Analysts anionic surfactant

Active matter (anionic surfactant) in AOS consists of alkene- and hydroxy-alkanemonosulfonates, as well as small amounts of disulfonates. Active matter (AM) content is usually expressed as milliequivalents per 100 grams, or as weight percent. Three methods are available for the determination of AM in AOS calculation by difference, the two-phase titration such as methylene blue-active substances (MBAS) and by potentiometric titration with cationic. The calculation method has a number of inherent error factors. The two-phase titration methods may not be completely quantitative and can yield values differing by several percent from those obtained from the total sulfur content. These methods employ trichloromethane, the effects from which the analyst must be protected. The best method for routine use is probably the potentiometric titration method but this requires the availability of more expensive equipment. 

CE has been used for the analysis of anionic surfactants [946,947] and can be considered as complementary to HPLC for the analysis of cationic surfactants with advantages of minimal solvent consumption, higher efficiency, easy cleaning and inexpensive replacement of columns and the ability of fast method development by changing the electrolyte composition. Also the separation of polystyrene sulfonates with polymeric additives by CE has been reported [948]. Moreover, CE has also been used for the analysis of polymeric water treatment additives, such as acrylic acid copolymer flocculants, phosphonates, low-MW acids and inorganic anions. The technique provides for analyst time-savings and has lower detection limits and improved quantification for determination of anionic polymers, compared to HPLC. 

M. Agudo, A. Rios, M. Valcarcel, Continuous liquid—liquid extraction with on-line monitoring for the determination of anionic surfactants in waters, Analyst 119 (1994) 2097. 

A. J. Frend, G. J. Moody, J. D. R. Thomas, and B. J. Birch, Flow Injection Analysis with Tubular Membrane Ion-Selective Electrodes in the Presence of Anionic Surfactants. Analyst, 108 (1983) 1357. 

Moody, G. J., J. O. Rutherford, J. D. R. Thomas, Sorption behavior of dodecylsulphate and other anionic surfactants on anion-exchange resins. Analyst, 1981,106, 537-546. lanniello, R. M., Organic acids in APE by anion exclusion HPLC, Anal. Lett., 1988,2 /, 87-99. Hiilskotter, F., M. Raulf, N. Buschmann, Titration of lipophilic ionic and nonionic surfactants, World Surfactants Congr., 4th, 1996,4,96-101. 

Chitikela, S., S. K. Dentel, H. E. Allen, Modified method for the analysis of anionic surfactants as methylene blue active substance.s. Analyst, 1995,120,2001-2004. 

Kasahara, 1., K. Hashimoto, T. Kawabe, A. Kunita, K. Magawa, N. Hata, S. Taguchi, K. Goto, Spectrophotometric determination of anionic surfactants in sea water based on ion-pair extraction with te 2-(5-trifluoromethyl-2-pyridylazo)-5-diethylaminophenolato cobalt(lll). Analyst, 1995,120,1803-1807. 

Higuchi, K., Y. Shimoishi, H. Miyata, K. Toei, T. Hayami, Spectrophotometric determination of anionic surfactants in river waters using l-(4-nitrobenzyl)-4-(4-diethylaminophenylazo)-pyridinium bromide. Analyst, 1980,105, 768-773. 

Yamamoto, K., S. Motomizu, Solvent extraction-spectrophotometric determination of anionic surfactants in sea water. Analyst, 1987,112,1405-1408. 

Kubota, H., M. Idei, S. Motomizu, Liquid-liquid distribution of ion associates of anions with 4-[4-aIkyI(aryI)aminophenylazo]pyridines and their use as spectrophotometric reagents for anionic surfactants. Analyst, 1990,115,1109-1115. 

Tsubouchi, M., J. H. Mallory, Differential determination of cationic and anionic surfactants in mixtures by two-phase titration. Analyst, 1983,108, 636-639. 

Alegret, S., J. Alonso, J. Bartrolf, J. Baro-Rom, J. Sdnchez, M. del Valle, Solid-state ion-selective electrode for automated titration of anionic surfactants. Analyst, 1994, 119,... 

Recalde Ruiz, D. L., A. L. Carvhalo Torres, E. Andrds Garcfa, M. E. Diaz Garcfa, Fluorimet-ric flow-injection method for anionic surfactants based on protein-surfactant interactions. Analyst, 1998, /2i, 2257-2261. 

