THEORY OF pH MEASUREMENT
Color Methods. Over the years, researchers have discovered dyes and chemicals that will change color at
prescribed pH values. Litmus paper is a good example of a commonly used indicator. In an alkaline solution, the paper
turns blue and in acid solution, the paper will turn pink. But there are two major drawbacks with the use of indicators. First
is the difficulty of detection in highly colored or turbid solutions and second is chemical interferences with the indicator,
invalidating the test. With the invention of the pH probe and meter, scientists were able to eliminate these drawbacks as
well as increase the precision of pH measurements.
Instrument Methods. There are three components of pH measurement. The measuring electrode, the
reference electrode, and the pH meter. Instrumental pH measurement can be performed relatively fast and with a high
degree of precision.
Measuring Electrode The key to the pH measuring system is the glass bulb at the end of the measuring
electrode. This glass bulb is manufactured from a special glass which is very sensitive and highly
selective to hydrogen ions. The pH measurement is then a function of a voltage charge across the bulb
which is directly related to the hydrogen ion concentration.
Reference Electrode A second electrode, the reference electrode, is then required to complete the
electrical circuit between the measuring electrode, through the meter, into the sample being measured.
The reference electrode completes this circuit by very, very slow seepage of KCI (K++ CI -), into the
sample through a porous junction. Clogging of this junction can cause erratic and incorrect pH
Combination Electrode Combination electrodes are electrodes which contain both a measuring and a
reference electrode in one probe.
pH Meter The input signals from the electrodes are displayed on the pH meter. A direct reading of the
voltage input is displayed when the meter is set on the millivolt scale. For the more common pH usage,
the meter converts the voltage input from the electrodes to a unit of pH which is displayed when the
meter is set on the pH scale.
Symptom Meter exhibits no response.
Check battery power. Plug the attached recharger into the appropriate power outlet (110 VAC or 220
VAC depending on model ordered) for at least 24 hours to insure full battery charge. Recheck function.
If there is still no response, then replace batteries.
Turn meter OFF.
Unplug battery charger.