South Fork Instruments Advances in pH/ORP Measurement – Polymeric Sensors Presented by John Daly 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 H 2 ions in the liquid being measured - the more H 2 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 ElectrodeReference Cell pH Sensitive Glass Liquid Junction KCl/AgCl Gel KCl/AgCl liquid

South Fork Instruments Process Internal Fill Solution How the pH Sensitive Glass Works n Lithium Ions in the pH sensitive glass act as current carriers n Positive Charged Free Hydrogen Ions (H + )Develop Positive mV Potential Relative to Internal Buffer l Acidic Solutions n Fewer Hydrogen Ions Relative to Internal Buffer Produce a Negative mV Potential l Alkaline Solutions H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ pH Glass External Gel Layer Internal Gel Layer H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ Li

South Fork Instruments pH Electrode with pH Sensitive Glass Internal Solution: H + is constant Acid solution < 7Alkaline solution > 7 Gel layer on both sides of glass Glass membrane thickness mm Glass Matrix Silver/Silver Chloride wire and billet

South Fork Instruments The Reference Electrode and Liquid Junction Electrolyte: Liquid, Gel or Polymer Silver Wire Silver/Silver Chloride Reference Billet Liquid Junction: Ceramic, Wood, or Plastic

South Fork Instruments Making a pH Measurement pH Measurement

South Fork Instruments 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 pH Sensor Reference Sensor

South Fork Instruments pH is a Potentiometric Measurement n The Measuring System consists of a pH Measuring Electrode and Reference Electrode n The Potential Difference Between the Two Electrodes is a Function of the pH Value of the Measured Solution n The Solution Must Be Conductive and is Part of the Electrical Circuit pH Measuring ElectrodeReference Electrode

South Fork Instruments Reference Cell Problems (1) n In conventional pH probe designs, the reference electrode and reference cell contents are in contact with the process fluid n Under certain conditions, the cell becomes depleted

South Fork Instruments Reference Cell Problems (2) n Under different conditions, the cell becomes poisoned n Reference cell depletion or poisoning leads to drift and probe failure

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

South Fork Instruments The Polymeric Solution n No Porous Junction to allow poisoning and depletion n Polymeric sleeve isolates the reference electrode from the process

South Fork Instruments Benefits of Polymeric Sensors n Long, long life l Plastic is ionic and takes a long time to deplete n Minimal Zero Drift l Reference is not depleting n Resiliant to fouling l No junction to plug up

South Fork Instruments Applications n Sour Water n Ultra Pure Water n Waste Water Treatment n Oily Water n Low Ionic Service n Vacuum Service

South Fork Instruments Summary n Polymeric sensors provide longer life and better stability in “difficult” applications than conventional technology probes. n In “standard” applications, the benefits of polymeric technology are clear – exceptional life and low drift/maintenance requirements South Fork Instruments (925)