The Measurement of pH,

1
Chapter 18

• The Measurement of pH,
• Selected Ions, and Conductivity

2
The Importance and Definition of pH

• Chemistry of life based on water
• Cells contain 80-90 water
• pH
• Measure of the acidity of an aqueous solution
• Intrinsically related to water chemistry
• Essential for biological systems
• Food processing
• Sewage treatment
• Water purification
• Pharmaceutical production
• Must be monitored and maintained

3
The Importance and Definition of pH

• Water naturally dissociated to form hydrogen (H)
  and hydroxide (OH-) ions
• H2O H OH-
• At 25OC, equilibrium is reached
• Concentration of H OH-
• 1 x 10-7 mole/L
• Neutral

4
The Importance and Definition of pH

• Acids
• Release hydrogen ions into solution when added to
  water
• Causes increase of H to gt1x10-7
• Bases
• Cause hydrogen ions to be removed from solution
  when added to water
• Causes decrease of H to lt1x10-7

5
The Importance and Definition of pH

• Strong acids and strong bases
• Completely dissociate in water
• Have strong effect on H concentration
• HCl H Cl-
• NaOH Na OH-
• Both are completely in ionic form in water
• Weak acids and weak bases
• Do not completely dissociate in water
• Have smaller effect on concentration of H in
  solution

6
The Importance and Definition of pH

• pH
• Convenient way to express H concentration in
  solution
• the negative log of the H concentration when
  concentration is expressed in moles per liter
• Practical range between 0 and 14
• Some acids and bases can fall between this
  range..VERY DANGEROUS!
• gt 7 bases
• lt 7 acids

7
The Importance and Definition of pH

• Ultimate importance of H concentration
• Profoundly affects properties of aqueous
  solutions
• Concentrated sulfuric acid
• Dissolve iron nails
• Citric acid (weak acid)
• Contributes to taste of foods
• pH scale
• Logarithmic
• pH of 5.0 10x more H than pH 6.0
• Methods to measure pH
• Indicator dyes
• pH meter/electrode

8
pH Indicators

• pH indicators
• Dyes whose color is pH dependent
• Change colors at certain pH values
• Directly dissolved in a solution
• Impregnated into strips
• Dipped into the solution to be tested
• Phenolphthalein
• Indicator dye (used to be found in some
  laxatives)
• Colorless to red between pH 8 and 10
• Litmus
• Extracted from lichens
• Oldest know pH indicator
• Pink in acidic solutions
• Blue in basic ones

9
pH Indicators

• pH indicators, cont..
• Do not change colors sharply at a single pH
• Change over a range of 1-2 pH units
• Broad indicators only
• Some have range of dyes to give more accurate
  readings
• Good for 0.3 pH units
• Uses
• Biological systems
• Low probability of contamination to mixtures
• Solution can be placed into a cell culture system
  to monitor cell wastes vs. environmental pHred
  ok.yellowish tinge becoming acidic
• Radioactive solutions
• Dip paper and dispose of it
• Inexpensive/simple to use

10
The Design of pH Meter/Electrode Measuring Systems

• Overview
• pH meter/electrode systems
• Most common method of measuring pH in the
  laboratory
• Accurate, sensitive and flexible vs. indicators
• Measure to the nearest 0.1 pH unit or better
• Used with a variety of samples
• Consists of
• Voltmeter
• Measures voltage
• 2 electrodes
• Sample
• Whats being measured
• pH meter
• H concentration is a measure of the magnitude of
  the electrical potential (of the sample)
• Measured by the voltmeter

11
The Design of pH Meter/Electrode Measuring Systems

• The Basic Design and Function of Electrodes
• Electrodes and the Measurement of pH
• Electrodes
• Heart of the pH system
• Actually two electrodes
• pH measuring electrode
• -thin, fragile glass bulbcalled glass electrode
  (fig. 18.2)
• - glass sensitive to H concentration
• - sets up electrical potential between inner and
  outer surfaces of the glass
• H concentration causes increase in magnitude of
  potentialpH
• - electode contains buffered chloride solution

