A highly sensitive and simple spectrophotometry for a small amount of long chain quaternary ammonium salts (R
4NX) was developed by using chlorogallein methylester (Cl. Gall. CH
3) as a reagent. Maximum difference of absorbance between Cl. Gall. CH
3 solution and Cl. Gall. CH
3-R
4NX solution is observed at 600 nm, and the absorbance is constant in the pH range from 4.3 to 5.4. The calibration curve was linear in the range of 5 to 20μg/10ml cetylpyridinium (CP), cetyltrimethylammonium (CTA) and benzethonium (Benz.). This method was applicable to the analyses of cationic surface-active agent in waste water. The recommended analytical procedure for R
4NX salts is as follows. A sample solution containing less than 20 μg of R
4NX salts (CP, CTA, Benz.) is taken in a 10 ml measuring flask, and then 2.0 ml of Walpole buffer solution (pH 4.6), 1.0 ml of 1.0% polyvinylalcohol (PVA) solution and 1.0 ml of 1.0 × 10
-3 M Cl. Gall. CH
3 methanol solution are added, and diluted to the mark with distilled water, and the solution is kept at (2025)°C for 30 min, and then the difference of absorbance of Cl. Gall. CH
3 solution and Cl. Gall. CH
3-R
4NX solution is measured at 600 nm. Bismuth(III), thorium (IV), zirconium(IV) and iron (III) could be masked by the addition of EDTA or CyDTA solution, and aluminum(III) and tin(IV) could be masked by sodium fluoride or sodium oxalate solution.
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