pH Theory

1
pH Theory

• Applied to Proper Hardware Selection

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
• 0 - 14 pH

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-

4
Ion Concentration (mol/l)
5
Typical pH Values
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
7
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

pH Measuring Electrode
Reference Cell
KCl Buffered to 7 pH
KCl Gel
Ag/AgCl Element
pH Sensitive Glass
Liquid Junction
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
pH Glass
Internal Gel Layer
External Gel Layer
Process
9
pH Temperature Slope

• 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

10
Temperature Error in pH Units
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 gt 10.0 pH
• Alkaline / Sodium Ion Error
• Coatings
• Slower Response
• Increase Zero Offset
• lt 50 Water
• Dehydration
• Steam Sterilization
• Dehydration
• Ag/AgCl Dissolves from Silver Reference Element

12
OHMs Law Applied to pH

• The pH Measuring Electrode Develops 59.2 mV per
  pH Unit
• pH Membrane Impedance is Approximately 100 Meg
  Ohms _at_ 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
100,000,000 Ohms
10-9 Nano Amps
13
Temperature Affects pH Electrode Response Time

• pH Glass Electrode Impedance is approximately 100
  MegOhms _at_ 25oC
• For approximately every 8oC Step Change from 25oC
  the pH Glass Impedance Doubles or Halves
• gt 25oC Faster Response
• lt 25oC Slower Response
• gt1000 MegOhms pH Generally Becomes Inoperable
• Recommendations
• Hold Distance Between Transmitter and Sensor to a
  Minimum
• Use Low Temperature pH Membrane

14
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

15
pH Glass Electrode Options

• Continuous Operation gt 11.0 pH
• HPH Option (High Alkaline)
• Continuous Operation gt 176oF (80oC)
• HT Option (High Temperature)
• Continuous Operation lt 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 _at_ 20oC for 140 hrs OK
• 1000 ppm HF _at_ 50oC for 100 hrs OK
• 10,000 ppm HF _at_ 20oC for 100 hrs Fail
• Total time 340 hours or 14 days

16
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 (H2S) Poisoning
• Plugs Liquid Junction
• Loss of Signal
• High Purity Water lt 100 uS/cm Conductivity
• Flow Sensitivity
• Low pH Values (lt1.0 pH) or High pH Values (gt13.0
  pH)
• Junction Potential Offsets

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-)

18
pH Control

• Start, Stop, Slow Down or Speed Up a Chemical
  Reaction

19
Acid Equivalents Chart
20
Base Equivalents Chart
21
Titration CurveStrong Acid with Strong Base
22
Two Position On - Off Control

• Measured variable will cycle around control point
  (7.0 pH)
• Guidelines for control
• Tank Retention Time gt 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
Proportional Integral (PI) Control

• Tank or Vessel retention time lt 5 minutes
• In-line mixing prior to sensor important
• Adjust to achieve 41 decay ratio after process
  upset