THE MEASUREMENT OF pH

I GENERAL PRINCIPLES Convenient way to express hydrogen ion concentration, or acidity pH = - log[ H + ] Where concentration is expressed in moles/liter

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

CONSIDER pH OF PURE WATER The [H + ] of pure water is 1 X mole/L What is pH?

pH = - log[ H + ] The log of 1 X is -7 The negative log of is -(-7) = 7 The pH of pure water is 7, which we call neutral

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

ANSWER a. pH = - log [H + ] = - log = -(-4) = 4 b. pH = - log [H + ] = - log 5.0 X = -(-5.3) = 5.3

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

ANSWER pH = - log [H + ] 9.0 = - log [H + ] = log [H + ] antilog (-9.0) = 1 X mole/L

DESIGN OF pH METER/ELECTRODE MEASURING SYSTEMS pH meter systems measure hydrogen ion concentration; electrochemical

pH Scale NEUTRAL STRONGLY ACIDIC STRONGLY BASIC Drain cleaner Milk of magnesia Tap water Mild shampoo Orange juice Vinegar Battery acid

ANIMATIONS online/PHM/theory.html

Design of pH meter system

II CHARACTERISTICS AND TYPES OF ELECTRODES AND pH METERS Many types of electrodes and meters Meters:  Conventional  Portable  Solid state  Microprocessor controlled

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

SEPARATE ELECTRODES Difficult samples Require different type of glass electrode Different type of junction  Junctions have different flow rates  Tendency to clog

REFERENCE ELECTRODES Two major types:  Ag/AgCl and calomel (mercury) General purpose usually silver/silver chloride

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

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

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

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

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

DIFFICULT SAMPLES 1. Non-aqueous solvents 2.High purity water. High purity water does not readily conduct current and it absorbs CO 2 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

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

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

CALIBRATION BUFFERS Accuracy of pH meter depends on calibration buffers Some buffers react with CO 2 from air  Keep buffer containers closed  Throw away buffer after use  pH 10.0 buffer is particularly sensitive to CO 2

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

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

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

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

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

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.

d. Adjust meter to read 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.

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 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.

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 pH units. Do not adjust meter to pH of control buffer. If pH of control buffer is not within required tolerance, service electrodes.

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

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.

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

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.

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.

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

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.

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

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.

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.