Editing PH meter (section)

==Design and use==
[[File:George Garcelon using Beckman pH meter 2004.012.020.tif| thumb|right| 300px| Using an early Beckman pH meter in a lab]]

===Principle of operation===
[[Potentiometric]] pH meters 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 electrodes, or alternatively a combination electrode, and some form of display calibrated in pH units. It usually has a [[glass electrode]] and a [[reference electrode]], or a combination electrode. The electrodes, or probes, are inserted into the solution to be tested.<ref>{{cite journal|first=Peter|last=Riddle|title=pH meters and their electrodes: calibration, maintenance and use|journal=The Biomedical Scientist|date=2013|volume=April|pages=202–205}}</ref> pH meters may also be based on the [[antimony electrode]] (typically used for rough conditions) or the [[quinhydrone electrode]].

In order to accurately measure the potential difference between the two sides of the glass membrane [[reference electrode]], typically a [[silver chloride electrode]] or [[calomel electrode]] are required on each side of the membrane. Their purpose is to measure changes in the potential on their respective side. One is built into the glass electrode. The other, which makes contact with the test solution through a porous plug, may be a separate reference electrode or may be built into a combination electrode. The resulting voltage will be the potential difference between the two sides of the glass membrane possibly offset by some difference between the two reference electrodes, that can be compensated for. The article on the [[glass electrode#Galvanic cell schematic representation|glass electrode]] has a good description and figure.

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 glass electrode for measuring the pH has a glass bulb specifically designed to be selective to hydrogen-ion concentration. On immersion in the solution to be tested, hydrogen ions in the test solution exchange for other positively charged ions on the glass bulb, creating an electrochemical potential across the bulb. The electronic amplifier detects the difference in electrical potential between the two electrodes generated in the measurement and converts the potential difference to pH units. 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.  This conductor is immersed in an electrolyte solution, typically potassium chloride, which comes into contact with the test solution through a porous ceramic membrane.<ref name="Seafriends">{{cite web|last1=Anthoni|first1=J. Floor|title=pH Meter Principles|url=http://www.seafriends.org.nz/dda/ph.htm|website=seafriends.org|publisher=Seafriends Marine Conservation and Education Centre|access-date=28 March 2017}}</ref> The display consists of a [[voltmeter]], which displays voltage in units of pH.<ref name="Seafriends" />

On immersion of the glass electrode and the reference electrode in the test solution, an [[electrical circuit]] is completed, in which there is a potential difference created and detected by the voltmeter. The circuit can be thought of as going from the conductive element of the reference electrode to the surrounding potassium-chloride solution, through the ceramic membrane to the test solution, the hydrogen-ion-selective glass of the glass electrode, to the solution inside the glass electrode, to the silver of the glass electrode, and finally the voltmeter of the display device.<ref name="Seafriends" /> The voltage varies from test solution to test solution depending on the potential difference created by the difference in hydrogen-ion concentrations on each side of the glass membrane between the test solution and the solution inside the glass electrode.  All other potential differences in the circuit do not vary with pH and are corrected for by means of the calibration.<ref name="Seafriends" />

For simplicity, many pH meters use a combination probe, constructed with the glass electrode and the reference electrode contained within a single probe. A detailed description of combination electrodes is given in the article on [[glass electrode]]s.<ref name="Vanysek">{{cite news|last1=Vanýsek|first1=Petr|title=The Glass pH Electrode|url=https://www.electrochem.org/dl/interface/sum/sum04/IF6-04-Pages19-20.pdf|access-date=3 April 2017|work=Interface|issue=Summer|publisher=The Electrochemical Society|date=2004|pages=19–20}}</ref>

The pH meter is [[calibrated]] with solutions of known pH, typically before each use, to ensure [[accuracy]] of measurement.<ref name=magub>[http://bitesizebio.com/8750/how-to-care-for-your-ph-meter/ Bitesize Bio: How to Care for Your pH Meter], Steffi Magub, 18 May 2012.</ref>  To measure the pH of a solution, the electrodes are used as probes, which are dipped into the test solutions and held there sufficiently long for the hydrogen ions in the test solution to [[Dynamic equilibrium|equilibrate]] with the [[ions]] on the surface of the bulb on the glass electrode. This equilibration provides a stable pH measurement.<ref>{{cite web|title=Theory and Practice of pH Measurement|url=http://www.emerson.com/resource/blob/70736/dc7766471ccd3e4d6fe257f75c4f2053/manual--theory-and-practice-of-ph-measurement-data.pdf|website=Emerson Process Management|date=December 2010|access-date=2017-04-03|archive-date=2016-10-20|archive-url=https://web.archive.org/web/20161020014006/http://www.emerson.com/resource/blob/70736/dc7766471ccd3e4d6fe257f75c4f2053/manual--theory-and-practice-of-ph-measurement-data.pdf|url-status=dead}}</ref>

