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

2. 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
Feb 2001
File Name: pH.ppt

3. 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

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

5. Typical pH Values
Feb 2001
File Name: pH.ppt

6. 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

7. 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

8. 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

9. 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

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

11. 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

12. 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 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

13. 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

14. pH Electrode Life is Temperature Dependent
Typical pH Electrode Life is 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

15. 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 20oC for 140 hrs = OK
1000 ppm 50oC for 100 hrs = OK
10,000 ppm 20oC for 100 hrs = Fail
Total time 340 hours or 14 days
Feb 2001
File Name: pH.ppt

16. 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

17. 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

18. 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

19. Acid Equivalents Chart
Feb 2001
File Name: pH.ppt

20. Base Equivalents Chart
Feb 2001
File Name: pH.ppt

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

22. 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

23. 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