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Cider Digest #1212
Subject: Cider Digest #1212, 3 April 2005
From: cider-request@talisman.com
Cider Digest #1212 3 April 2005
Forum for Discussion of Cider Issues
Dick Dunn, Digest Janitor
Contents:
pH meters (John Emmett)
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Subject: pH meters
From: John Emmett <jlemmett@iinet.com>
Date: Thu, 31 Mar 2005 21:39:41 -0800
Hello all,
I wanted to offer an additional point of view to that of Andrew Lea on
pH meters. Both Andrew and I have, in our professional careers,
access to the finest scientific instrumentation and have technicians
to keep it calibrated for us. While wonderful, it perhaps insulates
from other experiences. Since I retired several years ago, I have
built a rather sophisticated lab in my home which is largely for
research on matters unrelated to food. In the course of setting up the
lab I purchased a simple hand held pH meter more for completeness than
any plan at the time. The hand held unit I purchased is called " Heavy
Duty Waterproof pHTestr3+ with Temperature Readout" manufactured
Oakton. It is a double junction temperature compensated unit costing
$99.95. I purchased it and an extra electrode assembly from
Cole-Parmer Instrument Company.
I have occasion to use it perhaps one or two dozen times a year,
certainly not enough to keep it stored in liquid and calibrated. My
primary uses for the device are to measure the pH of apple juices,
pickles, salsas, and other canned foods, as well as the occasional pH
measurement of soils. Every now or then I will run a titration curve
with it. In using and storing this device, I note that when it is
manufactured, stored at the chemical supply house, and shipped to the
customer it is not wet. Therefore I use it with the following
protocol. When I take take it from its storage location, I place it in
a glass of tap water for 2-3 hours before I turn it on. Then I turn it
on and place it in pH 7.0 buffer solution and after three minutes
engage the calibration procedure. The calibration procedure is
automatic and does not require any screwdriver adjustment. I then
rinse it in tap water and place it in the pH 4.01 buffer solution, wait
three minutes, and engage the automatic calibration. I repeat this
procedure, rinsing before each change of solution, once or twice as
necessary to achieve the correct measurement at both pH's. I then
check the instrument with buffer solutions of pH 1.68 and 10.0.
Generally it is on at 10.0 and reads .04 high on the 1.68 buffer.
Inexpensive instruments such as this only provide for calibration at
two points, wheras high end scientific instruments allow several
calibration points. With this instrument and protocol I am confident
that in the pH range from 3.0 to 8.0 I am within plus or minus 0.025 or
better, and I have verified that with a high end instrument. During
the use of the instrument during a day, I will recheck the calibration
two or three times to assure that there is no drift. When not
immediately in use, I put it in tap water. When I am finished, I rinse
the unit in tap water, air dry it, replace the protective end cap, put
it back in its plastic box, and put it away. I have now been using
this unit for seven years with no difficulty. When I originally
purchased it, I purchased an extra electrode assembly which I have
never had to use. This instrument is far more reliable than I
anticipated.
Fore those who do not have experience with electronic pH meters, I
might offer a few observations. In what I have written above, I Keep
saying "tap water". Don't use the instrument with distilled water or,
more importantly, deionized water. The glass electrode of these
devices is not stable in a deionized water environment. The glass
electrodes will not take any mechanical abuse, so handle it very
carefully. When using buffer solutions, pour out some of the buffer
solution into another container and use that to make your tests.
Always rinse the instrument in tap water before placing it in a
different buffer or test solution. When finished, throw out the used
buffer solutions. Do not pour them back into the stock bottles. The
buffer solutions are inexpensive and are available at chemical supply
houses or from anyone who sells pH meters.
I have also tried to use the narrow range pH papers and find them
entirely useless. I cannot reproducibly measure 0.1 pH unit. I am not
sure whether the problem is the quality of my color vision, the type of
light source used for the color comparison, or the inability to
properly compare a wet and a dry color strip. Whatever it is, it
doesn't work for me. With colored solutions, it is even worse.
With regard to measurements, always remember that a bad measurement is
worse than no measurement, as one tends to use the bad measurement. If
pH is important to you, make a good measurement.
Good luck,
John
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End of Cider Digest #1212
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