pH Meters / pH Level Measuring Testers
A pH meter is an electronic instrument used to measure the pH (acidity or alkalinity) of a liquid (though special probes are sometimes used to measure the pH of semi-solid substances). A typical pH meter consists of a special measuring probe (a glass electrode) connected to an electronic meter that measures and displays the pH reading.
The first commercial pH meters were built around 1936 by Radiometer in Denmark and by Dr. Arnold Orville Beckman from the United States.
The pH probe measures pH as the activity of hydrogen ions surrounding a thin-walled glass bulb at its tip. The probe produces a small voltage (about 0.06 volt per pH unit) that is measured and displayed as pH units by the meter. For more information about pH probes, see glass electrode.
For very precise work the pH meter should be calibrated before and after each measurement. For normal use calibration should be performed at the beginning of each day. The reason for this is that the glass electrode does not give a reproducible e.m.f. over longer periods of time.
Calibration should be performed with at least two standard buffer solutions that span the range of pH values to be measured. For general purposes buffers at pH 4 and pH 10 are acceptable. The pH meter has one control (calibrate) to set the meter reading equal to the value of the first standard buffer and a second control (slope) which is used to adjust the meter reading to the value of the second buffer. A third control allows the temperature to be set. Standard buffer sachets, which can be obtained from a variety of suppliers, usually state how the buffer value changes with temperature.
The calibration process correlates the voltage produced by the probe (approximately 0.06 volts per pH unit) with the pH scale. After each single measurement, the probe is rinsed with distilled water or deionized water to remove any traces of the solution being measured, blotted with a clean tissue to absorb any remaining water which could dilute the sample and thus alter the reading, and then quickly immersed in another solution. When not in use, the probe tip must be kept wet at all times. It is typically kept immersed in an acidic solution of around pH 3.0. In an emergency, acidified tap water can be used, but distilled or deionized water must never be used for longer-term probe storage as the relatively ionless water "sucks" ions out of the probe through diffusion, which degrades it.