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pH Meters and Electrodes (long, as usual)
We've just replaced some aging pH electrodes in our three pH controllers and had
the usual annoying experience. Has anybody else done this or is it just us?
It is "common knowledge" that pH electrodes drift over time and eventually go
bad. It is therefore prudent to periodically calibrate the electrodes and
replace them every year or two years, depending on how paranoid you are.
Calibrating an old or new electrode involves using two strong buffer solutions,
pH 7.0 and either pH 4.0 or pH 10.0. The probe is alternately placed in each
solution (thoroughly rinsing in between) and the meter's "standard" and "slope"
adjustments are tweaked until the meter reads 7.0 and 4.0/10.0. There is some
interaction between the adjustments, so you go back and forth until they are
stable.
The instructions that came with our pH controllers suggested that the probes be
calibrated weekly. "Calibrating weekly" lasted about a month. They didn't seem
to drift that fast and it was a pain. We now check the water with a good pH test
kit every other week and recalibrate when needed. Even then, the recalibration
might be simply adjusting the meter to read what the test kit indicates the pH
should be. Why have we gotten so sloppy?
We got three new probes last week, one for each tank. We buy Broadley-James
"Silver" electrodes from Pet Warehouse ($34, www.petwhse.com). We have tried
more expensive electrodes but have found these to be about the best for
relatively soft water. This must be the first time in history that "cheapest"
was "best".
We carefully calibrated all three electrodes and put them in the tanks. All
three read properly compared to a narrow range pH test kit. The next day we
rechecked pH and all three were reading from 0.2 to 0.3 too high (what the meter
read as 7.0 was actually 6.8 or 6.7, thus we were injecting more CO2 than we
wanted). We groaned, "Here we go again".
We recalibrated the probe in one tank. It read 7.2 in the 7.0 cal solution (as
expected). We adjusted the "standard" to 7.0 and completed the cal procedure.
When we put the probe in the tank (still pH 6.8), the controller now read "6.3".
Arrrggh! We tried soaking the electrode in storage solution ("to rehydrate it").
It still read 7.0 in the cal solution and still read 6.3 in the tank. We
monitored it for two hours and it slowly drifted up to the correct reading. It
still reads correctly 2 days later. We did NOT recal the other two electrodes;
we simply changed the controller "set point" to make sure we are injecting the
right amount of CO2. We will deal with those later.
We seem to go through this same experience every time we recal the electrodes or
get new ones. Thus we tend to not recal very much.
We discussed this with Broadley-James some time ago. It appears the problem is
the "ionic strength" of the solution being tested. Most pH electrodes are used
to check buffers for proper pH. Buffers (and cal solutions) are strong ionic
strength solutions. Soft freshwater is a weak ionic strength solution.
Electrodes do not do well in weak ionic strength solutions. Bummer.
This is one reason we don't trust the pH pens. Electrodes are fussy things.
Labs keep electrodes in a storage solution between uses and calibrate before
every use. I suspect pH pens don't get this loving care. They might work well
out of the box but how about long term? How long do the electrodes last under
typical conditions? Are they checked periodically to see if they are still
accurate? How quickly do they respond to soft water?
Ain't high tech great?
Comments? Other horror stories?
George Booth in Ft. Collins, Colorado (booth at frii_com)
http://www.frii.com/~booth/AquaticConcepts