===pH electrode and reference electrode design===
Details of the fabrication and resulting microstructure of the glass membrane of the pH electrode are maintained as [[trade secrets]] by the manufacturers.<ref name="Galster">{{cite book|last1=Galster|first1=Helmuth|title=pH Measurement: Fundamentals, Methods, Applications, Instrumentation|date=1991|publisher=VCH Publishers, Inc.|location=Weinheim|isbn=978-3-527-28237-1}}</ref>{{rp|125}} However, certain aspects of design are published. Glass is a solid electrolyte, for which alkali-metal ions can carry current. The pH-sensitive glass membrane is generally spherical to simplify the manufacture of a uniform membrane. These membranes are up to 0.4 millimeters in thickness, thicker than original designs, so as to render the probes durable. The glass has [[silicate]] [[chemical function]]ality on its surface, which provides binding sites for alkali-metal ions and hydrogen ions from the solutions. This provides an ion-exchange capacity in the range of 10<sup>−6</sup> to 10<sup>−8</sup>&nbsp;mol/cm<sup>2</sup>. Selectivity for hydrogen ions (H<sup>+</sup>) arises from a balance of ionic charge, volume requirements versus other ions, and the coordination number of other ions. Electrode manufacturers have developed compositions that suitably balance these factors, most notably lithium glass.<ref name="Galster" />{{rp|113–139}}

The [[silver chloride electrode]] is most commonly used as a [[reference electrode]] in pH meters, although some designs use the [[saturated calomel electrode]]. The silver chloride electrode is simple to manufacture and provides high [[reproducibility]]. The reference electrode usually consists of a platinum wire that has contact with a silver/silver chloride mixture, which is immersed in a potassium chloride solution. There is a ceramic plug, which serves as a contact to the test solution, providing low resistance while preventing mixing of the two solutions.<ref name="Galster" />{{rp|76–91}}

With these electrode designs, the voltmeter is detecting potential differences of ±1400 millivolts.<ref name="Bye">{{cite journal|title=Potentiometric pH Meter|journal=Journal of Scientific Instruments|date=1962|volume=39|issue=6|page=323|doi=10.1088/0950-7671/39/6/442|last1=Ltd|first1=W G Pye and Co}}</ref> The electrodes are further designed to rapidly equilibrate with test solutions to facilitate [[ease of use]]. The equilibration times are typically less than one second, although equilibration times increase as the electrodes age.<ref name="Galster" />{{rp|164}}

===Maintenance===
Because of the sensitivity of the electrodes to contaminants, cleanliness of the probes is essential for [[accuracy and precision]]. Probes are generally kept moist when not in use with a medium appropriate for the particular probe, which is typically an aqueous solution available from probe manufacturers.<ref name=magub/><ref name=mrc/> Probe manufacturers provide instructions for cleaning and maintaining their probe designs.<ref name=magub/> For illustration, one maker of laboratory-grade pH gives cleaning instructions for specific contaminants: general cleaning (15-minute 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.<ref name=mrc>[http://mrclab.com/media/uploads/ph_electrode_maintenance.pdf MRC lab: How to Store, Clean, and Recondition pH Electrodes] {{Webarchive|url=https://web.archive.org/web/20150922044420/http://mrclab.com/media/uploads/ph_electrode_maintenance.pdf |date=2015-09-22 }}.</ref><ref>[http://www.ph-meter.info/pH-electrode-cleaning Cleaning electrodes].</ref>

===Calibration and operation===
[[File:PH Meter 01.jpg|thumb|5.739 pH/Ion at 23 °C temperature shown on photo. pH 7110 pH meter manufactured by inoLab]]
The [[German Institute for Standardization]] publishes a standard for pH measurement using pH meters, DIN 19263.<ref name="DIN">{{cite web|title=pH Measurement - pH Measuring Chains|url=https://www.beuth.de/de/norm/din-19263/96105863|website=Beuth publishing DIN|publisher=Beuth Verlag GmbH|access-date=28 March 2017}}</ref>

Very precise measurements necessitate that the pH meter is calibrated before each measurement. More typically calibration is performed once per day of operation. Calibration is needed because the glass electrode does not give reproducible [[electrostatic potential]]s over longer periods of time.<ref name="Galster" />{{rp|238–239}}

Consistent with principles of [[good laboratory practice]], calibration is performed with at least two standard [[buffer solution]]s that span the range of pH values to be measured. For general purposes, buffers at pH 4.00 and pH 10.00 are suitable. The pH meter has one calibration control to set the meter reading equal to the value of the first standard buffer and a second control to adjust the meter reading to the value of the second buffer. A third control allows the temperature to be set. Standard buffer sachets, available from a variety of suppliers, usually document the [[temperature dependence]] of the buffer control. More precise measurements sometimes require calibration at three different pH values. Some pH meters provide built-in temperature-coefficient correction, with temperature [[thermocouples]] in the electrode probes. The calibration process correlates the voltage produced by the probe (approximately 0.06&nbsp;volts per pH unit) with the pH scale. Good laboratory practice dictates that, after each measurement, the probes are 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 immersed in a storage solution suitable for the particular probe type.<ref name="all-about-pH">{{cite web|title=How to perform a pH meter calibration|url=http://www.all-about-ph.com/ph-meter-calibration.html|website=all-about-pH.com|access-date=14 December 2016}}</ref>