12
The Design of pH Meter/Electrode Measuring Systems

• The Basic Design and Function of Electrodes
• Electrodes and the Measurement of pH
• Reference electrode
• Stable, constant voltage
• Needed b/c not possible to measure potential of a
  single electrode
• Strip of metal and electrolyte solution in a
  glass/plastic tube
• electrolytes substances-acids, bases, salts-that
  release ions when dissolved in water
• --- metal and electrolyte react together to
  develop constant voltage

13
The Design of pH Meter/Electrode Measuring Systems

• The Basic Design and Function of Electrodes
• Electrodes and the Measurement of pH
• Reference electrode-pH events
• Electrical potential (voltage) of glass electrode
  varies depending on the samples H concentration
• Electrical potential of the reference electrode
  is constant
• The two electrodes are connected to one another
  through the voltage meter
• Meter measures the voltage difference between the
  reference electrode and the measuring electrode.
  This voltage is an electrical signal
• The electrical signal is amplified and converted
  to a display of pH

14
Operation of a pH Meter System

• Calibration
• Also called standardization
• Critical step in operation
• Tells the meter how to translate the voltage
  difference between the measuring and reference
  electrodes into units of pH
• Calibrate every day
• Response of electrodes declines over time
• Response between ph and voltage is linear
• Two buffers of known pH used for calibration
• 1st usually pH 7.00
• 2nd typically pH 4.00, 12.00, or 12.00
• If solution acidic, acid buffer used
• If basic, basic buffers used

15
Operation of a pH Meter System

• Calibration-Process
• Immerse electrodes in pH 7.00 buffer
• Set display to read 7.00
• Meter internally adjusts self so that pH of 7.00
  corresponds to 0 millivolts (mV)
• Called set, zero offset, or standardized
• Meter sets to 0 mV
• Immerse electrodes in second pH buffer
• Set display to read the pH of that buffer
• Meter uses the 2nd reading to establish
  calibration line with a particular slope
• Standard curve

16
Operation of a pH Meter System

• Calibration-Process
• Slope
• Measure of the response of the electrodes to pH
• Steeper the slope, more sensitive the electrodes
  are to H
• Should be close to -59.2 mV/pH units _at_ 25OC
• In older probes
• Less able to generate a potential
• Slope of the line declines (fig 18.9, pg 350)
• Can be used as indication of the condition of the
  electrodes
• Can be expressed as a percent of the theoretical
  value
• 97 slope -57.2 mV/pH unit
• Buffers
• Must be good quality
• Commonly purchased as ready to use solutions
• Often in colors

17
Operation of a pH Meter System

• Temperature
• Two effects
• Effects the potential that develops with the
  measuring electrodes response to pH
• pH of the solution being measured can
  increase/decrease with temperature changes

18
Operation of a pH Meter System

• Operation of a pH Meter System
• Most pH meters can compensate for temperature
  dependent changes in the electrode response
• Has temperature adjustment knob or built in
  thermometer
• Called ATC-automatic temperature compensating
  probes
• Temperature effects on pH of solutions
• Measure pH when solution at temperature at which
  it will be used
• Calibration
• Use room temperature standards

19
Operation of a pH Meter System

• Box 1, pg. 352 for operating a pH meter
• Allow meter to warm up as directed by the
  manufacturer
• Open the filling hole fill solution must be
  nearly to the top
• Set to pH mode to read the pH of a sample or
  standard

20
Operation of a pH Meter System

• Box 1, pg. 352 for operating a pH meter
• Calibrate system each day or before use
• Adjust meter temp setting to room temperature or
  use ATC probe
• Minimum of two standard buffers
• Rinse electrodes with DI water and BLOT dry
• Wiping creates static charge
• Immerse electrodes in RT buffer
• Junction is immersed
• Level of sample below level of filling solution
• Allow reading to stabilize

21
Operation of a pH Meter System

• Box 1, pg. 352 for operating a pH meter
• Adjust meter to read 7.00 with knob, dial,
  button, or other methods of adjustment
• Remove electrodes, rinse with distilled water,
  blot dry
• Can rinse with next solution and NOT blot
• Place electrodes in 2nd standard buffer, allow
  the reading to stabilize
• Set meter to temperature of sample or use ATC
  probe
• Place electrodes in the sample
• Allow pH reading to stabilize
• Record relevant information
• Remove electrodes from sample, rinse them, and
  store properly

