Abstract The protolytic properties of fat row carbonic acids in Triton X-100-stabilized “oil-in-water” (O/W) emul- sions were studied. The differential influence of emul- sions on the force of acids was shown to depend on the length of the hydrocarbon radical. The conditions for sep- arate alkalimetric determinations of acids with different hydrophobicities in their mixtures using emulsions stabi- lized by non-ionic surfactant were suggested. The proce- dure for the determination of 4-chloro-N-(2-furilmethyl)- 5-sulfamoylanthranilic acid (furosemide) main substance content in pharmaceuticals using Triton X-100-stabilized emulsion was proposed. Keywords O/W emulsions · Non-ionic surfactant · Triton X-100 · Carbonic acids · Protolytic properties · Alkalimetric determination · Furosemide Introduction Until recently aqueous micellar solutions of surfactants were considered as convenient and ecologically safe alter- native media for titrimetric determinations of hydropho- bic organic acids and bases due to the solubilization phe- nomena [1, 2, 3, 4, 5]. Traditionally toxic organic solvents such as benzene and chloroform were used for determina- tion of hydrophobic compounds using the acid–basic titra- tion method [6]. However, these solvents are hazardous to human health and the environment. On the other hand, aqueous micellar solutions of surfactants and emulsion media stabilized by surfactants are non-volatile and rela- tively harmless reagents [7, 8, 9, 10, 11, 12]. Usually, sur- factants increase the solubility of hydrophobic compounds making them suitable for reliable titrimetric determina- tions. However, for highly hydrophobic compounds it is practically impossible to achieve the threshold concentra- tion to achieve their precise determination. This obstacle can be overcome through substitution of aqueous micellar systems with O/W emulsions stabilized by surface-active compounds [7]. If nontoxic natural oils were used for preparing the emulsions then such media would have the advantage of both solubility and ecological safety. Emul- sions of oils stabilized by a non-ionic surfactant Triton X-100 are resistant to aggregation and sedimentation. Therefore, these emulsions become convenient for per- forming titration of hydrophobic organic acids. The objective of our study was to investigate protolytic characteristics of fat row carbonic acids in the emulsion media stabilized by Triton X-100. The use of such acids as model compounds allows us to trace the behavior of hydrophobic organic acids in the emulsion media and the influence of hydrocarbonic radical length of acids on their protolytic characteristics in O/W emulsions. We also planned to find and develop concentration conditions to allow titri- metric determinations of emulgated hydrophobic acids. Experimental Reagents and apparatus In our research we used fat row monocarbonic acids with general formula C n H 2n+1 COOH where n=1–17. These acids were from Reachem and were of analytical reagent grade. The solid acids were additionally purified by recrystallization from an ethanol/ace- tone mixture [13]. We used the vegetative sunflower oil (TYY 18.503–98), which has practically zero acidity, as the “oil” com- ponent of the emulsions. For the stabilization purposes we used non-ionic surfactant Triton X-100 (≥99%) purchased from Merck. All solutions were prepared by dissolving calculated masses of substances in distilled water. All pH measurements were per- formed on the standard scale pH meter (pH-340). Procedures Previously, we studied the stability of O/W emulsions stabilized by Triton X-100. We showed that the emulsion stability increases with Triton X-100 concentration and decreasing oil content. As the optimum emulsion media we have chosen the systems that contained 0.25 g of oil in 25 mL of the 2.5% Triton X-100 so- S. A. Kulichenko · G. M. Shevchenko Triton X-100-stabilized “oil-in-water” emulsions as suitable media for alkalimetric determination of hydrophobic organic acids Anal Bioanal Chem (2003) 375 : 255–258 DOI 10.1007/s00216-002-1647-6 Received: 26 June 2002 / Revised: 11 September 2002 / Accepted: 11 October 2002 / Published online: 7 December 2002 ORIGINAL PAPER S.A. Kulichenko (✉) · G.M. Shevchenko Analytical Chemistry Department, Taras Shevchenko Kyiv University, Volodymyrska Str. 61, 01033 Kyiv, Ukraine e-mail: kulichenko@mail.univ.kiev.ua © Springer-Verlag 2002