Surfactant sensitive electrode

The effect of hardness ions on surfactant activity is further illustrated in Figure. 4. Surfactant activity was monitored by means of a surfactant sensitive electrode as a function of hardness. The increase in potential in each curve between 900 and lOOOppm Ca indicates a reduction in activity. Only the sodium silicate maintained the surfactant activity at a constant level up to nearly 900ppm of Ga. The other alkalis resulted in a decrease in surfactant activity at about 400ppm. 

There are a number of reviews available for surfactants in specific industries [S7], and for specific surfactant classes. References [SJ-90] discuss methods for the determination of anionic surfactants, which are probably the most commonly encountered in the petroleum industry. Most of these latter methods are applicable only to the determination of sulfate- and sulfonate-functional surfactants. Probably the most common analysis method for anionic surfactants is Epton s two-phase titration method [9J, 92] or one of its variations [93, 94], Related, single-phase titrations can be performed and monitored by either surface tension [95] or surfactant-sensitive electrode [84, 85, 96-98] measurements. Grons-veld and Faber [99] discuss adaptation of the titration method to oleic phase samples. 

A promising method for quantitation of anionic process surfactants is by cationic surfactant (e.g., Hyamine) titration monitored by a surfactant-sensitive electrode. The basic approach is described in references [76, 77, S9-92]. This technique has found application in the analysis of formulated products in the cosmetic [9J ] and pharmaceutical [90] industries and may... 

Titrations of anionic, cationic and amphoteric surfactants with surfactants of opposite charge, using a surfactant-sensitive electrode. Titrations of cationic and amphoteric surfactants, and metal complexes of nonionic surfactants, with tetraphenylborate. 

Potentiometric titration with surfactants of opposite charge using a surfactant-sensitive electrode... 

Surfactant-sensitive electrodes are commercially available, and they are also easily made in the laboratory. They can be used to detect the end-point in titrations of anionic and cationic surfactants with surfactants of opposite charge. They are used in exactly the same way as a glass electrode in acid-base titrations, or a silver-silver chloride electrode in titrations of chloride with silver nitrate. The main advantages of potentiometric as opposed to two-phase titration are ... 

The construction of surfactant-sensitive electrodes presents no difficulty, and three types are described here. The first type is a concentration cell, the second is a conductor coated with a membrane containing a surfactant salt, and the third is a conductor coated with a membrane not containing a surfactant salt. Many other electrodes have been described in the literature over the last 20 years or so, but most if not all of them are of one of these basic types. 

With all types of surfactant-sensitive electrodes, high concentrations of electrolyte tend to depress the potential jump at the end-point, and some product types require separation of the surfactant from the product matrix. 

Pipette 25 ml into a 100 ml beaker. Insert a surfactant-sensitive electrode and a reference electrode. Add lOmlM hydrochloric acid. If necessary add water until the electrode tips are immersed. 

It is to be expected that other types of surfactant-sensitive electrodes can be used with equal or greater success, and it would not be surprising if the two-phase methods, e.g. that of Cross (section 7.1.3) could also be used, although the author is unaware of any published evidence. 

The concentration of the surfactant were determined by titration with cetylbenzyldimethylammonium chloride, which forms water-insoluble complexes with the anionic surfactant. Potentiometric titrations were carried out with a Titroprocessor 672 (Metrohm, Switzerland) using a surfactant-sensitive electrode [7]. The cationic polymer was analyzed by polyelectrolyte titration with poly-(ethylensulfonate) (PES). The equivalence point of the polyelectrolyte complexation reaction was indicated either by streaming potential (PCD 2, Mutek GmbH, FRG) [8] or, for lower concentrations of the cationic polymer, by a color change due to the cooperative binding of a metachromic dye on the excess chromotrope titrant [9, 10]. Brilliant Yellow was used as dye indicator. 

The product is acidified and titrated with sodium tetraphenylborate in aqueous solution using a surfactant-sensitive electrode or other electrode for end point detection, as described in Chapter 16. The addition of gum arabic to the titration vessel smooths the titration curve by preventing the deposition of the cationic/ tetraphenylborate precipitate on the electrode. 

Schulz, R., Potentiometric titration of nonionic surfactants with the surfactant-sensitive electrode (in German), SOFW-Jour., 1996,122,1022,1024-1028. 

Matesic-Puac R, Sak-Bosnar M, Bilic M, Grabaric S (2005) Potentiometric determination of anionic surfactants using a new ion-pair-based all-solid-state surfactant sensitive electrode. Sens Actuators B Chem 106 221-228... 

A ring test proved that surfactant-selective electrodes are suitable for quantitative determination of anionic surfactants including alkanesulfonates [21]. The precision of this method, however, does not yet correspond to the state-of-the-art of the two-phase titration. Therefore, further development is needed to enhance the reproducibility and competitiveness of surfactant-sensitive titration. 

Direct determination of surfactants in complex matrices can also be carried out using ion-selective electrodes. Depending on the membranes and additives used, the detergent electrodes are optimized for the detection of anionic surfactants [81], cationic surfactants [82], and even nonionic surfactants [83]. The devices are sensitive to the respective group of surfactants but normally do not exhibit sufficient stability and reproducibility for their use in household appliances. With further optimization of membrane materials, plasticizers and measurement technology, surfactant-selective electrodes offer high potential for future applications. 

Pipette 25 ml into a 100 ml beaker, insert surfactant-sensitive and calomel electrodes and commence stirring. 

