A figure expressing the acidity or alkalinity of a solution; more specifically, pH is a
measure of hydrogen ion concentration (potential of Hydrogen). The pH scale
runs from 0 to 14, with pure water, at 7.0, being defined as neutral. Readings of
pH below 7 are acidic (vinegar and battery acid); readings above 7 are basic,
or alkaline (ammonia and liquid drain cleaner).
Pure water is neutral
Who Cares About pH?
Knowing and controlling pH value makes
better milk, cheese, wine; it grows better
crops, lusher lawns; it helps in
sanitizing pools,
drinking water, and
wastewater; it keeps
fish alive; its use in
labs is almost unlimited.
Whether you know it or
not, pH plays a huge role
in your life.
Counting Ions
While the pH range extends from 0 to
14, most digital pH meters measure to
the nearest hundredths. They are counting
hydrogen ions… lots of them. To make
it more manageable, the pH scale is
logarithmic, meaning that the difference
between two adjacent whole numbers
is tenfold.
For example, a reading of 4 is ten
times more acidic than a 5, while a 3 is
100 times more acidic than a 5. Same is
true for alkalinity in the other direction.
A strong acidic solution can be one
hundred million million times more
acidic than a strong basic solution.
It’s easy to see why the logarithmic
scale makes sense. (And you may note
- that’s 14 zeros after the one.)
Types of pH Meters
Several types of meters are available
depending on how the meter is to be used.
They range from small pen-like pocket
testers, to portable meters with external
probes, to more sophisticated bench
meters for laboratory use.
Before Testing,
Do Your ...ations
That would be calibration and validation.
Calibration involves immersing the probe
(glass electrode) into a solution with a
known, stable pH value (a buffer); then
adjusting the meter until the readout
matches the solution value. Because
of the inherently unstable nature of all
pH probes, it is expected they MUST
be calibrated periodically. How often
depends on accuracy requirements. For
very precise measurements, you should
calibrate before each use. More typically,
users calibrate weekly, even monthly.
Quick & Easy Validation
Validation is a simple test to make sure
your probe is still alive, and should be
done any time your readings become
suspect. Milwaukee recommends
validating at least once a month. First,
immerse the probe in white, distilled
vinegar (available in the condiments
section of your local grocery)
and verify that the meter reads
2.4-2.5. Pull probe from the vinegar, and
immediately immerse in Windex
ammonia. Check to see that the meter
goes up quickly above 10. If not, most
likely, the probe is dead.
Cleaning & Storage
It is critical that probes be carefully
cleaned after each use with a special
cleaning solution or vinegar. Blot dry with
a scientific wipe being careful not to
scratch the glass bulb. When storing, you
absolutely must keep the probe WET at
all times. Use a special storage solution,
or a 4.01 or 7.01 buffer solution. NEVER
use distilled or deionized water as it will
draw ions from the probe’s ion-filled gel
and render it useless.
The Death of a Probe
Just like batteries, probes have a limited
lifespan. With excellent care, a probe may
last two years, but more typically, only
about 10 to 12 months. To
help extend its life, be
sure to clean after each
use and always store
the probe so it stays
wet. Even with a dead
probe, a meter may
still give a reading, and
if it’s always 6.5 to 7.8,
you’ll know it’s time
for a new probe.
Contrary to the advice of some, we don’t
recommend buying a spare because
the clock is ticking on your spare probe
whether you’re using it or not.