Quote:
Originally Posted by mabrungard
With temperature compensation, would it be possible to calibrate a pH meter at the refrigerated buffer temperature and then use the calibrated instrument at a more normal room temperature (say 25C)? I'm not sure that the electronics would properly compensate since buffer solutions are calibrated to a certain pH at a certain reference temperature (say 20C or 25C).
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Actually, they are calibrated over a reasonable range of temperatures and the pH values at each of those temperatures are listed on the packaging in some cases and in others they are not. But as they are standardized NIST buffers the pH values are stored in the meters and so it is possible to do as you suggest.
But (there is always a but) as we all know pH meter calibration uses two buffers to determine two parameters, the slope and offset, of the electrode. There is actually a third, the isoelectric pH which is assumed in every meter I have ever seen to be 7. If it is not 7 ATC doesn't function as it should. Error is fairly insensitive to pHi so that it is actually specified as 6.5 < pHi <7 and as long as it is in that range the error introduced is small. But it can approach the basic buffer accuracy of ±0.02 pH and you want the buffers, not the meter/electrode, to be the limiting factor. Thus I always recommend that buffers and sample be within a few degrees of one another.
Quote:
Originally Posted by mabrungard
I'm assuming that the pK's of the buffers could be used to back calculate what the buffer pH should be at the refrigerated buffer temperature?
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You'd have to know the pK's as a function of temperature and anyway a theoretical calculation isn't what is desired here. These are technical (secondary) standards and are compared to the primary standards that actually define the pH scale. It's by comparison to those that the data are obtained. Anyway, it has been done for you already.
Quote:
Originally Posted by mabrungard
I do like the thought that refrigeration could extend buffer life though!
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I think this is a case where it is best to follow manufacturers recommendations. Just looking at a bottle of 4.01 buffer with an expiration date of 5/14 (and I know I've had it over a year) that says expiry date is valid for storage between 10 and 25 °C. So putting that in the fridge would not be a good idea. Don't know why but I'll take them at their word.
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