Re: [Killietalk] pH (was Re: copper leaching from new plumbing)

[Ken Simolo <Simolo at chem_chem.rochester.edu>]

Hi Lee,

About 5 years ago, I switched to an ISFET solid state pen style pH 
meter (www.phmeters.com).  This was back in the days when I thought 
pH was routinely important.  As I have posted before, the ISFETs are 
stored dry and mine at least has not drifted off calibration by any 
large amount.  Testing with pH 4, 7, & 10 pre-made calibration 
solutions and making my own with the buffer capsules and distilled 
water has always shown little or no drift in calibration.  In the 
beginning, I was making many pH measurements a day.  Now, it is rare 
but even after 6 months of storage, it is unusual to see more than a 
0.1 pH drift in calibration when compared to the three standards. 
However,  as you mentioned, when I was using the gel pen style pH 
meters, those drifted all over the place even in just a short period 
of time.  This was especially true when I was using them in a variety 
of samples with varying pH and buffering capacity.

If someone really cares about pH, I would recommend the ISFET units 
but they have gone up a lot since I bought mine.  However, if you 
just want to do an occasional rough measurement, pH paper is fine.

Ken




>
>     I was a technical leader of a study at DuPont which ran an experiment
>with a group of trained laboratory technicians to determine the 
>sources of error
>in pH measurements. They each measured samples we provided them and used their
>own pH meters in the manner they normally used them. Results were dramatic.
>Unless they calibrated the meter just before using them, the results were all
>over the map. These were with mostly laboratory bench model pH meters -- all
>fairly expensive from reputable manufacturers. Only those who calibrated the
>meter before the test gave results close to the right value. These 
>test solutions
>were buffered solutions. Calibrating once a day was not sufficient either.
>Storing the electrode in pH 4 buffer is also important.
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