pH Sensor

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pH Sensor

• Blake Hunsaker
• ECE 5320
• Mechatronics
• Assignment 1

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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

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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.

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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.

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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.

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

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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).

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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
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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.

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The ideal pH electrode

• Zero volts output at neutral pH (7.0)
• Positive voltage in acids, pHlt7
• Negative voltages in bases, pHgt7
• 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.
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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.

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References

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

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To Explore Further

• www.sensorland.com
• www.honeywell.com
• www.globalspec.com
• www.vernier.com
• www.picotech.com
• www.oceanoptics.com

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THE END

• Thank You