Anionic surfactant concentrations quantitative determination

Physicochemical Basis of Quantitative Determination of Anionic Surfactant Concentrations by Using an Autotitrator... 

Similarly to quantitative determination of high surfactant concentrations, many alternative methods have been proposed for the quantitative determination of low surfactant concentrations. Tsuji et al. [270] developed a potentio-metric method for the microdetermination of anionic surfactants that was applied to the analysis of 5-100 ppm of sodium dodecyl sulfate and 1-10 ppm of sodium dodecyl ether (2.9 EO) sulfate. This method is based on the inhibitory effect of anionic surfactants on the enzyme system cholinesterase-butyryl-thiocholine iodide. A constant current is applied across two platinum plate electrodes immersed in a solution containing butyrylthiocholine and surfactant. Since cholinesterase produces enzymatic hydrolysis of the substrate, the decrease in the initial velocity of the hydrolysis caused by the surfactant corresponds to its concentration. Amounts up to 60 pg of alcohol sulfate can be spectrometrically determined with acridine orange by extraction of the ion pair with a mixture 3 1 (v/v) of benzene/methyl isobutyl ketone [271]. 

The development of an easy-to-handle method for the qualitative and quantitative determination of surfactants in consumer products was the goal for applying ESI in the FIA-MS(+/—) mode by direct infusion into the mass spectrometer. In this way Ci2, Ci4, Ci6 and Ci8 ASs could be determined besides other anionics (LASs, alkylcarboxylates), nonionics (alkyl polyglucosides (APGs)) and cationics (quats and ester-quats). The methods applied for concentration and determination (MS-MS) helped to identify the compounds and in addition deuterated internal standards were applied for confirmation [57]. 

Two independent analytical methods—LC-MS-MS and 19F-NMR— for the determination of perfluorinated anionic surfactants in environmental water samples were presented. Perfluorinated alkanesulfonates and perfluorocarboxylates were determined qualitatively and quantitatively because of an accidental release of perfluorosurfactant contaminated fire-fighting foam [55]. Ci8-SPE was applied for concentration of the compounds from water samples. Methanol was used for elution prior to ESI-LC-MS(—) analysis. The negatively recorded LC-MS-MS TIC for the determination of PFOS, PFHxS, PFOA, perfluor-oheptanoic acid (PFHpA), perfluorododecanoic acid (PFDoA internal standard) in water samples was presented [55]. 

Ion-pair extraction of an anionic surfactant with a cationic dye such as methylene blue from water into oil is often used for the quantitative analysis of the surfactant in water [57,58]. The surfactant concentration in water is then determined as the dye concentration in the oil or water phase by conventional absorption spectroscopy. Synthetic surfactants such as sulfates and sulfonates completely dissociate in water even at low pH. On the other hand, the association of fatty acid salts (traditional soaps) with H" " depends on the pH. Therefore, the quantitative analysis of surfactants in water is performed by the ion-pair extraction at various pHs. Although quantitative analysis and thermodynamic studies have been already reported for the anionic surfactant/cationic dye extraction, kinetic analysis of the ion-pair extraction has been rarely reported and the extraction mechanism is not discussed in detail. In this section, we describe the kinetic analysis of the extraction of a dodecyl sulfate anion with methylene blue as a typical example using the single microdroplet manipulation and microabsorption methods [59]. In particular, the pH dependence of the ion-pair extraction is discussed. 

Aliphatic carboxylic acids and their anions are known to absorb in the ultraviolet region as a result of u-tt and tt-tt transitions. Mukerjee et al. [64] found that long-chain perfluorocarboxylates, such as perfluorooctanoate (e = 344 L/mol/cm at 25 °C), have higher molar absorptivities in the 205-230-nm region than perflu-oroacetate (e = 57 L/mol/cm at 25°C). The absorptivity of perfluoroalkanoates is sufficient for a quantitative determination of the fluorinated surfactant down to the 10 M concentration range using a 10-cm cell. Mukerjee et al. [64] observed that below the critical micelle concentration (cmc), perfluoroheptanoate and perfluorooctanoate solutions obeyed the Beer-Lambert law within 1%. A somewhat bet-... 

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