From: SFPhysics@aol.com
Date: Fri Jul 25 2003 - 01:13:29 PDT
From: SFPhysics@aol.com Message-ID: <162.23a9740a.2c5240a9@aol.com> Date: Fri, 25 Jul 2003 04:13:29 EDT Subject: pH meters part 2...
<< 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 David:
It does measure the total acidity or alkalinity. The key word is total and
you must remember what pH is, a ratio of all the ions, positive and negative.
The perHydrion is a measurement of the net charges of all the soluble ions.
I hope I can remember this from some 40 years ago and relay it accurately.
The Haber Cell sets up a potential on it's surface. The positively charged
protons that determine acidity and other ions like the hydroxyl ions with their
negative charge add to or subtract from the half-cell chemical potential
through electrostatic induction. The glass is so thin as to be considered a
membrane albeit a nonporous one. Then the voltage measured is so many millivolts
per pH unit, looking it up it is 59.2 mV according to Van Nostrand's
"Scientific Encyclopedia" 5th Edition. Unlike metals making contact with a solution
there is no chemical interaction and the glass just picks up the mean charge of
the solution being tested. That is why, going back to the first question, you
can just wash the electrode off with "neutral" pure water at a pH of 7 and you
are ready for the next measurement.
Hope that clarifies things a bit and that I am not too far off base.
Remember, I only taught Chemistry when I absolutely had to!
Best wishes to all,
Al Sefl
Who like Balsamic acetic acid over his bitter alkaline lettuce...
And overanalyzes even a salad...
PS: The alkaline flocculant alum power added to our tap water supply is why
a lemon's acidity makes a glass of water taste better. It's all in the pH and
your tongue is a pH meter too!
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