LABORATORY METHODS MANUAL
Determination of pH in Waters
Version: 03
Page: 1 of 3
Issue Date: 12 July 1998
Authorised by : Marjorie Dickenson
1.
2.
3.
4.
SCOPE
This method covers the measurement of pH of natural water and industrial wastewater by means of a pH meter and combination glass pH electrode.
Natural waters usually have pH values within the range of 4 to 9, and most are slightly basic because of the presence of bicarbonates and carbonates.
PRINCIPLE
The pH (negative logarithm of the hydrogen ion concentration) of a sample is determined by measuring the potential between a glass electrode and a reference calomel or Ag/AgCl electrode.
The sensing membrane of the electrode is made of a glass that is selectively permeable to hydrogen ions. The glass electrode system displays a change of 1.0 pH unit equivalent to an electrical change of 59.16 mV at 25

C, corresponding to a ten-fold change in hydrogen ion concentration in solution.
INTERFERENCES
Below pH 1 the electrode is subject to an acid error and in alkaline solutions above pH 10 it may be subject to a sodium ion error. However, it is unlikely that during routine analyses these interferences will be encountered.
Temperature exerts a significant effect on pH measurements, but this may be compensated by the temperature correction facility of the pH meter.
NOTE : Unless stored wet between uses, pH electrode bulbs dry out resulting in crystallization of
KCl on the inside surface and leading to poor performance.
REAGENTS
5.
4.1. General Unless otherwise specified, all reagents shall be of analytical reagent grade, and high-purity water used shall comply with Type I water as specified in ISO 3696 (16.7 M
 cm resistivity; 0.06

S/cm conductivity).
4.2. Buffer Solutions (Commercially available solutions).
4.2.1. pH 4.0
4.2.2. pH 7.0
4.2.3. pH 10.0
4.2.4. Reference solution - pH 8.0
APPARATUS
5.1. pH meter
5.2. Combination glass pH electrode Store in pH 4 buffer (4.2.1), when not in use.
5.3. Magnetic stirrer
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LABORATORY METHODS MANUAL
5.4. Beaker 100mL
6.
7.
Determination of pH in Waters
Version: 03
Page: 2 of 3
Issue Date: 12 July 1998
Authorised by : Marjorie Dickenson
SAMPLING AND SAMPLE PREPARATION
Samples should be analysed in the field; however, under most circumstances this is not practical.
Consequently, samples must be stored at 4

C in the dark immediately after collection, and analysed as soon as possible on return to the laboratory. APHA “Standard Methods for the Examination of
Water and Wastewater” specifies no more than 2 h should elapse between collection and analysis.
PROCEDURE
8.
9.
10.
7.1. General
(a) pH measurements should always be carried out with the solution stirred magnetically or gently by hand. In either case, since the electrodes are relatively fragile, ensure that the glass sensing membrane does not contact the stirrer or the bottom or sides of the beaker since fine cracks may result and the electrode then will not function accurately.
(b) For all measurements, ensure that the glass bulb membrane is completely immersed in the agitated solution before accepting the meter reading; approximately 20mL of buffer or sample should be adequate for this purpose.
7.2. Quality Control
(a) Reference solution (4.2.4) should be measured within the batch of samples at the rate of one, plus every twenty (20) samples or job, if smaller.
(b) Sample Replicate Test samples should be replicated at the rate of one, plus every ten (10) samples or job, if smaller.
7.3. Calibration of pH Meter
The manufacturers instructions are followed when preparing and caring for the pH electrode as well as calibration of the meter. Remove the electrode from the pH 4 storage solution, rinse thoroughly with distilled water and blot dry. Set the pH 7 buffer first. Remove the electrode from the first buffer, rinse thoroughly with distilled water, blot dry and immerse in pH 4 buffer if calibrating with three (3) standards. Set the pH and remove from the buffer, rinse thoroughly with distilled water, blot dry and immerse in pH 10 buffer if calibrating with either two (2) or three (3) standards. Set the pH and remove from the buffer, rinse thoroughly with distilled water, blot dry.
7.4. Sample Measurement
Immerse the electrode in sample solution, stir and allow to equilibrate. Record the pH. Rinse thoroughly with distilled water, blot dry and immerse in the next sample.
CALCULATIONS
Nil
QUALITY CONTROL
The batch of samples is accepted if the measured pH of the reference solution (4.2.4) has a value of
8.00

0.10. Update the control chart in the LIMS InfoLAB.
REPORTING
Report the pH value to one decimal place and report the temperature at which the measurement was made.
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LABORATORY METHODS MANUAL
11. REFERENCES
(a)
(b)
(c)
Determination of pH in Waters
Version: 03
Page: 3 of 3
Issue Date: 12 July 1998
Authorised by : Marjorie Dickenson
APHA Standard Methods for the Examination of Water and Wastewater 20 th edition (1998)
Method 4500-H+ pH Meter/Electrode Operating Instructions.
ISO 3696:1987 - Water for analytical laboratory use; Specification and test methods.
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