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pH measurement at temperatures other than ambient

pH measurements of solutions are generally done with pH Meter calibrated with buffer solutions at ambient temperature, 25C.I n the instance where the process solution is maintained at higher temperature, say 55C there are 2 opinions on pH Measurement: 1. All measurements are done at 25C 2. pH Meter is calibrated with buffer at the temperature of the solution whose pH is to be measured and then pH of the solution is measured with temperature compensation for the same higher temperature.Can the first method be used because of its simplicity without any inaccuracies?
S.Ramajayam

Subramanian Ramajayam
consultant - Bangalore, India
April 19, 2009

Hi, Subramanian. If you are not governed by NADCAP or other hare-brained rules, I would suggest method 3 as the simplest: Calibrate against the buffer solutions at ambient temperature and measure the process solution at its operating temperature, relying on the temperature compensation circuitry of the pH meter.

Regards,

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
April 23, 2009

It depends on the accuracy of the measurement that you need. On Ted's method three, it is going to depend on the temp compensation method used by your equipment. There are significant differences between them for the purist, but are great for most applications.
Letting the solution cool to ambient is time consuming and may be bothered by boric acid crystallizing out, but works fine, particularly when you are taking the sample from the line back to the lab where you have the high priced meter/probe.

James Watts
- Navarre, Florida
April 27, 2009

Ted's probably right, allow the temperature-correction circuitry to do its thing. You can confirm this by setting the meter at the correct temperature, than heating (or cooling) the buffer solution and see if the reading is still accurate when the temperature is reset on the meter.

George Brackett III
- Maine
April 29, 2009

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