Ask A Scientist Chemistry Archive

   
name         Debbie R.
status       educator

Question -   Why does a pH meter calibrate a pH 4.00 buffer as 4.01?
Likewise for 10.00 buffers?
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Debbie,

The pH of a solution, as read by a calibrated pH meter, is also
dependent on the temperature of the solution.  In the case of the pH 4
buffer solution, I have a table that shows at 0 C the buffer pH is 4.00,
but at 90 C it is 4.21.  At a normal temperature of 25 C, the pH is
4.01.  The same hold true for the other buffers.  Many pH meters now
correct for temperature issues.

Bob Hartwell
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Various pH buffers are prepared from solutions of acids, bases and various
salts of the acids and bases. These recipes are put together not so much to
read an exact pH = 4.00000... but to give a pH in the vicinity of 4.00. In
the case of the buffer solution you are using the dial should read 4.01 in
order for the electronics in the meter to accurately measure the pH. Not the
other way around.

Vince Calder
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