pH meter
A pH meter is a
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.
Advances in the instrumentation and in
Design and use
Principle of operation
In order to accurately measure the potential difference between the two sides of the glass membrane
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.[9] The display consists of a voltmeter, which displays voltage in units of pH.[9]
On immersion of the glass electrode and the reference electrode in the test solution, an
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 electrodes.[10]
The pH meter is
pH electrode and reference electrode design
Details of the fabrication and resulting microstructure of the glass membrane of the pH electrode are maintained as
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.[13]: 76–91
With these electrode designs, the voltmeter is detecting potential differences of ±1400 millivolts.
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.[11][15] Probe manufacturers provide instructions for cleaning and maintaining their probe designs.[11] 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.[15][16]
Calibration and operation
The
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
Consistent with principles of
Types of pH meters
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.[19] 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.[20]
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
Specialty meters and probes are available for use in special applications, such as harsh environments
History
The concept of pH was defined in 1909 by S. P. L. Sørensen, and electrodes were used for pH measurement in the 1920s.[24]
In October 1934,
: 131–135On April 8, 1935, Beckman's renamed
The 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."[24]
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,[29] which led to broader acceptance in a wide range of industries including pharmaceutical production.[30]
Beckman marketed a portable "Pocket pH Meter" as early as 1956, but it did not have a digital read-out.[31] 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 Corporation.[32]
Building a pH meter
Specialized manufacturing is required for the electrodes, and details of their design and construction are typically trade secrets.[13]: 125 However, with purchase of suitable electrodes, a standard multimeter can be used to complete the construction of the pH meter.[33] However, commercial suppliers offer voltmeter displays that simplify use, including calibration and temperature compensation.[7]
See also
- Antimony electrode
- Ion-selective electrodes
- ISFET pH electrode
- Potentiometry
- Quinhydrone electrode
- Saturated calomel electrode
- Silver chloride electrode
- Standard hydrogen electrode
References
- ^ "Beckman Coulter Product Milestones" (PDF). Beckman Coulter. Retrieved 5 April 2017.
- ^ "pH meter". Encyclopædia Britannica Online. 2016. Retrieved 10 March 2016.
- ISBN 9780198529170
- ^ a b "pH Measurement and Value". Global Water. Xylem, Inc. Retrieved 21 March 2017.
- ^ Bell, Ronald Percy. "Acid-Base Reaction". Encyclopaedia Britannica. Encyclopaedia Britannica, Inc. Retrieved 21 March 2017.
- ^ PMID 20419443.
- ^ a b "pH Measurement Handbook" (PDF). PragoLab. Thermo Scientific, Inc. Retrieved 22 March 2017.
- ^ Riddle, Peter (2013). "pH meters and their electrodes: calibration, maintenance and use". The Biomedical Scientist. April: 202–205.
- ^ a b c d Anthoni, J. Floor. "pH Meter Principles". seafriends.org. Seafriends Marine Conservation and Education Centre. Retrieved 28 March 2017.
- ^ Vanýsek, Petr (2004). "The Glass pH Electrode" (PDF). Interface. No. Summer. The Electrochemical Society. pp. 19–20. Retrieved 3 April 2017.
- ^ a b c Bitesize Bio: How to Care for Your pH Meter, Steffi Magub, 18 May 2012.
- ^ "Theory and Practice of pH Measurement" (PDF). Emerson Process Management. December 2010. Archived from the original (PDF) on 2016-10-20. Retrieved 2017-04-03.
- ^ ISBN 978-3-527-28237-1.
- .
- ^ a b MRC lab: How to Store, Clean, and Recondition pH Electrodes Archived 2015-09-22 at the Wayback Machine.
- ^ Cleaning electrodes.
- ^ "pH Measurement - pH Measuring Chains". Beuth publishing DIN. Beuth Verlag GmbH. Retrieved 28 March 2017.
- ^ "How to perform a pH meter calibration". all-about-pH.com. Retrieved 14 December 2016.
- ^ "What is a pH Meter and How Does it Work?". Mettler-Toledo LLC. Retrieved 21 July 2021.
- ^ "A guide to pH Measurement Theory and Practice". Mettler-Toledo LLC. Retrieved 21 July 2021.
- ^ Olson, Vickie (2015-04-15). "How to Select a pH Sensor for Harsh Process Environments". automation.isa.org. International Society for Automation. Retrieved 31 March 2017.
- S2CID 96257175.
- ^ "pH Electrode". pH-meter.info. Retrieved 30 March 2017.
- ^ ISBN 978-0-7923-4890-0. Retrieved 29 May 2015.
- ^ ISBN 978-0-941901-23-9.
- ^ a b "Development of the Beckman pH Meter". National Historic Chemical Landmarks. American Chemical Society. Retrieved March 25, 2013.
- ^ Luther, Claudia (May 19, 2004). "Arnold O. Beckman, 104". Chicago Tribune News. Retrieved 8 March 2014.
- ^ Jaehnig, Kenton G. Finding Aid to the Beckman Historical Collection 1911 - 2011 (Bulk 1935 - 2004 ). Retrieved 30 October 2015.
Click on 'Beckman Historical Collection Finding Aid' to go to full document.
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ignored (help) - ^ 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
- ^ Dr. A. Fiechter, Dr. W. Ingold und A. Baerfuss, Chemie-Ingenieur-Technik 10 (1964) 1000-1004: "Die pH-Kontrolle in der mikrobiologischen Verfahrenstechnik"
- ^ "Here's the new Beckman Pocket pH Meter". Science History Institute. 1956. Retrieved 6 August 2019.
- ^ Buie, John. "Evolution of the pH Meter". Lab Manager. Retrieved October 7, 2010.
- ^ "Building the Simplest Possible pH Meter". 66pacific.com. Retrieved 29 March 2017.
External links
- Introduction to pH measurement – 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
- pH Measurement Handbook - A publication of the Thermo-Scientific Co.