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A pH Meter is a scientific instrument that measures the hydrogen-ion concentration (or pH) in a solution, indicating its acidity or alkalinity. The pH meter measures the difference in electrical potential between a pH electrode and a reference electrode. It usually has a glass electrode plus a calomel reference electrode, or a combination electrode. In addition to measuring the pH of liquids, a special probe is sometimes used to measure the pH of semi-solid substances.
Knowledge of pH to greater or lesser accuracy is useful or critical in a great many situations, including of course chemical laboratory work. pH meters of various types and quality can be used for soil measurements in agriculture; water quality for water supply systems, swimming pools, etc.; brewing, industrially or domestically; healthcare, to ensure that solutions are safe when applied to patients or lethal as sterilants and disinfectants; and many other applications.
Circuit and operation
Basic potentiometric pH meters simply measure the voltage between two electrodes and display the result converted into the corresponding pH value. They comprise a simple electronic amplifier and a pair of probes, or a combination probe, and some form of display calibrated in pH. The probe is the key part: it is a rod-like structure usually made of glass, with a bulb containing the sensor at the bottom. Frequent calibration with solutions of known pH, perhaps before each use, ensures the best accuracy. To measure the pH of a solution, the probe is dipped into it.
Probe care and cleaning
Probes need to be kept clean of contamination as far as possible, and not touched by hand. Probes are best kept moist with a medium appropriate for the particular probe (distilled water, which can encourage diffusion out of the electrode, is undesirable) when not in use. If the bulb becomes contaminated with use it can be cleaned in the manner recommended by the manufacturer; a quick rinse in distilled water immediately after use, blotted (not wiped) off may be sufficient. One maker of laboratory-grade equipment gives different cleaning instructions for general cleaning (15' soak in a solution of bleach and detergent), salt (hydrochloric acid solution followed by sodium hydroxide and water), grease (detergent or methanol), clogged reference junction (KCl solution), protein deposits (pepsin and HCl, 1% solution), and air bubbles.
Calibration and use
For very precise work the pH meter should be calibrated before 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 4.00 and pH 10.00 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 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. For more precise measurements, a three buffer solution calibration is preferred. As pH 7 is essentially, a "zero point" calibration (akin to zeroing or taring a scale or balance), calibrating at pH 7 first, calibrating at the pH closest to the point of interest (e.g. either 4 or 10) second and checking the third point will provide a more linear accuracy to what is essentially a non-linear problem. Some meters will allow a three-point calibration and that is the preferred scheme for the most accurate work. Higher quality meters will have a provision to account for temperature coefficient correction, and high-end pH probes have temperature probes built in. 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 scientific wipe to absorb any remaining water which could dilute the sample and thus alter the reading, and then quickly immersed in a solution suitable for storage of the particular probe type.
Types of pH meters
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. Specialty meters and probes are available for use in special applications, harsh environments, etc. There are also holographic pH sensors, which allow pH measurement colorimetrically.
In October 1934 Arnold Orville Beckman registered the first patent for a complete chemical instrument for the measurement of pH, U.S. Patent No. 2,058,761, for his "acidimeter", later renamed the pH meter. 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). On April 8, 1935, Beckman's renamed National Technical Laboratories focused on the making of scientific instruments, with the Arthur H. Thomas Company as a distributor for its pH meter.:131–135 In its first full year of sales, 1936, the company sold 444 pH meters for $60,000 in sales. In years to come, it would bring in millions.
Radiometer in 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. Instead American instrument makers successfully developed industrial pH-meters with a wide variety of applications, such as in breweries, paper works, alum works, and water treatment systems." In 2004 the Beckman pH meter was designated an ACS National Historic Chemical Landmark in recognition of its significance as the first commercially successful electronic pH meter.
In the 1970s Jenco Electronics of Taiwan designed and manufactured the first portable digital pH meter. This meter was sold under Cole-Parmer's label.
Building a pH meter
A basic pH meter essentially measures the potential difference between two electrodes and displays the result calibrated in pH; the electronic circuit is very simple and easily built with a few cheap standard electronic components, plus the specialized pH probe.
- Ion-selective electrodes
- ISFET pH electrode
- Quinhydrone electrode
- Saturated calomel electrode
- Silver chloride electrode
- Standard hydrogen electrode
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- Cleaning electrodes
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