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

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Presentation on theme: "File Name: pH.pptFeb 2001 pH Theory Applied to Proper Hardware Selection."— Presentation transcript:

1 File Name: pH.pptFeb 2001 pH Theory Applied to Proper Hardware Selection

2 File Name: pH.pptFeb 2001 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 pH

3 File Name: pH.pptFeb 2001 Acids and Bases  Acid dissolves in water to furnish H + ions HCl H + + Cl - HNO 3 H + + NO 3 - HF H + + F -  Base dissolves in water to furnish OH - ions NaOH Na + + OH - KOHK + + OH - NH 4 OHNH OH -

4 File Name: pH.pptFeb 2001 Ion Concentration (mol/l)

5 File Name: pH.pptFeb 2001 Typical pH Values

6 File Name: pH.pptFeb 2001 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 ElectrodeReference Electrode

7 File Name: pH.pptFeb 2001 pH Sensor Components  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 Ag/AgCl Element pH Measuring ElectrodeReference Cell pH Sensitive Glass Liquid Junction KCl Gel KCl Buffered to 7 pH

8 File Name: pH.pptFeb 2001 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+ 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

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

10 File Name: pH.pptFeb 2001 Temperature Error in pH Units

11 File Name: pH.pptFeb 2001 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 Dehydration Ag/AgCl Dissolves from Silver Reference Element

12 File Name: pH.pptFeb 2001 OHM’s Law Applied to pH  The pH Measuring Electrode Develops 59.2 mV per pH Unit  pH Membrane Impedance is Approximately 100 Meg 25 o C  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 IR.059 V/pH 100,000,000 Ohms10 -9 Nano Amps

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

14 File Name: pH.pptFeb 2001 pH Electrode Life is Temperature Dependent  Typical pH Electrode Life is Months  Life is Reduced Approximately 50% for Every 25 o C Increase in Operating Temperature  Recommendations Sample Cooling Intermittent vs Continuous Measurement

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

16 File Name: pH.pptFeb 2001 pH Reference Cell  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 (H 2 S) Poisoning Plugs Liquid Junction Loss of Signal High Purity Water < 100 uS/cm Conductivity Flow Sensitivity Low pH Values ( 13.0 pH) Junction Potential Offsets

17 File Name: pH.pptFeb 2001 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 - )

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

19 File Name: pH.pptFeb 2001 Acid Equivalents Chart

20 File Name: pH.pptFeb 2001 Base Equivalents Chart

21 File Name: pH.pptFeb 2001 Titration Curve Strong Acid with Strong Base

22 File Name: pH.pptFeb 2001 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

23 File Name: pH.pptFeb 2001 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


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