USGS Test Method I-2587-85

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Method Name:
pH of Water Samples, Automated. Official Name: pH, Electrometric, Glass-Electrode

Samples are analyzed on a standardized pH-selective electrode. This procedure may be automated with commercially available instrumentation.

This method may be used to determine the pH of any natural or treated water, and any industrial or other wastewater. The pH electrode is standardized with buffer solutions of pH 4.00, 7.00 and 9.00. If samples with pH less than 4 or greater than 9 are analyzed, standardization with additional buffers will be necessary.

Methods for the Determination of Inorganic Substances in Water and Fluvial Sediments, Techniques of Water-Resources Investigations of the United States Geological Survey, Book 5, Chapter A1 Edited by Marvin J. Fishman and Linda C. Friedman

The determination is not affected by the presence of color or turbidity, or of organic or colloidal material. Oxidizing and reducing substances do not impair the accuracy of method. The pH measurement is temperature dependent, and a significant error results if the temperatures of the buffers and samples differ appreciably. However, a variation of less than 5oC has no significant effect except in the most exacting work. For samples having abnormally high sodium levels, corrections may be necessary. This correction varies with the type of electrodes used; hence, see the manufacturer's instructions for the necessary computations.

QC Requirements:
Quality-control samples area analyzed at a minimum of one in every ten samples. These QC samples include at least one of each of the following: blanks, quality control samples, third party check solutions, replicates, and spikes. Correlation coefficients for calibration curves must be at least 0.99. QC samples must fall within 1.5 standard deviations of the mean value. If all of the data-acceptance criteria in the SOPs are met, then the analytical data are acceptable.

Maximum Holding Time:
180 days from sampling



4.0 - 9.0 (for normal samples)

Precision for pH for five of the 36 samples is as follows: 27, 59, 33, 48 and 60 laboratories analyzed samples finding a mean of 6.21, 7.14, 7.52, 8.00 and 8.54 pH units and 0.26, 0.32, 0.15, 0.21 and 0.15 standard deviation (in pH units), respectively. Using automated instrumentation, analysis of two test samples by a single laboratory for 25 replicates of each resulted in mean values of 7.58 and 8.07 pH units and standard deviations of 0.05 and 0.03 pH units, respectively.

Method detection limits (MDLs) were determined for this method using the procedures of the U.S. Environmental Protection Agency (1997).

Revision Number:

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