USGS Test Method I-2371

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Method Name:
Iodide, dissolved in water by colorimetry. Official Name: Iodide, colorimetric, ceric-arsenious oxidation, automated-segmented flow

The method is based on the catalytic effect of iodide on the ceric-arsenious oxidation reaction in acid solution. In the presence of a small amount of iodide, the reaction follows first-order reaction-rate kinetics and at a given temperature and for a given reaction time, the extent of reduction of ceric- ion is directly proportional to the iodide concentration. Photometric measurement of the absorbance of the solution permits evaluation of the extent of the reaction (Mitchell, 1966).

This method may be used to determine iodide in fresh water and brines containing from 0.001 to 0.060 mg/L of iodide. Greater concentrations need to be reduced by dilution; however, samples containing concentrations greater than 1 mg/L need to be analyzed by the bromine oxidation method (method I-1370).

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

Most substances normally present in natural water do not interfere. Bromide ion does not interfere. Low iodide values will be obtained if the pH of the water samples is less than 3 or if the iron concentration exceeds 500 ug/L.

QC Requirements:
Calibrate instrument using calibration standards (CAL). Quality control samples (QCS) and laboratory blanks (LB) analyzed at a minimum of I each after every 10 samples

Maximum Holding Time:
180 days



0.001 to 0.060 mg/L


Precision data are not available for this method.

Revision Number:

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