pH Meter

A pH meter, typically consisting of a special measuring probe connected to an electronic meter, is an electronic instrument used to measure the pH (acidity or alkalinity) of a liquid. The probe measures pH as the hydrogen ion activity that surrounds a thin-walled glass bulb at its tip. A small voltage is produce by the probe that measures and displays pH units by the meter.

The meter circuit displays voltmeter in pH units instead of volts. The circuit of the pH meter consists of operational amplifiers in an inverting configuration that converts the small voltage produce by the probe into pH units that are offset by seven volts to give a reading on a pH scale.

Two adjustments performed at calibration set the gain and offset of the inverting amplifier. To be accurate the pH meter should be calibrated before each measurement since the glass electrode does not give a reproducible e.m.f. over longer periods of time.

Calibration needs to be performed with two standard buffer solutions that span the range of pH values to be measured. pH 4 and pH 10 are acceptable for general purpose buffers. The meter has one control (calibrate) to set the meter reading equal to the value of the first standard buffer and a second control (slope) that is used to adjust the meter reading to the value of the second buffer. The last control allows the temperature to be set.

Through the calibration process the voltage produced is correlated by the probe with the pH scale. After each used the probe must be rinsed with de-ionized water to remove any traces of the solution being measured. When not in use the probe needs to be kept wet all the time in order to avoid the pH sensing membrane dehydrating. However distilled nor de-ionized water must never be used for long term storage of a probe. Occasionally the probe should be cleansed using a pH-electrode solution. If the glass membrane performance has been degraded due to membrane poisoning, it can be quickly etched with diluted hydrofluoric acid or ammonium fluoride.

Meters can range form simple and cheap to complex and expensive with computer interfaces and several inputs for indicators, reference electrodes, and temperature sensors. Lower cost meters can require that temperature measurements be entered in order to adjust for any variation in pH caused by temperature. There are also pocket pH meters as cheap as ten dollars that can automatically compensate for temperature.

Arnold Orville Beckman created the first commercial pH meters around 1936. He was asked to create a quick, accurate method for measuring the acidity of lemon juice for the California Fruit Growers Exchange. His meter helped him to start Beckman Instruments Company and later, in 2004, his meter was designated an ACS National Historical Chemical Landmark in recognition of its significance as the first commercially successful electronic pH meter.

Jenco Electronics designed and manufactured the first portable digital pH meter in 1970s.

Due to the simplicity of a basic pH meter it is possible to build one from available parts at a local electronics store.