From: SFPhysics@aol.com
Date: Wed Jul 23 2003 - 11:55:35 PDT
From: SFPhysics@aol.com Message-ID: <114.26a85dc4.2c503427@aol.com> Date: Wed, 23 Jul 2003 14:55:35 EDT Subject: pH Meter Technology
>>One of my engineer friends is curious how a pH meter accomplishes its task:
>what's in the electrode; what is(are) the reaction(s) involved and what
>produces the electrical signal; why is rinsing off the electrode judged
>sufficient to avoid cross-contamination of samples (I seem to see a hole
>through which SOMETHING could pass and get stuck)?
>Anyone out there familiar with the nitty-gritty of these meters?
Thanks for your time,
Tim Merrill
>>
Hi Tim:
Guess I'll tackle this one since no one else has.
PH meters used in chem labs and for water testing are common pieces of test
instrumentation. Measuring pH is needed because many of the chemical reactions
commonly found are dependant on pH. Swimming pools are a good example. The
pH must be maintained or all kinds of problems occur.
The pH meter itself is composed of a pH probe containing a reference
electrode and a pH electrode attached to a high impedance millivolt meter or a digital
measuring system. In the old days the meters were very sensitive and
expensive D'Arsenval movements but now the digital readout units are quite inexpensive
made with the majority of the cost going into the glass electrode/reference
cell.
pH electrodes are a type of ion-selective sensor that reacts to the ion
concentrations within the solution surrounding the probe without a chemical
interaction. In fact pH electrodes are not a new technology but were invented in
1901 by German chemist Fritz Haber who constructed the first pH electrode after
he discovered that the voltage on certain glass surfaces varied proportionally
to the acidity of a solution. The internal cell often uses a mercury sulfuric
acid reaction with the leaded glass for the cell potential. That same design
is in use today and the electrode is referred to as a Haber Cell. In fact,
the pH electrode contains two electrochemical cells, a reference and a
measurement cell. The cells are constructed of a special glass filled with
electrolyte. The two cells can be combined in an electrode called a combination
electrode. Combination electrodes are the most commonly used pH electrodes. The
electrical potential (voltage) developed between the reference and measurement
cells is proportional to the hydrogen ion activity. Because the electrode is
glass all around there is no direct chemical activity between the cells
electrodes and the solution being testing. Thus, you can just wash off the electrode
with de-ionized or distilled water and you have a clean start for the next
measurement.
Hope that helps,
Al Sefl
Off to balance the pH in his pool with some HCl...
Which is hard to do by a guy who is off balance himself...
And thinks he can rid of his acidic personality...
By taking three packs of Tums a day...
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