Download Advances in pH/ORP Measurement

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
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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 Measuring Electrode
Reference Cell
KCl/AgCl liquid
KCl/AgCl Gel
Ag/AgCl Element
pH Sensitive Glass
Liquid Junction
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pH probes consist of
two parts – the
measurement cell
and the reference
cell
How the pH Sensitive Glass Works

Internal Fill Solution

H+
H+
H+
H+
H+
H+
H+
Li
Internal Gel Layer
Li
pH Glass
Li
Li
External Gel Layer
Li
H+
H+
H+
Process
H+
Li
H+

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
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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
+
+ - Glass Matrix
+ +
+ + +
+ + + ++ Acid solution < 7 + +
Alkaline solution > 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
pH Sensor
Reference
Sensor
E 7 Half-cell voltage Ag/AgCl || KCl
(reference electrode), voltage depends on
electrolyte concentration
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pH is a Potentiometric Measurement
pH Measuring Electrode
Reference Electrode



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
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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


Tortuous Path Electrodes


An additional junction is installed to slow down
depletion around the electrode itself and keep
poisons out longer
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


Minimal Zero Drift


Plastic is ionic and takes a long time to
deplete
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
South Fork Instruments