An electrode sensitive to both cationic and anionic surfactants can be made from a glass tube sealed at the bottom with a membrane of poly(vinylchloride) containing a high percentage of plasticizer, 40% or more. Dissolved in the plasticized PVC is a surfactant ion pair, such as hexadecyltrimethylammonium dodecylsulfate. The tube is filled with a dilute solution of anionic surfactant with a small amount of a chloride salt, into which a silver/ silver chloride electrode is inserted. For potential measurements in solutions, the circuit is completed with a reference electrode such as saturated calomel. Commercial surfactant-selective electrodes based on PVC membranes have been available since approximately 1990. 

This approach will not be practical for some time to come. The fundamental properties of surfactants (micelle formation, enrichment at interfaces) mean that the activity of a surfactant will usually differ from its absolute concentration (1). Just as serious is the technical problem that current surfactant-selective electrodes suffer from response which varies with their past and recent history they are also sensitive to the concentration of nonsurfactant ions. The result is that quantitative applications use electrodes not in direct measurements relating potential to concentration, but as indicators of the end point of a titration. In this latter application, it is not important that the electrode potential be exactly reproducible, but only that the potential change sharply as the surfactant concentration changes. For the titration of an anionic surfactant with a cationic surfactant, the electrode used for end point detection can be chosen to respond to either surfactant. Because of the drift in electrode potential, titrations must be conducted to an inflection in the titration curve rather than to a specific millivolt value. Details of the potentiometric titration methods can be found earlier in this chapter. The electrodes have also been demonstrated as detectors for flow injection analysis. 

InvestigatiOTis of surfactant-sensitive potentiometric electrodes began in the 1970s. Since surfactant ion-selective electrodes have been developed by Gavach and Seta, the development of potentiometric surfactant sensors is an area of interest. Several excellent articles " "" review the use of different types of electrodes for surfactant analysis. When compared with other analytical methods, ion-selective electrodes (ISEs) are simple, relatively inexpensive, robust, durable, and ideal for their use in field environments. Some other advantages involve that they can be used very rapidly, that they allow cmitinuous monitoring, and that they are not affected by turbidity or color of a sample. 

Buschmann N, Schultz R (1993) Comparison of different ion-sensitive electrodes for the titrimetric determination of ionic surfactants. Toiside Surf Det 30 18-23... 

Buschmann N, Schultz R (1992) Development of an ion-sensitive electrode fin the titration of surfactants. Comun Jom Com Esp Dctcrg 23 323—328... 

Surface heterogeneity may be inferred from emission studies such as those studies by de Schrijver and co-workers on P and on R adsorbed on clay minerals [197,198]. In the case of adsorbed pyrene and its derivatives, there is considerable evidence for surface mobility (on clays, metal oxides, sulfides), as from the work of Thomas [199], de Mayo and co-workers [200], Singer [201] and Stahlberg et al. [202]. There has also been evidence for ground-state bimolecular association of adsorbed pyrene [66,203]. The sensitivity of pyrene to the polarity of its environment allows its use as a probe of surface polarity [204,205]. Pyrene or ofter emitters may be used as probes to study the structure of an adsorbate film, as in the case of Triton X-100 on silica [206], sodium dodecyl sulfate at the alumina surface [207] and hexadecyltrimethylammonium chloride adsorbed onto silver electrodes from water and dimethylformamide [208]. In all cases progressive structural changes were concluded to occur with increasing surfactant adsorption. 

Before use, condition the new electrode by employing it in the titration of a sodium lauryl sulfate solution with a solution of Hyamine 1622 or benzetho-nium chloride solution, 0.04 M using a silver-silver chloride reference electrode. During this conditioning, a layer of an insoluble cation-anion complex becomes firmly attached to the plasticized PVC coating rendering it sensitive to both anionic and cationic surfactants. 

Specific-ion electrodes are expensive, temperamental and seem to have a depressingly short life when exposed to aqueous surfactants. They are also not sensitive to some mechanistically interesting ions. Other methods do not have these shortcomings, but they too are not applicable to all ions. Most workers have followed the approach developed by Romsted who noted that counterions bind specifically to ionic micelles, and that qualitatively the binding parallels that to ion exchange resins (Romsted 1977, 1984). In considering the development of Romsted s ideas it will be useful to note that many micellar reactions involving hydrophilic ions are carried out in solutions which contain a mixture of anions for example, there will be the chemically inert counterion of the surfactant plus the added reactive ion. Competition between these ions for the micelle is of key importance and merits detailed consideration. In some cases the solution also contains buffers and the effect of buffer ions has to be considered (Quina et al., 1980). 

The ditetrazolium salts (309) have been patented for use in electrochromic electrodes which are used in display devices <89JAP01230026) and tetrazolium salts have also been developed for cell bioassays for neurotoxins active on voltage-sensitive sodium channels <93MI 417-03). Tests for inhibition of corrosion of zinc and brass carried out on 5-aminotetrazole showed it to be ineffective relative to other azoles <86MI 417-01). A number of tetrazoles including the 5-amino, 5-methyl, and 5-phenyl derivatives have been separately incorporated into surfactants used for corrosion inhibition with copper in water <9lMl4l7-07). Photopolymerizable resin compositions which are highly resistant... 

Glasses exist that fnnction as selective electrodes for many different monovalent and some divalent cations. Alternatively, a hydrophobic membrane can be made semiper-meable if a hydrophobic molecnle called an ionophore that selectively binds an ion is dissolved in it. The selectivity of the membrane is determined by the structnre of the ionophore. Some ionophores are natnral products, such as gramicidin, which is highly specific for K+, whereas others such as crown ethers and cryptands are synthetic. Ions such as, 1, Br, and N03 can be detected using quaternary ammonium cationic surfactants as a lipid-soluble counterion. ISEs are generally sensitive in the 10 to 10 M range, but are not perfectly selective. The most typical membrane material used in ISEs is polyvinyl chloride plasticized with dialkylsebacate or other hydrophobic chemicals. 

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