pHメータ、pHメーター

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stamp with a meter indicating the postage; "meter the mail"
the basic unit of length adopted under the Systeme International dUnites (approximately 1.094 yards) (同)metre, m
rhythm as given by division into parts of equal duration (同)metre, time
any of various measuring instruments for measuring a quantity
(prosody) the accent in a metrical foot of verse (同)metre, measure, beat, cadence
measure with a meter; "meter the flow of water"
send a message from one computer to another to check whether it is reachable and active; "ping your machine in the office"
a sharp high-pitched resonant sound (as of a sonar echo or a bullet striking metal)
contact, usually in order to remind of something; "Ill ping my accountant--April 15 is nearing"
hit with a pinging noise; "The bugs pinged the lamp shade"
make a short high-pitched sound; "the bullet pinged when they struck the car"
(from potential of Hydrogen) the logarithm of the reciprocal of hydrogen-ion concentration in gram atoms per liter; provides a measure on a scale from 0 to 14 of the acidity or alkalinity of a solution (where 7 is neutral and greater than 7 is more basic and less than 7 is more acidic); (同)pH scale

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(ガス・水・電気などの)『計器』,メーター / (時間・速度・距離・強度などを測定,記録する)メーター,計器
(小銃弾などの)ピュー(ブーン)という音 / ピュー(ブーン)と音がする
ペーハー(水素イオン濃度指数)
《文》(程度に応じて)〈賞・罰など〉‘を'割りあてる《+『out』+『名,』+『名』+『out』》
meetの過去・過去分詞

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出典(authority):フリー百科事典『ウィキペディア（Wikipedia）』「2013/01/25 18:25:49」(JST)

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A pH meter

A pH meter is an electronic device used for measuring 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 probe

The pH probe measures pH as the activity of the hydrogen cations 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.

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 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. 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 another solution.

Storage conditions of the glass probes

When not in use, the glass probe tip must be kept wet at all times to avoid the pH sensing membrane dehydration and the subsequent dysfunction of the electrode.

A glass electrode alone without combined reference electrode is typically stored immersed in an acidic solution of around pH 3.0. In an emergency, acidified tap water can be used, but distilled or deionised water must never be used for longer-term probe storage as the relatively ionless water "sucks" ions out of the probe membrane through diffusion, which degrades it.

Combined electrodes (glass membrane + reference electrode) are better stored immersed in the bridge electrolyte (often KCl 3 M) to avoid the diffusion of the electrolyte (KCl) out of the liquid junction.

Cleaning and troubleshooting of the glass probes

Occasionally (about once a month), the probe may be cleaned using pH-electrode cleaning solution; generally a 0.1 M solution of hydrochloric acid (HCl) is used,^[1] having a pH of one.

Inn damaged film of glass. Alternatively a dilute solution of ammonium fluoride (NH₄F) can be used. To avoid unexpected problems, the best practice is however to always refer to the electrode manufacturer recommendations or to a classical textbook of analytical chemistry.

Types of pH meters

A simple pH meter

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 slight variation in pH caused by temperature. Specialty meters and probes are available for use in special applications, harsh environments, etc.

History

The first commercial pH meters were built around 1936 by Radiometer in Denmark and by Arnold Orville Beckman in the United States. While Beckman was an assistant professor of chemistry at the California Institute of Technology, he was asked to devise a quick and accurate method for measuring the acidity of lemon juice for the California Fruit Growers Exchange (Sunkist). Beckman's invention helped him to launch the Beckman Instruments company (now Beckman Coulter). 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.^[2]

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

Because the circuitry of a basic pH meter is 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 acquired and must usually be ordered from a scientific instrument supplier.) For a walkthrough of how to build the simplest possible pH meter or a detailed description of how to build a pH meter/pH controller, see The pH Pages. The application note for the LM6001 chip at the National Semiconductor web site also has a very simple demonstration circuit. Although the application note is for a specialty IC, 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.

References

^ Cleaning electrodes
^ Development of the Beckman pH Meter

External links

Introduction to pH – Excellent overview of pH and pH measurement at the Omega Engineering website
Development of the Beckman pH Meter – National Historic Chemical Landmark of the American Chemical Society
DigitalAnalogPrinter – Freeware compatible with the device shown in the article to monitor the pH in real time

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