Applied to Proper Hardware Selection pH Theory Applied to Proper Hardware Selection Feb 2001 File Name: pH.ppt

pH Theory pH is a Unit of Measurement pH = Power of Hydrogen (H+) Defined as the Negative Logarithm of Hydrogen Ion Activity pH = log (1/H+) Used for Determining the Acidity or Alkalinity of an Aqueous Solution Practical pH Scale for Industrial Instrumentation 0 - 14 pH Feb 2001 File Name: pH.ppt

Acids and Bases Acid dissolves in water to furnish H+ ions HCl H+ + Cl- HNO3 H+ + NO3- HF H+ + F- Base dissolves in water to furnish OH- ions NaOH Na+ + OH- KOH K+ + OH- NH4OH NH4+ + OH- Feb 2001 File Name: pH.ppt

Ion Concentration (mol/l) Feb 2001 File Name: pH.ppt

Typical pH Values Feb 2001 File Name: pH.ppt

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 Feb 2001 File Name: pH.ppt

pH Sensor Components Feb 2001 File Name: pH.ppt pH Measuring Electrode Purpose is to Develop a Millivolt Potential Directly Proportional to the Free Hydrogen Ion Concentration in an Aqueous Solution Reference Cell Purpose is to Maintain a Constant Reference Potential Regardless of pH Change or Other Ionic Activity in the Solution Reference Cell Liquid Junction Purpose is to Maintain Electrical Contact Between the Reference Electrode and the Measuring Electrode by way of the Solution pH Measuring Electrode Reference Cell KCl Buffered to 7 pH KCl Gel Ag/AgCl Element pH Sensitive Glass Liquid Junction Feb 2001 File Name: pH.ppt

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 H+ H+ H+ H+ H+ H+ H+ pH Glass Internal Gel Layer Li Li Li Li Li External Gel Layer Li Process H+ H+ H+ H+ H+ Feb 2001 File Name: pH.ppt

pH Temperature Slope Feb 2001 File Name: pH.ppt Acids = Positive mV Signal Base = Negative mV Signal 7.0 pH = 0 mV Output Sensor Output Changes with Temperature 0o C ~ 54.2 mV/pH 25oC ~ 59.2 mV/pH 50oC ~ 64.1 mV/pH Sensor Output is Corrected to 25oC with Automatic Temperature Compensation Feb 2001 File Name: pH.ppt

Temperature Error in pH Units Feb 2001 File Name: pH.ppt

pH Measuring Electrode Purpose is to Develop a Millivolt Potential Directly Proportional to the Free Hydrogen Ion Concentration in an Aqueous Solution Process Effects High Temperature Faster Response / Lower Impedance Accelerates Aging, Lithium Ions Leached from Membrane Short Span Low Temperature Slower Response / Higher Impedance Measurement > 10.0 pH Alkaline / Sodium Ion Error Coatings Slower Response Increase Zero Offset < 50% Water Dehydration Steam Sterilization Ag/AgCl Dissolves from Silver Reference Element Feb 2001 File Name: pH.ppt

E I R OHM’s Law Applied to pH Feb 2001 File Name: pH.ppt The pH Measuring Electrode Develops 59.2 mV per pH Unit pH Membrane Impedance is Approximately 100 Meg Ohms @ 25oC Recommendations Shielded Cable is Required to Transmit a Reliable Signal Use Self-Powered Unity Gain Preamplifier Mount the Transmitter 15 feet or Less from Sensor E .059 V/pH I R 10-9 Nano Amps 100,000,000 Ohms Feb 2001 File Name: pH.ppt

Temperature Affects pH Electrode Response Time pH Glass Electrode Impedance is approximately 100 MegOhms @ 25oC For approximately every 8oC Step Change from 25oC the pH Glass Impedance Doubles or Halves > 25oC Faster Response < 25oC Slower Response >1000 MegOhms pH Generally Becomes Inoperable Recommendations Hold Distance Between Transmitter and Sensor to a Minimum Use Low Temperature pH Membrane Feb 2001 File Name: pH.ppt

pH Electrode Life is Temperature Dependent Typical pH Electrode Life is 12 - 18 Months Life is Reduced Approximately 50% for Every 25oC Increase in Operating Temperature Recommendations Sample Cooling Intermittent vs Continuous Measurement Feb 2001 File Name: pH.ppt

pH Glass Electrode Options Continuous Operation > 11.0 pH HPH Option (High Alkaline) Continuous Operation > 176oF (80oC) HT Option (High Temperature) Continuous Operation < 41oF (5oC) LT Option (Low Temperature to -13oF (-25oC) Hydrofluoric Acid HF glass in 1N HCl (0.1pH) Test Protocol (Same Electrodes) 1000 ppm HF @ 20oC for 140 hrs = OK 1000 ppm HF @ 50oC for 100 hrs = OK 10,000 ppm HF @ 20oC for 100 hrs = Fail Total time 340 hours or 14 days Feb 2001 File Name: pH.ppt

pH Reference Cell Feb 2001 File Name: pH.ppt Purpose is to Maintain a Constant Reference Potential Regardless of pH Change or Other Ionic Activity in the Solution Process Effects Dilution of the Electrolyte Air Entrapment Normal Process Temperature and Pressure Fluctuations Sugar, Organics such as Methylene Chloride, Chlorine Gas Heavy Metal Poisoning from Lead, Mercury, Silver Drift Sulfide (H2S) Poisoning Plugs Liquid Junction Loss of Signal High Purity Water < 100 uS/cm Conductivity Flow Sensitivity Low pH Values (<1.0 pH) or High pH Values (>13.0 pH) Junction Potential Offsets Feb 2001 File Name: pH.ppt

Reference Cell Contamination Silver Reacts with Sulfides Forming a Non-Conductive Precipitant at the Liquid Junction Result is an Open Electrical Circuit Between the Measuring and Reference Other Undesirable Reactions that Change the Chemistry of the Reference Half Cell are Caused by: Silver (Ag) Lead (Pb) Mercury (Hg) Copper (Cu) Bromide (Br-) Iodide (I-) Cyanide (Cn-) Feb 2001 File Name: pH.ppt

Start, Stop, Slow Down or Speed Up a Chemical Reaction pH Control Start, Stop, Slow Down or Speed Up a Chemical Reaction Feb 2001 File Name: pH.ppt

Acid Equivalents Chart Feb 2001 File Name: pH.ppt

Base Equivalents Chart Feb 2001 File Name: pH.ppt

Titration Curve Strong Acid with Strong Base Feb 2001 File Name: pH.ppt

Two Position On - Off Control Measured variable will cycle around control point (7.0 pH) Guidelines for control Tank Retention Time > 5 minutes Good Chemical Mixing, Tank turnover every two minutes Use alarm dead band and/or delay time on/off to compensate for minor variations in control Feb 2001 File Name: pH.ppt

Proportional + Integral (PI) Control Tank or Vessel retention time < 5 minutes In-line mixing prior to sensor important Adjust to achieve 4:1 decay ratio after process upset Feb 2001 File Name: pH.ppt