Color Change of Sudan III against Concentrated Sulfuric Acid in Acetonitrile and Quantification for a Small Amount of Concentrated Sulfuric Acid

SAKURAI Takao, KURATA Shoji, OGINO Kenji

doi:10.2116/analsci.32.129

Color Change of Sudan III against Concentrated Sulfuric Acid in Acetonitrile and Quantification for a Small Amount of Concentrated Sulfuric Acid

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SAKURAI Takao

Criminal Investigation Laboratory, Metropolitan Police Department Graduate School of Bio-applications and Systems Engineering, Tokyo University of Agriculture and Technology
KURATA Shoji

Criminal Investigation Laboratory, Metropolitan Police Department
OGINO Kenji

Graduate School of Bio-applications and Systems Engineering, Tokyo University of Agriculture and Technology

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The color-changing phenomenon of hydrophobic bisazo dye, Sudan III in an acetonitrile solution against the addition of concentrated sulfuric acid has been discovered and the chromic properties investigated. Based on observations, a novel quantification method of concentrated sulfuric acid has been developed. Sudan III changes its color from orange to blue against a small volume of sulfuric acid, and the acetonitrile solution of Sudan III is the most suitable for observing the color-change phenomenon. ¹H-NMR and UV-Vis spectroscopic studies showed that the color-change mechanism of Sudan III against sulfuric acid is due to the protonation of the dye by sulfuric acid. This phenomenon is applicable to the quantification of concentrated sulfuric acid by introducing the Hammett acidity function. The proposed method requires only a small amount of the sample, 0.04 mL, and enables rapid quantification.