Advances in pH/ORP Measurement

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Advances in pH/ORP Measurement Polymeric Sensors
Presented by John Daly South Fork Instruments
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Introduction
pH probes are generally considered to be
consumable items in many plants, with high
maintenance overhead and high associated cost.
This session introduces Solid State Polymeric
sensor technology as an alternative, comparing it
to conventional pH sensor technology and describe
why challenging and extreme pH applications can
be solved using it
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Overview

• pH probes are electochemical devices that react
  to H2 ions in the liquid being measured - the
  more H2 ions, the more acidic the liquid.
• In order to operate correctly, the probe must be
  in intimate contact with the process fuid, and
  that is where the problems start..

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How pH probes work

• pH probes consist of two parts the measurement
  cell and the reference cell

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How the pH Sensitive Glass Works

• Lithium Ions in the pH sensitive glass act as
  current carriers
• Positive Charged Free Hydrogen Ions (H)Develop
  Positive mV Potential Relative to Internal Buffer
• Acidic Solutions
• Fewer Hydrogen Ions Relative to Internal Buffer
  Produce a Negative mV Potential
• Alkaline Solutions

Internal Fill Solution
pH Glass
Internal Gel Layer
External Gel Layer
Process
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pH Electrode with pH Sensitive Glass
Silver/Silver Chloride wire and billet
Glass membrane thickness 0.2-0.5 mm
Gel layer on both sides of glass
Internal Solution H is constant


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Glass Matrix



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Acid solution lt 7
Alkaline solution gt 7
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The Reference Electrode and Liquid Junction
Silver Wire
Electrolyte Liquid, Gel or Polymer
Silver/Silver Chloride Reference Billet
Liquid Junction Ceramic, Wood, or Plastic
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Making a pH Measurement
pH Measurement
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The pH Combination Electrode Circuit

• E 1 Half-cell voltage Ag/AgCl KCl (pH
  electrode), voltage depends on electrolyte
  concentration
• E 2 Potential of internal buffer, inside glass
  membrane
• E 3 Potental voltage across glass membrane
• E 4 Variable potential on the outside of the
  membrane
• E 5 Flow diffusion potential
• E 6 Reference diffusion potential
• E 7 Half-cell voltage Ag/AgCl KCl
• (reference electrode), voltage depends on
  electrolyte concentration

Reference Sensor
pH Sensor
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pH is a Potentiometric Measurement

• The Measuring System consists of a pH Measuring
  Electrode and Reference Electrode
• The Potential Difference Between the Two
  Electrodes is a Function of the pH Value of the
  Measured Solution
• The Solution Must Be Conductive and is Part of
  the Electrical Circuit

pH Measuring Electrode
Reference Electrode
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Reference Cell Problems (1)

• In conventional pH probe designs, the reference
  electrode and reference cell contents are in
  contact with the process fluid
• Under certain conditions, the cell becomes
  depleted

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Reference Cell Problems (2)

• Under different conditions, the cell becomes
  poisoned
• Reference cell depletion or poisoning leads to
  drift and probe failure

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Reference Cell Strategies

• Double Porous Junction Probes
• An additional junction is installed to slow down
  depletion around the electrode itself and keep
  poisons out longer
• Tortuous Path Electrodes
• A long poisoning path is built into the probe to
  prolong life
• Flowing Junction Electrodes
• Electrolyte is flowed through the porous junction
  to prevent ingress of poisons and to maintain
  electrolyte concentration

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The Polymeric Solution

• No Porous Junction to allow poisoning and
  depletion
• Polymeric sleeve isolates the reference electrode
  from the process

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Benefits of Polymeric Sensors

• Long, long life
• Plastic is ionic and takes a long time to deplete
• Minimal Zero Drift
• Reference is not depleting
• Resiliant to fouling
• No junction to plug up

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Applications

• Sour Water
• Ultra Pure Water
• Waste Water Treatment
• Oily Water
• Low Ionic Service
• Vacuum Service

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Summary

• Polymeric sensors provide longer life and better
  stability in difficult applications than
  conventional technology probes.
• In standard applications, the benefits of
  polymeric technology are clear exceptional life
  and low drift/maintenance requirements

South Fork Instruments (925) 461
5059 www.southforkinst.com