System of indicators for acid-base titration in surfactant-stabilized emulsions

Abstract

In recent years, the development of procedures utilizing surfactant-based organized media has attracted increasing attention of analysts. These media are convenient and pollution-free alternatives to toxic organic solvents in the titrimetric determination of hydrophobic protolytes [1][2][3][4][5][6][7]. The use of aqueous surfactant solutions allows one to change the acid-base properties of solubilized compounds in a controlled way. In this case, the performance characteristics of procedures improve as compared to titration in organic solvents. A logical development of this trend is to use surfactant-stabilized "oil-in-water" type emulsions (o/w-type emulsions) as the titration media. The use of emulsions frequently makes it possible to produce micellar media with a solubilization capacity (relative to the most hydrophobic organic compounds) high enough for titrimetric determinations [4][5][6][7]. Optimized emulsions are easy to work with and are stable against aggregation and sedimentation [8,9]. The emulsion media are environmentally safe when prepared using nontoxic oils. To develop procedures utilizing o/w-type emulsions as media for acid-base titration, one needs data on the protolytic equilibria of the indicators used to detect the titration end-point.The goal of this paper is to find the effective p T values (p T eff ) of some most frequently used acid-base indicators (HR) in emulsions based on various types of surfactants. The choice of the surfactants was governed by their high water solubility and their capacity to stabilize emulsions. The indicators were chosen based on their p K a values, which varied in a wide pH range. This experimental arrangement should provide the choice of the best indicator for the detection of the titration endpoint, depending on the nature and concentration of the analyte. EXPERIMENTAL Reagents and instruments.We used the following acid-base indicators of analytical grade: bromophenol blue, bromocresol blue, bromocresol purple, bromothymol blue, phenol red (PR), cresol red, thymol blue (TB), xylenol blue, methyl orange (MO), and phenolphthalein. We used sunflower oil of the TUU 18.503-98 specification, which has virtually zero intrinsic acidity, as an oil component in preparing the emulsions. To stabilize the emulsions, we used an anionic surfactant sodium dodecylsulfate (SDS), a cationic surfactant dodecylpyridinium chloride (DDPC), and a nonionic surfactant Triton X-100. The surfactants with the major content of at least 99.5% were from Merck. The working surfactant solutions were prepared by dissolving the corresponding weighted portions in distilled water. Indicator solutions were prepared according to the procedure [10]. The acidity of the solutions was monitored using a pH-340 pH-meter. Emulsion preparation.To prepare the emulsions, 0.25 g of the oil ( m oil ) and 25 mL of the surfactant solution were placed in a 50-mL beaker and stirred with a magnetic stirrer for 30 min. It was shown earlier that 0.1 M surfactant solutions ensure an acceptable stability and emulsifyin...

Determination of pH in Regions of the Midguts of Acaridid Mites

“…The selection of indicators was based on their pT and/or p K A values and transition ranges ( Holzbecher and Churacek 1987 , Senese 2000 , Shevchenko and Kulichenko 2005 , Sabnis 2008 ). All indicators were purchased from Sigma-Aldrich.…”