THE MEASUREMENT OF pH

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I GENERAL PRINCIPLES

Convenient way to express hydrogen ion concentration, or acidity

pH = - log[ H+]

Where concentration is expressed in moles/liter

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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

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CONSIDER pH OF PURE WATER

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

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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

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QUESTIONS

a. What is the pH of a solution with an H+ ion concentration of 10-4 mole/L?

b. What is the pH of solution with an H+ ion concentration of 5.0 X 10-6 mole/L?

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ANSWER

a. pH = - log [H+] = - log 10-4 = -(-4) = 4

b. pH = - log [H+] = - log 5.0 X 10-6 =

-(-5.3) = 5.3

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QUESTION

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

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ANSWER

pH = - log [H+]

9.0 = - log [H+]

- 9.0 = log [H+]

antilog (-9.0) = 1 X 10-9 mole/L

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DESIGN OF pH METER/ELECTRODE MEASURING SYSTEMS

pH meter systems measure hydrogen ion concentration; electrochemical

pH S

cale

0

7

14

NEUTRAL

STRONGLY ACIDIC

STRONGLY BASIC

Drain cleaner

Milk of magnesia

Tap water

Mild shampoo

Orange juiceVinegar

Battery acid

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ANIMATIONS

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

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Design of pH meter system

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II CHARACTERISTICS AND TYPES OF ELECTRODES AND pH METERS

Many types of electrodes and meters Meters:

Conventional Portable Solid state Microprocessor controlled

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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

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SEPARATE ELECTRODES

Difficult samples Require different type of glass electrode Different type of junction

Junctions have different flow rates Tendency to clog

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REFERENCE ELECTRODES

Two major types: Ag/AgCl and calomel (mercury)

General purpose usually silver/silver chloride

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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

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III PROPER OPERATION; AVOIDING ERROR Consider electrodes Consider sample Consider meter operation

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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

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ELECTRODES

Storage. Consult the manufacturer's instructions Don’t store electrodes in distilled water New combination electrodes often stored dry

Conditioned before use by soaking at least 8 hours in pH 7 buffer

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SAMPLE

Important part of the system

Must be homogenous

May be chemical changes in sample

May be temperature effects

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DIFFICULT SAMPLES

1. Non-aqueous solvents

2. High purity water. High purity water does not readily conduct current and it absorbs CO2 from the atmosphere

3. High salt samples. Sample ions compete with the reference filling solution ions

4. 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

5. Slurries, sludges, viscous and colloidal samples. Use a fast flowing junction. Keep measuring electrode bulb clean

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OPERATION; CALIBRATION

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

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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

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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

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BUFFERS

For best accuracy, do not use buffers after their expiration date

Avoid contamination

pH of a buffer will change as its temperature changes

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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

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COMPENSATION FOR 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

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TEMPERATURE

The pH of some solutions changes with temperature

Usually measure pH of samples at the temperature at which they will be used

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A CONVENTIONAL METHOD FOR 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

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4. Calibrate each day or before use:

a. Adjust temperature setting.

b. Rinse electrodes and blot dry. Don’t wipe.

c. 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.

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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.

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.

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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.

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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.

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SAMPLE

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

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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.

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9. Record all relevant information.

10. Remove electrodes from sample, rinse and blot, store properly with filling hole closed (refillable electrodes only).

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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.

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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. 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.

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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

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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.

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

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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.

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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.