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South Fork Instruments
Advances in pH/ORP Measurement – Polymeric Sensors
Presented by
John Daly
South Fork Instruments
South Fork Instruments
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
South Fork Instruments
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…..
South Fork Instruments
How pH probes work
pH probes consist of two parts – the measurement cell and the reference cell
Ag/AgCl Element
pH Measuring Electrode Reference Cell
pH Sensitive Glass Liquid Junction
KCl/AgCl Gel
KCl/AgCl liquid
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Process
Internal Fill Solution
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
H+H+H+H+ H+ H+
pH GlassExternal Gel Layer
Internal Gel Layer
H+
H+ H+ H+ H+ H+
Li Li
Li
Li Li
Li
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pH Electrode with pH Sensitive Glass
++
+++
--
--
Internal Solution: H+ is constant
Acid solution < 7 Alkaline solution > 7
Gel layer on both sides of glass
Glass membrane thickness 0.2-0.5 mm
Glass Matrix
Silver/Silver Chloride wire and billet
+++ + + +
+++
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The Reference Electrode and Liquid Junction
Electrolyte: Liquid, Gel or Polymer
Silver Wire
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 CircuitE 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
pH Sensor Reference 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