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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, such as cheese). 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.
History The first commercial pH meters were built around 1936 by Dr. Arnold Orville Beckman in the United States and by Radiometer in Denmark. Beckman was an assistant professor of chemistry at the California Institute of Technology when he was prompted to build a pH meter by his old college friend Glen Joseph who was a chemist at a research laboratory run by the California Fruit Growers Exchange (Sunkist). The Exchange’s citrus growers wanted to get an accurate and rapid measure of the acidity of lemon juice. Beckman's invention helped him to launch the Beckman Instruments company (now known as Beckman Coulter). In 2004 the Beckman pH meter was designated a ACS National Historical Chemical Landmark in recognition of its significance as the first commercially successful electronic pH meter.* The Probe The pH probe measures pH as the concentration 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. The Meter The meter circuit is fundamentally no more than a voltmeter that displays measurements in pH units instead of volts. The input impedance of the meter must be very high because of the high resistance—approximately 20 to 1000 MΩ—of the glass electrode probes typically used with pH meters. The circuit of a simple pH meter usually consists of operational amplifiers in an inverting configuration, with a total voltage gain of about -17. The inverting amplifier converts the small voltage produced by the probe (+0.059 volt/pH in basic solutions, -0.059 volt/pH in acid solutions) into pH units, which are then offset by 7 volts to give a reading on the pH scale. For example: The two basic adjustments performed at calibration (see below) set the gain and offset of the inverting amplifier. Calibration and Use Calibration with at least two buffer solution standards is usually performed every time a pH meter is used. One of the buffers has a pH of 7.00 (neutral pH) and the second buffer solution is selected to match the pH range in which the measurements are to be taken: usually pH 10.00 for basic solutions and pH 4.01 for acidic solutions. The gain and offset settings of the meter are adjusted repeatedly as the probe is alternately placed in the two calibration standards until accurate readings are obtained in both solutions. This calibration process correlates the voltage produced by the probe (approximately 0.06 volts per pH unit)*) with the pH scale. After calibration, the probe is rinsed in distilled, deionized water to remove any traces of the buffer solution, 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 the sample. Between uses, the probe tip, which must be kept wet at all times, is typically kept immersed in a small volume of pH 7.0 calibration solution. 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 (ion-sensitive, redox), reference electrodes, and temperature sensors such as thermoresistors or thermocouples. Cheaper models sometimes require that temperature measurements be entered to adjust for the slight variation in pH caused by temperature. Specialty meters and probes are availble for use in special applications, harsh environments, etc. Building a pH Meter -- pH Meter Schematic Diagram Because the circuitry of a pH meter is fundamentaly quite simple, it is possible to build a serviceable pH meter or pH controller with parts available at a neighborhood electronics retailer. (pH probes, however, are not so easily come by and must usually be ordered from a scientific instrument supplier.) For a schematic diagram of a simple pH meter, see the application note for the LM6001 chip at the National Semiconductor web site. The probe is connected to the input of this circuit and the output can be read with any inexpensive voltmeter. Although the application note is for a specialty chip, serviceable pH meters can be built from any operational amplifier with a high input impedance, such as the common and inexpensive National Semiconductor TL082 or its equivalent. See also | ||||||||
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