A more efficient method of isolating anionic surfactants is extraction as part of an ion pair (33). An inorganic salt is added to decrease the solubility of the ion pair in the aqueous phase. Sometimes, the methylene blue spectrophotometric method described in Chapter 12 is used as the cleanup step. This permits the analyst to estimate the amount of surfactant isolated before proceeding with more definitive analytical techniques. Methylene blue may be removed from the surfactant extract by passage through a cation exchange column (56). If concentration is performed by liquid-liquid extraction of the ion pair with an alkyl quaternary compound, the UV spectrum of the ion pair is identical to that of LAS alone (55). 

Nowadays, it is rare for infrared techniques to be used for qualitative or quantitative analysis of environmental materials. In either case, exhaustive separation of the surfactant from other materials must first be made. It is possible for inexperienced practitioners to go far wrong when identifying materials by IR, a technique best applied to pure compounds. Most environmental extracts, even after substantial cleanup, are mixtures which give complex spectra. It requires an experienced analyst to obtain useful information from the spectrum of a mixture containing unknown materials. As a general rule, a compound cannot be said to be present unless all of its characteristic absorbance bands are exhibited by the mixture. A once-common use of IR spectroscopy was confirmation of the identity of anionic surfactants isolated by the methylene blue spectrophotometric method. By proper choice of workup procedures and bands, this approach permitted exact determination of individual types of surfactants (78). 

Szczepaniak W (1990) Mercuiated polystyrene as a sensor for anionic surfactants in ion-selective polymeric membrane electrodes. Analyst 115 1451-1455... 

Surfactants are surface-active compounds, which are used in industrial processes as well as in trade and household products. They have one of the highest production rates of all organic chemicals. Commercial mixtures of surfactants consist of several tens to hundreds of homologues, oligomers and isomers of anionic, non-ionic, cationic and amphoteric compounds. Therefore, their identification and quantification in the environment is complicated and cumbersome. Detection, identification and quantification of these compounds in aqueous solutions, even in the form of matrix-free standards, still poses the analyst considerable problems. 

Numerous applications have been shown to exist that overcome the general problems of lack of volatility and instability at higher temperatures that principally hamper direct analysis of surfactants by GC methods. Thus, a whole suite of derivatisation techniques are available for the gas chromatographist to successfully determine anionic, non-ionic and cationic surfactants in the environment. This enables the analyst to combine the high-resolution chromatography that capillary GC offers with sophisticated detection methods such as mass spectrometry. In particular, for the further elucidation of the complex mixtures, which is typical for the composition of many of the commercial surfactant formulations, the high resolving power of GC will be necessary. 

Amphoteric surfactants can function either as anionic or cationic surfactants, depending on the pH of the system. They contain both anionic and cationic functions in the same moiecuie. More costiy to produce than ionic surfactants, amphoteric surfactants represent oniy about 3% of surfactant voiume in Europe and less than 1% in the United States. They are less irritating than other materials and are largely used in personal care products. A distinction can be made between amphoteric and zwitterionic surfactants. This distinction does not affect their analysis. For the analyst, a more important distinction is between am-photerics with a secondary or tertiary amine group and those containing a quaternary amine function. The former only have cationic properties when protonated at low pH, while the quaternary amines have cationic properties even under alkaline conditions. 

Since commercial surfactant electrodes are available, there is no reason for the analyst to prepare membranes and construct electrodes. Considerable know-how is involved in producing durable electrodes which behave reliably, so we should leave this to the instrument manufacturers. To illustrate this point, we may mention a study in which six electrodes were compared for use in end point detection of a titration of a cationic with an anionic. Two commercial surfactant-selective electrodes, two tetrafluoroborate selective electrodes, a nitrate selective electrode, and a homemade PVC-membrane electrode incorporating a tetraphenylborate salt were tested (135). Only one electrode, one of the commercial models, was traly suited for routine use, giving smooth potentiometric curves without reconditioning for over 100 titrations. The relative standard deviation of the end point was about 0.5%, while that for the other electrodes was 1.3-2.3%. The standard deviation of the end point potential was 3 mV for this electrode, compared to 8 mV or more for the other electrodes. Besides this, those electrodes not designed as surfactant electrodes required reconditioning (i.e., soaking in a dispersion of a surfactant ion pair) after 25 titrations in order to remain usable. 

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