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Field Test Methods
pH of Water
AOAC Official Method 973.41
A. Principle
- pH, which is accepted measure of
acidity or alkalinity, is determined by change in potential of glass
saturated calomel electrodes, as measured by commercial apparatus
standardized against standard buffer solutions whose pH values are assigned
by NIST. pH of most natural H2 O falls within 4–9.
- Majority of waters are slightly basic
from presence of CO3 -HCO3 system.
- Method is applicable to drinking,
surface, and saline waters, and domestic and industrial wastes. Oils and
greases, by coating elec-trodes, may cause sluggish response.
B. Apparatus and Reagent
- (a) pH meter.Commercial instrument with
flow type electrodes (preferred for relatively unbuffered samples such as
condenses) or immersion electrodes. Operate in accordance with manufacturer’s
instructions.
- (b) Standard buffer solutions.
C. Determination
- Thoroughly wet electrodes and prepare
in accordance ith manufacturer’sinstructions. Standardize instrument with
standard buffer with pH near that of sample and then with 2 thers to check
linearity of electrode response.
- Analyze sample as soon as possible,
preferably within few hours.
- Do not open sample bottle before
analysis. With immersion electrodes, wash 6–8 times with portions of
sample, particularly when unbuffered solution follows buffered solution.
Equilibrium, as shown by absence of drift, must be established before
readings are accepted.
D. References
- Methods for Chemical Analysis of Water
and Wastes, 1983 (available from National Technical Information Service,
5285 Port Royal Road, Springfield, VA 22161, Stock No. NTIS PB84-128677);
- FWPCA Method Study 1; Mineral and
Physical Analyses, June 1969 (available from National Technical Information
Service, 5285 Port Royal Road, Springfield,
- VA 22161, PB-230827); JAOAC 56,
295(1973).
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