Improving pH Control Using Differential Sensor Technology

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Improving pH Control Using Differential Sensor
Technology

• Ryan Richardson
• Hach Company

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Presenter

• Ryan Richardson has worked in the instrumentation
  industry for 6 years, first with Orbisphere and
  now as the Regional Sales Manager for industrial
  accounts in Georgia for Hach Company. Ryan
  graduated summa cum laude from Georgia Institute
  of Technology and resides in Cumming, GA with his
  wife and son.

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Take Home Messages

• pH is an electrochemical measurement of hydrogen
  ion concentration, useful in many applications.
• pH measurement can be plagued by many problems
  associated with electrochemical cells.
• Differential pH probes eliminate many of the
  causes of measurement error and probe failure.
• Proper probe maintenance is essential

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Outline

• Understanding pH
• Measuring pH
• Benefits of Differential
• Questions

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Understanding pH
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pH Theory

• pH is a measurement of the relative acidity of an
  aqueous solution
• pH is a measurement of hydrogen ion H
  concentration

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pH Scale
Ammonia
Vinegar
Pure Water
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Measuring pH
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How Does a pH Probe Work?

• Electrochemical cell
• Two electrodes in probe pH measuring half-cell
  reference half-cell

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METER
Reference Electrode
pH Measuring Electrode
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pH Measuring Electrode
Principle of Operation
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Reference Electrode
Principle of Operation
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Nernst Equation
Em
Er
E E0 - 2.3RT log ai nf E
59.16 mV per Decade _at_ 25C
E Em - Er
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Temperature Effects
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Probe Types

• Conventional Method
• Combination Sensor
• Differential Sensor

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pH Measurement
Conventional Method
Em
Er
pH Glass Electrode
pH Reference Electrode
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Combination Sensor
Em
Er
pH Reference Electrode
pH Glass Measuring Electrode
Reference Junction
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Measuring Electrode

• Typical Problems
• pH membrane coating
• Slow response due to high impedance
• Abrasion and/or breakage
• Temperature shock

pH Glass Measuring Electrode
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Reference Electrode

• Typical Problems
• Plugging junction
• Poisoning of the electrode
• Rapid electrolyte depletion
• Ground loops

Reference electrodes are large of failures in
pH measurement
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Differential Sensor
Solution Ground Electrode (Eg)
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Buffered pH 7 vs. KCL
Ground electrode
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Differential Sensor
Salt Bridge
pH Electrode
Ground Electrode
LCP
pHD
Reference Electrode
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Differential Sensor

• Summary of benefits
• Glass reference electrode vs. exposed Ag/AgCl
  wire
• Easy to clean no poisoning
• Reference solution is highly buffered pH 7, not
  KCl
• Harder to contaminate and shift reference
  potential
• Ground electrode eliminates ground loop currents
• Currents dont affect reference potential
• Replaceable, double junction salt bridge
• Further resists contamination of reference
  solution (Buffered pH 7)
• Preamp allows for greater distance between sensor
  and controller

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Questions?

• Questions?