Standard Methods: 4500-H+B:  pH in Water by Potentiometry

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Official Method Name
pH Value in Water by Potentiometry Using a Standard Hydrogen Electrode
Current Revision
Standard Methods Online
Media
WATER
Instrumentation
pH Meter
Method Subcategory
Inorganic
Method Source
  Standard Methods
Citation
  Standard Methods Online - Standard Methods for the Examination of Water and Wastewater
Brief Method Summary
Follow the manufacturer's instructions for pH meter and for storage and preparation of electrodes. Tap water is better than distilled water for storage of electrodes, but pH 4 buffer is best for the single glass electrode and saturated KCl is best for a calomel and Ag/AgCl reference electrode and a combination electrode. Keep electrodes wet with above solutions while pH meter is not in use. Remove electrode from storage solution, rinse, blot dry, and place in an initial buffer solution to set the isopotential point before use. Rinse, blot dry, and place in a second buffer that is within 2 pH units of the sample pH, and is at the same temperature as the sample. Record temperature and adjust the temperature dial on the meter. Remove electrode from the second buffer, rinse, blot dry, and place in a third buffer < pH 10 and within about 3 pH units from the second buffer. Meter response should be within 0.1 pH unit from the expected value. Above standardization adjusts the response of the glass electrode to the instrument. Standardize the instrument before each measurement when measuring only occasionally. Establish equilibrium between sample and electrode by stirring sample to homogeneity and stirring sample to release carbon dioxide. For samples of high ionic strength or those buffered, dip electrodes into the sample for 1 minute to condition, blot dry, immerse in a fresh aliquot of sample, and read pH. For dilute, poorly buffered samples, dip electrodes into 3 or 4 successive aliquots of the sample to equilibrate. Blot dry, immerse in a fresh aliquot of sample, and read pH.
Scope and Application
This method allows for determination of water pH.
Applicable Concentration Range
None given.
Interferences
(1) Sodium error may bias results at pH > 10 unless a "low sodium error" electrode is used. (2) Coatings of oily material can impair electrode. (3) Temperature effects.
Quality Control Requirements
See Standard Methods Section 1020.
Sample Handling
Collect sample in a plastic or glass container. No preservation is needed. Analyze within 2 hours.
Maximum Holding Time
2 hours
Relative Cost
Less than $50
Sample Preparation Methods