View source for PH meter

{{short description|Instrument that indicates acidity or alkalinity in water-based solutions, expressed as pH}}
{{lowercase title|title= pH meter}}
[[File:Beckman Model M pH Meter 2006.072.002.tif|thumb|right|Beckman Model M pH Meter, 1937<ref>{{cite web|title=Beckman Coulter Product Milestones|url=https://www.beckmancoulter.com/ucm/idc/groups/public/documents/webasset/glb_bci_149519.pdf|website=Beckman Coulter|access-date=5 April 2017|archive-url=http://web.archive.org/web/20170406110720/https://www.beckmancoulter.com/ucm/idc/groups/public/documents/webasset/glb_bci_149519.pdf|archive-date=2017-04-06}}</ref>]]
[[File:Beckman model 72 pH meter jw827b87k.tiff|thumb|right|Beckman model 72 pH meter, 1960]]
[[Image:PH Meter.jpg|thumb|781 pH/Ion Meter pH meter by Metrohm]]
A '''pH meter''' is a [[scientific instrument]] that measures the [[hydrogen-ion]] [[Thermodynamic activity|activity]] in [[Aqueous solution|water-based solutions]], indicating its [[acidity]] or [[alkalinity]] expressed as [[pH]].<ref name="EB">{{cite web|title=pH meter|date=2016|website=Encyclopædia Britannica Online|url=http://www.britannica.com/technology/pH-meter|access-date=10 March 2016}}</ref> The pH meter measures the difference in [[electrical potential]] between a pH electrode and a reference electrode, and so the pH meter is sometimes referred to as a "potentiometric pH meter". The difference in electrical potential relates to the acidity or pH of the solution.<ref>Oxford Dictionary of Biochemistry and Molecular Biology (2 ed.), ed. Richard Cammack, Teresa Atwood, Peter Campbell, Howard Parish, Anthony Smith, Frank Vella, and John Stirling, Oxford University Press 2006, {{ISBN|9780198529170}}</ref> Testing of pH via pH meters ('''pH-metry''') is used in many applications ranging from [[Experimentation|laboratory experimentation]] to [[quality control]].<ref name="Global">{{cite web|title=pH Measurement and Value|url=http://www.globalw.com/support/ph-measurement.html|website=Global Water|publisher=Xylem, Inc.|access-date=21 March 2017}}</ref>
<!-- this diagram is inadequate as there should be three knobs: calibrate, slope and temperature.
[[Image:PH_Meter_01.JPG|thumbnail|A simple pH meter with its probe immersed in a mildly basic solution. The two knobheads are used to calibrate the instrument.]] -->
==Applications==
The rate and outcome of chemical reactions taking place in water often depends on the acidity of the water, and it is therefore useful to know the acidity of the water, typically measured by means of a pH meter.<ref name="Brittanica1">{{cite web|last1=Bell|first1=Ronald Percy|title=Acid-Base Reaction|url=https://www.britannica.com/science/acid-base-reaction|website=Encyclopaedia Britannica|publisher=Encyclopaedia Britannica, Inc.|access-date=21 March 2017}}</ref>  Knowledge of pH is useful or critical in many situations, including [[chemical laboratory]] analyses. pH meters are used for [[soil]] measurements in [[agriculture]], [[water quality]] for [[municipal water]] supplies, [[swimming pools]], [[environmental remediation]]; [[brewing]] of wine or beer; [[manufacturing]], healthcare and clinical applications such as [[blood chemistry]]; and many other applications.<ref name="Global" />
Advances in the instrumentation and in [[detection]] have expanded the number of applications in which pH measurements can be conducted. The devices have been [[miniaturized]], enabling direct measurement of pH inside of [[living cells]].<ref name="MMB2010">{{cite book|last1=Loiselle|first1=F.B.|last2=Casey|first2=J.R.|title=Membrane Transporters in Drug Discovery and Development |chapter=Measurement of Intracellular pH |date=2010|volume=637|pages=311–31|doi=10.1007/978-1-60761-700-6_17|pmid=20419443|series=Methods in Molecular Biology|isbn=978-1-60761-699-3}}</ref> In addition to measuring the pH of liquids, specially designed electrodes are available to measure the pH of semi-solid substances, such as foods. These have tips suitable for piercing semi-solids, have electrode materials compatible with ingredients in food, and are resistant to clogging.<ref name="Handbook">{{cite web|title=pH Measurement Handbook|url=http://www.pragolab.cz/files/aktuality/2015-03/pH-Measurement-Handbook.pdf|website=PragoLab|publisher=Thermo Scientific, Inc.|access-date=22 March 2017}}</ref>
==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>
==Types of pH meters==
