From: David Lauter (djlauter@hotmail.com)
Date: Thu Jul 24 2003 - 14:42:18 PDT
From: "David Lauter" <djlauter@hotmail.com> Subject: Re: Pinhole Digest #1271 - 07/24/03 Date: Thu, 24 Jul 2003 14:42:18 -0700 Message-ID: <Law15-F11q9StmITZo20000719b@hotmail.com>
Dear Al,
But why would the voltage across a glass electrode specifically respond to
pH and not all of the other soluble ions?
Lauter
George Washington High
>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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