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Re: ISFET Ph pens and controllers

> Date: Thu, 17 Aug 2000 08:26:34 -0600 (MDT)
> From: George Booth <booth at lvld_agilent.com>
> Subject: Re: ISFET Ph pens and controllers
> > How many ways of measuring Ph electronically are there anyway?
> I'm clueless. "6"?

Naah. There's really only one, AFAIK.

Professor Arnold O. Beckman, of Cal Tech, invented it in the mid 30s and
registered it as
"Apparatus for Testing Acidity," U.S. Patent No. 2,058,761.

Basically it was based on the fact that dissimilar metals in an electrolytic
solution will always act like a battery. The voltage, positive or negative
can be linearly related to the log of the Hydrogen ion concentration -- pH.
Good instruments are based on selecting extremely stable conductors that
take acid and alkali well, and amplifying and scaling their voltages to a
usable meter. Thermocouple effects, etc. must also be eliminated in the
design, of course.

The various types of probes are different just to make them more stable, to
read quicker, or be more resistant to corrosive damage. [Temperature
compensation,etc., are bells and whistles we can usually ignore, as George
has indicated.]

When George had trouble with the really soft snow run-off in CO, it was
because the few ions present failed to give enough current to accurately
read the voltage (FETs -- Field-Effect Transistors -- are a huge help,
there, BTW). Higher tds water is easier to read and probably gives better
repeatability. If he had some really pure NaCl to add to the water, I bet it
would have given better (more repeatable) readings.

The metal electrodes seemingly need to be imbedded in a wettable matrix,
like a gel or porous glass. IDK why. That slows readings down by varying
amounts. That factor is where the problems usually arise, also. I've never
needed to replace a probe because of a trick I learned so long ago I can't
even cite the source.

I use three solutions to calibrate. They are buffered to pH 4, 7, and 10.
When calibration seems to get squirrely, or readings stabilize too slowly,
it usually is a buildup of contamination in the porous material of the
probe. That's easy in my less-than-pristine tanks.

I just let the probe soak for up to a few hours in the pH=4 solution. The
acid seems to clear out most of the contamination and restore activity to
near normal. You could probably use vinegar or dilute pool acid to get the
same results. You don't want to etch away the conductors, so do it only
until the desired stable operation is reached.

It works fine for me, but don't complain if you have a different brand and
the probe is destroyed. OK?


Wright Huntley, Fremont CA, USA, 510 494-8679  huntleyone at home dot com

                There are two rules for success in life:
             Rule 1: Don't tell people everything you know.

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