[[Image:2009-03-30 Red pH meter reads 4.96.jpg|thumb|right|A simple pH meter]]
[[Image:Soil pH meter.jpg|thumb|right|[[Soil pH]] meter]]
In general there are three major categories of pH meters.  Benchtop pH meters are often used in laboratories and are used to measure samples which are brought to the pH meter for analysis.  Portable, or field pH meters, are handheld pH meters that are used to take the pH of a sample in a field or production site.<ref>{{cite web |title=What is a pH Meter and How Does it Work? |url=https://www.mt.com/global/en/home/products/Laboratory_Analytics_Browse/pH-meter.html#custom4 |publisher=Mettler-Toledo LLC |access-date=21 July 2021}}</ref>  In-line or in situ pH meters, also called pH analyzers, are used to measure pH continuously in a process, and can stand-alone, or be connected to a higher level information system for process control.<ref>{{cite web |title=A guide to pH Measurement Theory and Practice |url=https://www.mt.com/global/en/home/library/guides/process-analytics/ph-measurement-guide.html |publisher=Mettler-Toledo LLC |access-date=21 July 2021}}</ref>
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. The output can be digital or analog, and the devices can be [[battery-powered]] or rely on [[line power]]. Some versions use telemetry to connect the electrodes to the voltmeter display device.<ref name="Galster" />{{rp|197–215}}
Specialty meters and probes are available for use in special applications, such as harsh environments<ref name="Olson">{{cite web|last1=Olson|first1=Vickie|title=How to Select a pH Sensor for Harsh Process Environments|url=http://automation.isa.org/2015/04/how-to-select-a-ph-sensor-for-harsh-process-environments/|website=automation.isa.org|publisher=International Society for Automation|access-date=31 March 2017|date=2015-04-15}}</ref> and biological microenvironments.<ref name="MMB2010" /> There are also holographic pH sensors, which allow pH measurement [[Colorimetry (chemical method)|colorimetrically]], making use of the variety of [[pH indicators]] that are available.<ref>{{cite journal |author1=AK Yetisen |author2=H Butt |author3=F da Cruz Vasconcellos |author4=Y Montelongo |author5=CAB Davidson |author6=J Blyth |author7=JB Carmody |author8=S Vignolini |author9=U Steiner |author10=JJ Baumberg |author11=TD Wilkinson |author12=CR Lowe |title=Light-Directed Writing of Chemically Tunable Narrow-Band Holographic Sensors |journal= Advanced Optical Materials |volume=2 |issue=3 |pages=250 |year=2013 |doi= 10.1002/adom.201300375 |s2cid=96257175 |url=https://www.repository.cam.ac.uk/handle/1810/293246 }}</ref> Additionally, there are commercially available pH meters based on [[ISFET|solid state electrodes]], rather than conventional glass electrodes.<ref name="solidstate">{{cite web|title=pH Electrode|url=http://www.ph-meter.info/pH-electrode-solid-state|website=pH-meter.info|access-date=30 March 2017}}</ref>
==History==
[[File:Here's the new Beckman Pocket pH Meter 2012.002 b49f13 001.tif | thumb|right| "Here's the new Beckman Pocket pH Meter", 1956]]
The concept of pH was defined in 1909 by [[S. P. L. Sørensen]], and electrodes were used for pH measurement in the 1920s.<ref name=Determinants>{{cite book|last1=Travis|first1=Anthony S.|last2=Schröter|first2=H.G.|last3=Homburg|first3=E. |author3-link=Ernst Homburg|last4=Morris|first4=P.J.T.|title=Determinants in the evolution of the European chemical industry : 1900-1939 : new technologies, political frameworks, markets and companies|date=1998|publisher=Kluwer Acad. Publ.|location=Dordrecht|isbn=978-0-7923-4890-0|page=332|url=https://books.google.com/books?id=gvP7CAAAQBAJ&pg=PA332|access-date=29 May 2015}}</ref>
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 Growers, Incorporated|Sunkist]]).<ref name="hundred"/>{{rp|131–135}}
