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pH Sensor

pH Sensor. Blake Hunsaker ECE 5320 Mechatronics Assignment #1. Outline. Introduction Definition pH Scale pH Color Chart Concentration of Hydrogen ions compared to distilled water How does pH measurement work? Combination pH Electrode How does the pH electrode work? Ag/AgCl wire

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pH Sensor

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  1. pH Sensor Blake Hunsaker ECE 5320 Mechatronics Assignment #1

  2. Outline • Introduction • Definition • pH Scale • pH Color Chart • Concentration of Hydrogen ions compared to distilled water • How does pH measurement work? • Combination pH Electrode • How does the pH electrode work? • Ag/AgCl wire • The ideal pH electrode • But the electrode is practically never ideal

  3. Introduction: • The pH concept was introduced in 1909 by the Danish chemist Sorensen. • The pH of a solution indicates how acidic or basic (alkaline) it is.

  4. Definition: • pH is the negative logarithm of the hydrogen ion activity. • The pH term translates the values of the hydrogen ion concentration - which ordinarily ranges between about 1 and 10 x -14 gram-equivalents per liter - into numbers between 0 and 14.

  5. pH Scale • If the level of H+ ions increases, the substance is considered an acid and the pH number is below 7. • A neutral solution such as water has a pH of approximately 7. • If the level of OH- ions increases, the substance is considered to be alkaline or base and the pH number is above 7.

  6. pH Color Chart

  7. Concentration of Hydrogen ions compared to distilled water “k12science.ati.stevens-tech.edu/curriculum/ waterproj/phconcentrations.html “

  8. How does pH measurement work? • The pH measurement is obtained using 2 electrodes which have in recent years been combined to form a single probe which is termed a 'combination pH electrode'. • A millivolt reading is then converted to a pH reading returned by the sensor through a BNC or DIN connector. “accessories.picotech.com/ph_sensor.html”

  9. Combination pH Electrode • Consists of a pH half cell (or measurement cell), whose voltage varies proportionally to the Hydrogen ion activity of the solution being measured. • The other half of the probe is called a reference half cell, which provides a stable and constant reference voltage. • The pH meter is connected to the pH combination electrode and measures the difference between the pH half cell and the reference half cell in millivolts direct current.

  10. How does the pH electrode work?(Slide A) • The measurement part of an electrode consists of a thin membrane of Hydrogen sensitive glass blown on the end of an inert glass tube. • This tube is filled with electrolyte, and the signal is carried through a Ag/AgCl wire (silver-silver chloride wire).

  11. Ag/AgCl wire • The body of the electrode is made from porous glass and serves as the ionic conducting electrical pathway between the inside of the reference electrode and the bulk of your cell. “www.consultrsr.com/resources/ref/agcl.htm “

  12. How does the pH electrode work?(Slide B) • A similar tube (but without the sensitive glass) makes up the reference part of the electrode. • A small hole is formed in the reference electrode tubing and a filter inserted within the hole. • This filter allows the current to pass through and thus forms a circuit to the wire within the measurement cell via the hydrogen sensitive glass bulb.

  13. The ideal pH electrode • Zero volts output at neutral pH (=7.0) • Positive voltage in acids, pH<7 • Negative voltages in bases, pH>7 • Generates -59.16 millivolts per pH unit at room temperature (="Nernst potential"). “www.emesystems.com/OL2ph.htm “ Nernst potential: calculation of the exact electrical potential at equilibrium that is generated for a known concentration difference in a specific ion, separated by a membrane permeable to that ion.

  14. But the electrode is practically never ideal • The slope (Nernst potential) and offset will drift slowly with time as the electrode ages. • Therefore, it is necessary to calibrate and standardize the pH monitoring and recording instrument from time to time. • The pH measurement is temperature dependent. • -0.002mV per degree C change in the slope factor.

  15. References • Hanna Digital Catalogue 2003 Version A • Electronically Monitored Ecosystems, Berkeley CA U.S.A. • www.consultrsr.com • www.sensorland.com

  16. To Explore Further • www.sensorland.com • www.honeywell.com • www.globalspec.com • www.vernier.com • www.picotech.com • www.oceanoptics.com

  17. THE END Thank You

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