South Fork Instruments Advances in pH/ORP Measurement – Polymeric Sensors Presented by John Daly South Fork Instruments.

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

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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

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

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(925) 461 5059

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