pH measurement temperature ranges

[biochemist]

Hello,

I recently got cited by an auditor for not mentioning the allowed temperature ranges when preparing / adjusting the pH of a buffer or mobile phase.

Can someone help me out with the allowed range, i.e. 23 - 27, 20 - 25, 15 - 35 degC?

Do you have a reference for this, or is this based on normal practice?

Thanks!

[scottythree]

Do a search for NIST pH tables. They have different pH/Temp range tables for different buffers.

[biochemist]

Thanks Scotty . Are you referring to the standard pH calibration buffers? We do use tables (for verification) and the ATC (automatic temperature compensation) function of the meter (for calibration).

And we also have a fixed temperature range for sample measurement according to the USP (25 +/- 2 deg C)

However, this specification (23 - 27 C) seems to be unnecessarily tight and difficult to achieve, for prepared buffers and mobile phases, which are to be used for analyses. For example, if adding an acid or alkali to a solution for pH adjustment, the temperature can rise to about 30 deg C, if it's an exothermic reaction.

Does anyone thermostat their buffers or reagents when doing this?

[tom jupille]

Good gravy!

If you have good system suitability criteria, and you can demonstrate reasonable robustness with respect to pH, then the temperature should not have a significant impact. USP, EP, and the FDA allow pH to be adjusted by 0.2 units so long as you meet system suitability. Even the worst-case NIST buffer varies by less than that over a 20-degree range!
( http://www.coleparmer.com/techinfo/tech ... erTips.htm )

My suggestion would be to write up an SOP with a 15 - 30 degree C range, and cite the NIST tables plus the FDA default adjustment limits on page 14 of ORA LAB 5.4.5 as justification.
(http://www.fda.gov/downloads/ScienceRes ... 092147.pdf )

[LC_labrat]

What type of auditor? I find some auditors need to justify their worth by noting the smallest non-relevant issues. I would just justify it as not being a critical parameter and ambient temperature is suitable, unless you work in some unusually hot/cold environment. As ambient temperature is how the mobile phase is used. Some auditors just check off the box, and others actually think it through.

[unmgvar]

i think that somewhere EP, USP say it should be done at 23 or 25 degrees
many analysts i know simply warm the temp probe with their fingers so that the print out is at the correct temp

[biochemist]

Thanks all