pH meter
pH meters are used for soil measurements in agriculture, water quality for municipal water supplies, swimming pools, environmental remediation; brewing of wine or beer; manufacturing, healthcare and clinical applications such as blood chemistry; and many other applications.
These have tips suitable for piercing semi-solids, have electrode materials compatible with ingredients in food, and are resistant to clogging.
The design of the electrodes is the key part: These are rod-like structures usually made of glass, with a bulb containing the sensor at the bottom.
The magnitude of the electrochemical potential across the glass bulb is linearly related to the pH according to the Nernst equation.
The reference electrode is insensitive to the pH of the solution, being composed of a metallic conductor, which connects to the display.
[12] Details of the fabrication and resulting microstructure of the glass membrane of the pH electrode are maintained as trade secrets by the manufacturers.
Electrode manufacturers have developed compositions that suitably balance these factors, most notably lithium glass.
Calibration is needed because the glass electrode does not give reproducible electrostatic potentials over longer periods of time.
[13]: 238–239 Consistent with principles of good laboratory practice, calibration is performed with at least two standard buffer solutions that span the range of pH values to be measured.
Some pH meters provide built-in temperature-coefficient correction, with temperature thermocouples in the electrode probes.
[20] pH meters range from simple and inexpensive pen-like devices to complex and expensive laboratory instruments with computer interfaces and several inputs for indicator and temperature measurements to be entered to adjust for the variation in pH caused by temperature.
[13]: 197–215 Specialty meters and probes are available for use in special applications, such as harsh environments[21] and biological microenvironments.
Beckman developed the prototype as an assistant professor of chemistry at the California Institute of Technology, when asked to devise a quick and accurate method for measuring the acidity of lemon juice for the California Fruit Growers Exchange (Sunkist).
[25]: 131–135 On April 8, 1935, Beckman's renamed National Technical Laboratories focused on the manufacture of scientific instruments, with the Arthur H. Thomas Company as a distributor for its pH meter.
[26] The Radiometer Corporation of Denmark was founded in 1935, and began marketing a pH meter for medical use around 1936, but "the development of automatic pH-meters for industrial purposes was neglected.
Dr. Werner Ingold began to industrialize the production of single-rod measuring cells, a combination of measurement and reference electrode in one construction unit,[29] which led to broader acceptance in a wide range of industries including pharmaceutical production.
[31] In the 1970s Jenco Electronics of Taiwan designed and manufactured the first portable digital pH meter.
[32] Specialized manufacturing is required for the electrodes, and details of their design and construction are typically trade secrets.
[13]: 125 However, with purchase of suitable electrodes, a standard multimeter can be used to complete the construction of the pH meter.
[33] However, commercial suppliers offer voltmeter displays that simplify use, including calibration and temperature compensation.
