[phth]

We are about to get a new pH meter due to the old one being an old POS. A coworker started measuring mixed aqeuous-orgainc solutions with it, and the sensitivity seemed to sharply drop off.  After a few days in 3M KCl, it seemed to get better, but not back to what it used to be.  Are repair procedures fruitless at this point? It is exhibiting 15% error with new calibration buffers..

[Arkcon]

Yes.  Its not likely worth your effort.  First of all, observe the bulb:  is it scratched, or chipped?  This is very common, the stir bar has to only jump once to damage the bulb.  They make them strong enough these days to not crack or break, but if the surface is marred, its not forming a pristine gel layer for the delicate equilibrium that makes the potential that the meter can detect.

You should of course, refill the inside of the probe with fresh electrolyte.  Soaking the outside is good, but the inside is also likely contaminated, and needs replacement.  Maybe a few times -- once to rinse out the old, and recondition the inside of the glass, then again to be fresh.

The frit can be plugged or discolored with some crap.  You'll want to clean that with soaking, or something stronger, like EDTA or strong nitric acid.  Old timers used to use HF or any acidic fluoride solution to make a fresh glass surface.  Or maybe its a little safer to use concentrated hydroxide -- I  prefer ammonia over KOH or NaOH, the cations may contaminate the glass and affect its function, and I figure ammonia is better in that regard.

'Course, you'll have to rinse thoroughly between these cleaning steps, then recondition with filling solution.  Multiple times.  How does your hourly rate of pay compare with the cost of a new electrode now?

I know why you want to do this -- just like me, you have a little free time, and you want to know: is the glass damaged, or plugged coated, or is the filling solution contaminated.  So maybe its worth it to screw around like this.

Also, I assume this is a silver-silver chloride electrode, the only other option being calomel, and no one likes mercury.  Do you use TRIS buffers?  TRIS forms a complex with silver.  So gradually, your Ag||AgCl₂ becomes an Ag||Ag-TRIS||AgCl₂ electrode, which will behave differently.

And this is why we check calibration daily, and track the change over time.

*SIGH*  I know all this stuff, and more about pH electrodes, and you can read more here: http://www.ph-meter.info/  I know not to wipe the bulb, but only blot the rinse water away after through rinsing.  Yet, after I frequently use a pH meter, it starts taking longer and longer to stabilize, and its calibration drifts farther and farther from ideal.  Just for me, not anyone else.  Dunno why -- either I'm doing something very bad, or other people are more lax with their concerns.

[wildfyr]

pH meters are the very devil. For something so simple, they are notoriously unreliable and drift all over the place, especially if youre in the pH 6-8 range. In our factory we substitute conductivity meters whenever possible, despite the obvious drawbacks.