USGS Test Method I-2781-85

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Method Name:
Specific Conductance of Water, Automated. Official Name: Specific Conductance, Electrometric, Wheatstone Bridge

Specific conductance is determined by using a Wheatstone bridge in which a variable resistance is adjusted so that it is equal to the resistance of the unknown solution between platinized electrodes of a standardized conductivity cell. The ability of a solution to conduct an electric current is a function of the concentration and charge of the ions in the solution and also depends on the rate at which the ions can move under the influence of an electrical potential. As the number of ions per unit volume of solution increases, the rate at which individual ions can move decreases, because of interionic attraction and other effects, For this reason, a graph of total-ion concentration versus specific conductance, even for solutions of a single salt, is a simple straight line only for rather dilute solutions, As specific conductance increases beyond about 5,000 uS/cm at 25oC, the regression line curves, and beyond 50,000 uS/cm, the specific conductance is generally an unsatisfactory index of solute-ion concentration.Additional information on the theory and practice of specific conductance measurements can be found in Daniels and Alberty (1966), and in Scofield (1932). This procedure may be automated by the addition of commercially available instrumentation.

This method may be applied to all natural, treated, and industrial water.

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


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



12,900 (undiluted)

Precision for specific conductance for 36 samples within the range of 9.0 to 2080 uS /cm can be expressed as follows: S = 0.054X - 3.80, where S is overall precision in microsiemens per centimeter at 25oC and X is specific conductance in microseimens per centimenter at 25oC. The correlation coefficient is 0.9005. Precision for specific conductance for five of the 36 samples is: 32, 31, 28, 36 and 55 laboratories analyzed the samples with mean values of 9.0, 112, 557, 1170 and 2080 uS/cm at 25oC and 19, 4, 3, 5 and 47 %-RSD, 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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