22
Operation of a pH Meter System

• Measuring the pH of Difficult Samples
• Sample is the key to the measurement system
• Must be homogeneous to avoid variation and drift
• Equilibrated to a particular temperature
• Chemical reactions can cause pH to change over
  time
• Among substances in the sample
• With CO2 from the air
• Some samples inherently difficult to pH-require
  special techniques or types of electrodes
• Nonaqueous
• High purity water, containing little salt
• High salt concentrations
• High protein concentrations
• S-2, Br-, or I-
• Tris buffers
• Viscous
• Turbid

23
Trouble-Shooting pH Measurements

• General Considerations
• pH meter range response
• 100 to 10-14 moles/L
• To very low concentrations of H
• To very small changes to H concentration
• Detect H in the presence of other ionic species
• Meters
• Must have a wide range of response
• Must be VERY sensitive
• Must be VERY specific

24
Trouble-Shooting pH Measurements

• General Considerations
• 1st step..recognizing that there is a problem
• Most can be quite subtle
• Sometimes overlooked.
• Symptoms of pH problems
• Reading drifts in one direction
• Will not stabilize
• Takes an unusually long time to stabilize
• Reading fluctuates
• Meter cannot be adjusted to both calibration
  buffers
• Apparent pH value for a buffer or sample seems to
  be wrong
• No reading at all

25
Trouble-Shooting pH Measurements

• The Components of a pH Measuring System and their
  Potential Problems
• Problems can arise from
• Reference electrode
• Measuring electrode
• Meter
• Calibration buffers
• The sample

26
Trouble-Shooting pH Measurements

• The Components of a pH Measuring System and their
  Potential Problems
• Reference electrode junction
• Most common source of problems
• Dirty
• Partially occluded
• Long stabilization time
• Reading drifts slowly toward the correct pH
• Slow equilibration can also be caused by
• -changes in temperature of the sample
• -reactions within the sample
• -incompatibility between the reference electrode
  and the sample
• -checking for occlusion in table 18.9, pg. 356

27
Trouble-Shooting pH Measurements

• The Components of a pH Measuring System and their
  Potential Problems
• Measuring electrode bulb
• Must be clean
• Needed for contact with solution
• Sample residues
• High levels of protein
• Age of electrode
• Attack to the glass by aqueous solutions
• Form gel layer on bulb surface
• Thickens over time..becomes sluggish and slope
  of calibration line decreases

28
Trouble-Shooting pH Measurements

• The Components of a pH Measuring System and their
  Potential Problems
• Meter
• Complete lack of response
• Caused by meter itself
• Least likely component of the system to cause
  problems
• Buffers
• Can be checked by using solution of known pH
• Sample not homogenous/temperature not stable
• Ph readings will fluctuate or drift

29
Trouble-Shooting pH Measurements

• Tips
• Simple mistakes
• Always look for and correct simple/embarrassing
  mistakes first
• Electrode measuring bulb and the junction not
  immersed in the sample
• Meter not turned on or plugged in or electrode
  cables not connected to the meter
• Reference electrode not filled with electrolyte
• Reference electrode filling hole is closed
• Sample not well-stirred
• Calibration buffers not fresh
• Electrode is cracked or broken and needs to be
  replaced

30
Trouble-Shooting pH Measurements

• Tips
• Trouble-Shooting based on symptoms
• Tables 18.9-18.12, pg 356

31
Trouble-Shooting pH Measurements

• Procedures for Cleaning and Maintaining
  Electrodes
• Follow manufacturers instructions
• Commonly recommended suggestions
• Boxes 3-6, pg 357-58

32
Other types of Selective Electrodes

• Indicator electrode
• Responds selectively to a specific molecule in
  solution
• Dissolved CO2
• Dissolved O2
• During fermentation
• Growth of microorganisms sensitive to oxygen
  levels