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Calibrating Dupla pH-electrode

Yes it does matter. The 7.0 or (6.88) must be used
first to set the zero point of the probe. Other
solutions away from 7 are used to set the 
slope. If the slope is set before the zero, it
comes out as being the
wrong slope. I would also have a look at the
information in the Krib,
regarding the calibration of pH probes as it is
also worthwhile testing
that the reference is ok and is not blocked or
that the KCl in the 
barrel is still of the right strength. How to do
this is all outlined
in the Krib. Also remember that there is a
temperature effect with
calibrating probes and that it needs to be taken
into account when
calibrating. (the reference solutions do have a
temperature coefficient,
meaning that they slightly change their pH as the
temperature changes)

It really depends on how accurate you want to be.
For me, being within
0.2 of a pH is close enough for jazz for tank
control and Im a bit 
slack with calibration of that probe, but for
environmental testing,
Im very picky and calibrate as accurately as I


PS (George Booth - did you get my email re:

> Subject: Calibrating Dupla pH-electrode
> Does it matter which solution (pH 7.0 or pH 9.0) is used first while
> calibrating the Dupla pH-electrode? Is it possible to first use the solution
> with pH 7.0, tune it and then use the solution with ph 9.0? And the other
> way around?