THE MEASUREMENT OF pH

1
THE MEASUREMENT OF pH
2
General Principles

• Convenient way to express hydrogen ion
  concentration, or acidity
• pH - log H
• Where concentration is expressed in moles/liter

3
Ph SCALE

• Is logarithmic
• Example, pH 5.0 solution has ten times more
  hydrogen ions than pH 6.0 solution
• As hydrogen ion concentration, or acidity,
  increases, the pH value decreases

4
Consider Ph OfPure Water

• The H of pure water is 1 X 10-7 mole/L What is
  pH?

5
Consider Ph OfPure Water Cont

• pH - log H
• The log of 1 X 10-7 is -7
• The negative log of 10-7 is -(-7) 7
• The pH of pure water is 7, which we call neutral

6
Questions

• What is the pH of a solution with an H ion
  concentration of 10-4 mole/L?
• What is the pH of solution with an H ion
  concentration of 5.0 X 10-6 mole/L?

7
Answer

• pH - log H - log 10-4 -(-4) 4
• pH - log H - log 5.0 X 10-6
• -(-5.3) 5.3

8
Question

• What is the concentration of H ions in a
  solution with a pH of 9.0?

9
Answer

• pH - log H
• 9.0 - log H
• - 9.0 log H
• antilog (-9.0) 1 X 10-9 mole/L

10
Design OfpH Meter/ElectrodeMeasuring Systems

• pH meter systems measure hydrogen ion
  concentration electrochemical

11
(No Transcript)
12
Animations

• http//www.umd.umich.edu/casl/natsci/slc/slconline
  /PHM/theory.html

13
Characteristics Types of Electrodes Ph Meters

• Many types of electrodes and meters
• Meters
• Conventional
• Portable
• Solid state
• Microprocessor controlled

14
Electrodes

• Heart of the system
• Can buy separate measuring and reference
  electrodes or
• Combination both electrodes are in one housing
• Combination almost always found in biology labs
• But not necessarily right

15
Separate Electrodes

• Use them for difficult samples that
• Require different type of glass electrode or
• Require a different type of junction
• Junction is small hole from which filling
  solution slowly flows out of electrode
• Different types of junctions have different flow
  rates
• Tendency to clog

16
ReferenceElectrodes

• Two major types
• Ag/AgCl and calomel (mercury)
• General purpose usually silver/silver chloride

17
ReferenceElectrodes Cont

• Calomel electrodes are said to be more accurate
  for measuring pH of Tris buffers
• But calomel electrodes cannot be used in
  solutions above 80o C and contain mercury
• Both types are available in combination
  electrodes or separately

18
ProperOperation Avoiding Error

• Consider electrodes
• Consider sample
• Consider meter operation

19
Electrodes

• Maintain electrodes properly
• Filling Solution. Filling solution, often
  saturated KCl or AgCl/KCl
• Refillable electrodes are periodically filled to
  nearly the top
• Filling hole must be open to pH and closed for
  storage
• Gel-filled electrodes contain gelled filling
  solution, never refilled

20
(No Transcript)
21
Electrodes

• Storage. Consult the manufacturer's instructions
• Dont store electrodes in distilled water
• New combination electrodes often stored dry
• Then are conditioned before use by soaking at
  least 8 hours in pH 7 buffer

22
Sample

• Important part of the system
• Must be homogenous
• May be chemical changes in sample
• May be temperature effects

23
Difficult Samples

• Non-aqueous solvents
• High purity water. High purity water does not
  readily conduct current and it absorbs CO2 from
  the atmosphere
• High salt samples. Sample ions compete with the
  reference filling solution ions

24
Difficult Samples Cont

• Sample-electrode compatibility. Tris buffer,
  sulfides, proteins, Br-, and I- , can complex
  with silver in Ag/AgCl electrodes leading to a
  clogged junction. A calomel electrode is often
  recommended
• Slurries, sludges, viscous and colloidal samples.
  Use a fast flowing junction. Keep measuring
  electrode bulb clean

25
OperationCalibration

• User calibrates frequently
• Two buffers of known pH forms calibration line.
  Try this in lab
• Older pH meter first calibration buffer should be
  pH 7.00
• For acidic samples, second is pH 4.00
• For basic samples, second is pH 10.00

26
OperationCalibration Cont

• Newer, microprocessor-controlled meters
• Any two standard buffers that bracket pH of
  samples may be used
• Some microprocessor-controlled meters allow use
  of more than two standard buffers

27
The Relationship Between pH andMillivolt Readings
The X axis is pH and the Y axis is millivolts
28
Calibration Buffers

• Accuracy of pH meter depends on calibration
  buffers
• Some buffers react with CO2 from air
• Keep buffer containers closed
• Throw away buffer after use
• pH 10.0 buffer is particularly sensitive to CO2

29
Buffers

• For best accuracy, do not use buffers after their
  expiration date
• Avoid contamination
• pH of a buffer will change as its temperature
  changes