On April 8, 1935, Beckman's renamed [[National Technical Laboratories]] focused on the manufacture of scientific instruments, with the Arthur H. Thomas Company as a distributor for its pH meter.<ref name="hundred">{{cite book | isbn = 978-0-941901-23-9 |author1=Arnold Thackray |author2=Minor Myers, Jr. |name-list-style=amp |others = foreword by James D. Watson. | year = 2000 | publisher = Chemical Heritage Foundation | location = Philadelphia, Pa. | title = Arnold O. Beckman : one hundred years of excellence}}</ref>{{rp|131–135}} In its first full year of sales, 1936, the company sold 444 pH meters for $60,000 in sales.<ref name="Landmarks"/> In years to come, the company sold millions of the units.<ref name=Chicago>{{cite news|last=Luther|first=Claudia|title=Arnold O. Beckman, 104|url=https://www.chicagotribune.com/2004/05/19/arnold-o-beckman-104/|access-date=8 March 2014|newspaper=Chicago Tribune News|date=May 19, 2004}}</ref><ref name=Jaehnig>{{cite book|last1=Jaehnig|first1=Kenton G.|title=Finding Aid to the Beckman Historical Collection 1911 - 2011 (Bulk 1935 - 2004 )|url=http://othmerlib.sciencehistory.org/record=b1072260~S6|website=Science History Institute|quote="Click on 'Beckman Historical Collection Finding Aid' to go to full document."|access-date=30 October 2015}}</ref> In 2004 the Beckman pH meter was designated an [[National Historic Chemical Landmarks|ACS National Historic Chemical Landmark]] in recognition of its significance as the first commercially successful electronic pH meter.<ref name="Landmarks">{{cite web |url = http://www.acs.org/content/acs/en/education/whatischemistry/landmarks/beckman.html |title = Development of the Beckman pH Meter |publisher = American Chemical Society |work = National Historic Chemical Landmarks |access-date= March 25, 2013}}</ref>
The [[Radiometer (company)|Radiometer]] Corporation of 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."<ref name=Determinants/>
In the 1940s the electrodes for pH meters were often difficult to make, or unreliable due to brittle glass. Dr. [[Werner Ingold]] began to industrialize the production of single-rod measuring cells, a combination of measurement and reference electrode in one construction unit,<ref>15.3.1957: English Patent – Measuring assemblies for the determination of ion concentrations and redox potentials, particularly suitable for carrying out measurements at elevated temperatures. Patent No. 850177</ref> which led to broader acceptance in a wide range of industries including pharmaceutical production.<ref>Dr. A. Fiechter, Dr. W. Ingold und A. Baerfuss, Chemie-Ingenieur-Technik 10 (1964) 1000-1004: "Die pH-Kontrolle in der mikrobiologischen Verfahrenstechnik"</ref>
Beckman marketed a portable "Pocket pH Meter" as early as 1956, but it did not have a digital read-out.<ref>{{cite web |title=Here's the new Beckman Pocket pH Meter |date= 1956 |url=https://digital.sciencehistory.org/works/4q77fr32b |website=Science History Institute |access-date=6 August 2019}}</ref>
In the 1970s Jenco Electronics of [[Taiwan]] designed and manufactured the first portable digital pH meter.  This meter was sold under the label of the [[Cole-Parmer|Cole-Parmer Corporation]].<ref name="Buie">{{cite web|last1=Buie|first1=John|title=Evolution of the pH Meter|url=http://www.labmanager.com/lab-product/2010/10/evolution-of-the-ph-meter|website=Lab Manager|access-date=October 7, 2010}}</ref>
==Building a pH meter==
Specialized manufacturing is required for the electrodes, and details of their design and construction are typically trade secrets.<ref name="Galster" />{{rp|125}} However, with purchase of suitable electrodes, a standard [[multimeter]] can be used to complete the construction of the pH meter.<ref name="Simple">{{cite web|title=Building the Simplest Possible pH Meter|url=http://www.66pacific.com/ph/simplest-ph.aspx|website=66pacific.com|access-date=29 March 2017}}</ref>  However, commercial suppliers offer voltmeter displays that simplify use, including calibration and temperature compensation.<ref name="Handbook" />
==See also==
* [[Antimony electrode]]
* [[Ion-selective electrode]]s
* [[ISFET]] pH electrode
* [[Potentiometry]]
* [[Quinhydrone electrode]]
* [[Saturated calomel electrode]]
* [[Silver chloride electrode]]
* [[Standard hydrogen electrode]]
==References==
{{Reflist}}
==External links==
{{commons category|{{not a typo|㏗ meters}}|pH meters}}
* [https://www.omega.com/en-us/resources/ph-meter Introduction to pH measurement] – Overview of pH and pH measurement at the Omega Engineering website
* [http://www.acs.org/content/acs/en/education/whatischemistry/landmarks/beckman.html Development of the Beckman pH Meter] – National Historic Chemical Landmark of the American Chemical Society
* [http://www.pragolab.cz/files/aktuality/2015-03/pH-Measurement-Handbook.pdf pH Measurement Handbook] -  A publication of the Thermo-Scientific Co.
{{Laboratory equipment}}
{{DEFAULTSORT:Ph Meter}}
[[Category:Acid–base chemistry]]
[[Category:Electrochemistry]]
[[Category:Measuring instruments]]
[[Category:Scientific instruments]]