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==Overview==
{{lowercase|title= pH meter}}
[[Image:PH meter.JPG|thumb|A pH meter. Notice the temperature and pH probes on the right.]]

A '''pH meter''' is an electronic instrument used to measure the [[pH]] ([[acid]]ity or [[base (chemistry)|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. [[Image:PH_Meter_01.JPG|thumbnail|A simple pH meter with its probe immersed in a mildly alkaline solution. The two knobheads are used to calibrate the instrument.]]

==The probe==
The pH probe measures pH as the activity of [[hydrogen ion]]s 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#Construction|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 [[Electrical impedance|impedance]] of the meter must be very high because of the high [[electrical resistance|resistance]] — approximately 20 to 1000 M[[ohm|Ω]]  — of the [[glass electrode]] probes typically used with pH meters. The circuit of a simple pH meter usually consists of [[operational amplifier]]s 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 seven volts to give a reading on the pH scale. For example:

* At neutral pH (pH 7) the voltage at the probe's output is 0 volts. 0 * 17 + 7 = 7.
* At alkaline pH, the voltage at the probe's output ranges from > 0 to +0.41 volts (7 * 0.059 = 0.41). So for a sample of pH 10 (3 pH units from neutral), 3 * 0.059 = 0.18 volts), the output of the meter's amplifier is 0.18 * 17 + 7 = 10.
* At acid pH, the voltage at the probe's output ranges from -0.7 volts to < 0. So for a sample of pH 4 (also 3 pH units from neutral, but in the other direction), 3 * +0.059 = +0.18 volts, the output of the meter's amplifier is -0.18 * 17 + 7 = 4.

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, but preferably three, [[buffer solution]] standards is usually performed every time a pH meter is used, though modern instruments will hold their calibration for around a month. One of the buffers has a pH of 7.01 (almost neutral pH) and the second buffer solution is selected to match the pH range in which the measurements are to be taken: usually pH 10.01 for basic solutions and pH 4.01 for acidic solutions (It should be noted that the pH of the calibration solutions is only valid at 25°C). 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.  Modern instruments have completely automated this process and only require immersing in each solution once, or at worst, twice.

The 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 storage solution, which is an acidic solution of around pH 3.0.  Alternatively, the pH 7.01 calibration solution can be used, but this results in a need for more frequent calibration. In an emergency, 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, which degrades it.

Occasionally (about once a month), the probe should be cleaned using pH-electrode cleaning solution; generally a 0.1 [[Molarity#Molarity|M]] solution of Hydrochloric Acid (HCl) is used <ref>[http://www.ph-meter.info/pH-electrode-cleaning] Cleaning electrodes.</ref>, having a pH of about one.

==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-selective electrode|ion-sensitive]], [[redox electrode|redox]]), [[reference electrode]]s, and temperature sensors such as [[thermoresistor]]s or [[thermocouple]]s. 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 available for use in special applications, harsh environments, etc.  Pocket pH meter are readily available today for a few tens of dollars that automatically compensate for temperature (ATC, Automatic Temperature Compensation)

==History==

The first commercial pH meters were built around 1936 by Radiometer in Denmark and by Dr. [[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 Growers, Incorporated|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 Historical Chemical Landmarks|ACS National Historical Chemical Landmark]] in recognition of its significance as the first commercially successful electronic pH meter.<ref>[http://acswebcontent.acs.org/landmarks/landmarks/phmeter/phmeter.html] The Beckman pH Meter
</ref>

==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 [http://www.66pacific.com/ph/simplest_ph.aspx how to build the simplest possible pH meter]or a detailed description of [http://www.66pacific.com/ph/ph_1.htm how to build a pH meter/pH controller], see [http://www.66pacific.com/ph/ph.aspx '''The pH Pages''']. [http://www.national.com/ds/LM/LMC6001.pdf 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 [http://www.national.com/pf/TL/TL082.html TL082] or its equivalent.

== See also ==
*[[pH#Measurement|pH measurement]]
*[[pH indicator]]

==References==
<References />

==External links==
* [http://www.ijenco.com/ph-c-177.html Examples of pH meters] - Good example of how pH theories are applied to commercial products
* [http://www.omega.com/techref/ph.html Introduction to pH] - Excellent overview of pH and pH measurement at the Omega Engineering website
* [http://www.jp.horiba.com/story_e/ph/index.htm The story of pH made easy] - An introduction to the basics of pH and pH measurement
*[http://www.ph-meter.info www.ph-meter.info]- Theoretical and practical information about pH meters
*[http://acswebcontent.acs.org/landmarks/landmarks/phmeter/phmeter.html The Beckman pH Meter] - National Historic Chemical Landmark of the American Chemical Society
*[http://www2.iq.usp.br/docente/gutz/Curtipot_.html CurTiPot software] - All-in-one freeware for pH and acid-base equilibrium calculations and for simulation and analysis of POTentiometric TItration CURves

[[Category:Acid-base chemistry]]
[[Category:Electrochemistry]]
[[Category:Measuring instruments]]

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