30
Temperature And pH

• Temperature has two important effects
• Measuring electrode's response to pH is affected
  by the temperature
• pH of solution that is being measured change as
  its temperature changes

31
Effect of Temperature on pH Readings
32
CompensationFor Temperature

• Meter needs to "know" temperature of solution
• Can measure solution temperature with thermometer
  and "tell" pH meter
• Alternatively, use ATC (automatic temperature
  compensating) probes that is connected to the pH
  meter
• Compact devices may have a temperature probe
  built into the electrode housing

33
Temperature

• The pH of some solutions changes with temperature
• Usually measure pH of samples at the temperature
  at which they will be used

34
A ConventionalMethod or Measuring pH

• 1. Warm-up meter
• 2. Open filling hole check filling solution
  level
• 3. If meter has a "standby mode, use it when
  the electrodes are not immersed in sample

35
A ConventionalMethod or Measuring pH cont

• 4. Calibrate each day or before use
• A. Adjust temperature setting.
• B. Rinse electrodes and blot dry.
• C. Dont wipe.Immerse electrodes in pH 7.00
  calibration buffer. Junction must be immersed
  and level of sample must be below filling
  solution level. Allow reading to stabilize.

36
A ConventionalMethod or Measuring pH cont

• D. Adjust meter to read 7.00
• E. Remove electrodes, rinse, blot dry.
• F. Place electrodes in second standardization
  buffer Adjust meter to pH. Remove, rinse, and
  blot.

37
A ConventionalMethod or Measuring pH cont

• G. Older meters Recheck pH 7.00 buffer and
  readjust as necessary. Recheck second buffer and
  readjust as necessary.
• H. Readjust as needed up to three times. If
  readings are not within 0.05 pH units of what
  they should be, electrode probably needs
  cleaning.

38
Quality Control

• 5. Optional Quality control checks.Try this
  in lab.
• A. Linearity Check. Take reading of a third
  calibration buffer. For example, if you
  calibrated with pH 7.00 and 10.00 buffers, check
  pH 4.00 buffer.
• B. Allow the reading to stabilize and record
  value. Do not readjust meter. If reading is
  outside proper range, service electrodes.

39
Quality Control

• C. Check pH of a control buffer whose pH is
  known and that has a pH close to the pH of the
  sample.
• Common to set maximum allowable error of control
  buffer to 0.10 pH units.
• Do not adjust meter to pH of control buffer.
• If pH of control buffer is not within required
  tolerance, service electrodes.

40
Sample

• 6. Set meter to temperature of sample or use
  ATC probe.

41
Sample

• 7. Place electrodes in sample allow reading to
  stabilize.
• A. Wait too long, pH of some samples will
  change due to air, chemical reactions, etc.
• B. Difficult solutions may require longer to
  stabilize.
• C. Many new pH meters have an autoread
  feature.

42
Sample

• 9. Record all relevant information.
• 10. Remove electrodes from sample, rinse and
  blot, store properly with filling hole closed
  (refillable electrodes only).

43
Iv Trouble-shooting

• First step is to know you have trouble.
• Symptoms of pH system problems include
• Reading drifts and won't stabilize.
• Reading fluctuates.
• Meter cannot be adjusted to both calibration
  buffers.
• pH value for a buffer or sample seems wrong.

44
Trouble-shooting Tips

• Look for and correct simple (embarrassing)
  mistakes
• Electrode measuring bulb and junction are not
  immersed in sample.
• Meter is not turned on or plugged in, or the
  electrode cables are not connected to meter.

45
Trouble-shootingTips Cont.

• Reference electrode is not filled with
  electrolyte.
• Reference electrode filling hole is closed.
• Sample is not well-stirred.
• Calibration buffers are not good.
• Electrode is cracked or broken.

46
Next

• What is most likely cause of problems?
• Problems can arise in
• The reference electrode
• The measuring electrode
• The calibration buffers
• The sample
• The meter

47
Reference Electrode

• Reference electrode junction most common source
  of problems.
• Occluded junction causes long stabilization time
  reading drifts slowly towards correct pH.
• If junction is completely plugged, reading may
  never stabilize.
• This is also caused by broken electrodes and by
  some problems within meter.

48
Reference Electrode cont

• Slow equilibration also caused by changes in
  sample temperature, reactions in sample, or
  sample-electrode incompatibility.

49
Reference Electrode cont

• Poor calibration buffers will cause inaccuracy.
  This may be detected if solution of known pH is
  checked.
• If sample is not homogenous, or if its
  temperature is unstable, then pH readings will
  fluctuate or drift.
• Difficult samples may be slow to equilibrate or
  give incorrect readings.

50
Reference Electrode cont

• Complete lack of response likely caused by
  problems with meter, but meter is least likely
  component to cause problems.
• To check which component is at fault substitute
  in a new reference or combination electrode.