not the only one who's noticed that these Atlas buffer solutions are off either:[/QUOTE]
They could be. Let's find out.
QUOTE=cheesebach;7871124]Thanks for the suggestion - I'll take a look at these. It would be nice to have something like this on hand as a sanity check if I start to doubt the accuracy of any of the cheaper solutions that I buy online or from the LHBS in the future.[/QUOTE]
Compare the Atlas buffers against the Hach buffers using a meter that has passed the stability test and if you find a difference then shout it from the rooftops.