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National Water Quality Laboratory

Saturday July 21, 2018

USGS Open-File Report 95-189

Methods of Analysis by the U.S. Geological Survey National Water Quality Laboratory — Determination of Methylene Blue Active Substances by Spectrophotometry

Mark R. Burkhardt, Pete J. Cinotto, Galen W. Frahm, Mark T. Woodworth, and Jeffrey W. Pritt

1995

A method for the determination of methylene blue active substances in whole-water samples by liquid-liquid extraction and spectrophotometric detection is described. Sulfate and sulfonate-based surfactants are reacted with methylene blue to form a blue-colored complex. The complex is extracted into chloroform, back-washed with an acidified phosphate-based buffer solution, and measured against external standards with a probe spectrophotometer. The method detection limit for routine analysis is 0.02 milligram per liter. The precision is ±10 percent relative standard deviation. The positive bias from nitrate and chloride in U.S. Geological Survey method O-3111-83 for methylene blue active substances is minimized by adding a back-washing step.

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