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pH SensorOutlineIntroduction:Definition:pH ScalepH Color ChartConcentration of Hydrogen ions compared to distilled waterHow does pH measurement work?Combination pH ElectrodeHow does the pH electrode work? (Slide A)Ag/AgCl wireHow does the pH electrode work? (Slide B)The ideal pH electrodeBut the electrode is practically never idealReferencesTo Explore FurtherTHE ENDpH SensorpH SensorBlake HunsakerBlake HunsakerECE 5320 ECE 5320 Mechatronics Mechatronics Assignment #1Assignment #1OutlineOutline•IntroductionIntroduction•DefinitionDefinition•pH ScalepH Scale–pH Color ChartpH Color Chart–Concentration of Hydrogen ions compared to distilled waConcentration of Hydrogen ions compared to distilled waterter•How does pH measurement work?How does pH measurement work?–Combination pH ElectrodeCombination pH Electrode•How does the pH electrode work?How does the pH electrode work?–Ag/Ag/AgClAgCl wire wire•The ideal pH electrodeThe ideal pH electrode•But the electrode is practically never idealBut the electrode is practically never idealIntroduction:Introduction:•The pH concept was introduced in The pH concept was introduced in 1909 by the Danish chemist 1909 by the Danish chemist Sorensen.Sorensen.•The pH of a solution indicates how The pH of a solution indicates how acidic or basic (alkaline) it is. acidic or basic (alkaline) it is.Definition:Definition:•pH is the negative logarithm of the pH is the negative logarithm of the hydrogen ion activity.hydrogen ion activity. •The pH term translates the values of the The pH term translates the values of the hydrogen ion concentration - which hydrogen ion concentration - which ordinarily ranges between about 1 and 10 ordinarily ranges between about 1 and 10 x -14 gram-equivalents per liter - into x -14 gram-equivalents per liter - into numbers between 0 and 14. numbers between 0 and 14.pH ScalepH Scale•If the level of H+ ions increases, the If the level of H+ ions increases, the substance is considered an acid and the pH substance is considered an acid and the pH number is below 7.number is below 7.•A neutral solution such as water has a pH of A neutral solution such as water has a pH of approximately 7. approximately 7. •If the level of OH- ions increases, the If the level of OH- ions increases, the substance is considered to be alkaline or substance is considered to be alkaline or base and the pH number is above 7.base and the pH number is above 7.pH Color ChartpH Color ChartConcentration of Hydrogen ions Concentration of Hydrogen ions compared to distilled watercompared to distilled water“k12science.ati.stevens-tech.edu/curriculum/ waterproj/phconcentrations.html “How does pH measurement How does pH measurement work? work? •The pH measurement The pH measurement is obtained using 2 is obtained using 2 electrodes which have electrodes which have in recent years been in recent years been combined to form a combined to form a single probe which is single probe which is termed a 'combination termed a 'combination pH electrode'.pH electrode'. •A millivolt reading is A millivolt reading is then converted to a pH then converted to a pH reading returned by reading returned by the sensor through a the sensor through a BNC or DIN connector.BNC or DIN connector. “accessories.picotech.com/ph_sensor.html”Combination pH ElectrodeCombination pH Electrode•Consists of a pH half cell (or measurement cell), Consists of a pH half cell (or measurement cell), whose voltage varies proportionally to the whose voltage varies proportionally to the Hydrogen ion activity of the solution being Hydrogen ion activity of the solution being measured. measured. •The other half of the probe is called a reference The other half of the probe is called a reference half cell, which provides a stable and constant half cell, which provides a stable and constant reference voltage. reference voltage. •The pH meter is connected to the pH The pH meter is connected to the pH combination electrode and measures the combination electrode and measures the difference between the pH half cell and the difference between the pH half cell and the reference half cell in millivolts direct current.reference half cell in millivolts direct current.How does the pH electrode How does the pH electrode work?work?(Slide A) (Slide A) •The measurement part of an electrode The measurement part of an electrode consists of a thin membrane of Hydrogen consists of a thin membrane of Hydrogen sensitive sensitive glass blown on the end of an inert glass tube. glass blown on the end of an inert glass tube. •This tube is filled with electrolyte, and the This tube is filled with electrolyte, and the signal signal is carried through a Ag/AgCl wire (silver-silver is carried through a Ag/AgCl wire (silver-silver chloride wire).chloride wire).Ag/AgCl wireAg/AgCl wire•The body of the The body of the electrode is made electrode is made from porous glass from porous glass and serves as the and serves as the ionic conducting ionic conducting electrical pathway electrical pathway between the inside between the inside of the reference of the reference electrode and the electrode and the bulk of your cell. bulk of your cell. “www.consultrsr.com/resources/ref/agcl.htm “How does the pH electrode How does the pH electrode work?work?(Slide B)(Slide B)•A similar tube (but without the sensitive glass) A similar tube (but without the sensitive glass) makes up the reference part of the electrode. makes up the reference part of the electrode. •A small hole is formed in the reference A small hole is formed in the reference electrode tubing and a filter inserted within the electrode tubing and a filter inserted within the hole. hole. •This filter allows the current to pass through and This filter allows the current to pass through and thus forms a circuit to the wire within the thus forms a circuit to the wire within the measurement cell via the hydrogen sensitive measurement cell via the hydrogen sensitive glass bulb.glass bulb.The ideal pH electrodeThe ideal pH electrode•Zero volts output at neutral Zero volts output at neutral pH (=7.0) pH (=7.0) •Positive voltage in acids, Positive voltage in acids, pH<7 pH<7 •Negative voltages in bases, Negative voltages in bases, pH>7 pH>7 •Generates -59.16 millivolts Generates -59.16 millivolts per pH unit at room per pH unit at room temperature (="Nernst temperature (="Nernst potential"). potential"). Nernst potential:


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USU ECE 5320 - PH Sensor

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