Metal-cored wire electrodes are much cheaper and easier to manufacture than solid electrodes. They can be made in a shorter turnaround time and at a lower cost than special-ordered solid electrodes.
Anionic, cationic, and amphoteric surfactants were potentiometrically titrated using a coated-wire electrode. The coated-wire electrode and a silver-silver chloride reference electrode were immersed in sample aqueous solutions with different concentrations of simulated commercial detergents and raw material surfactants.
Several thermodynamic studies of the copolymer+surfactant micellar aggregates are available from the literature [39-49]. These analyses were based on isothermal titration calorimetry (ITC) experiments.
The ITC data demonstrates that the enthalpy of injection (DHi) depends on the surfactant molarity and the hydration state of its polar heads. At low molarity, the DHi values are close to those observed for the pure surfactant and at high molarity, the DHi value decreases.
Moreover, ITC results indicate that the dependence of DHi on the surfactant molarity is more pronounced than that of molarity in water for sodium decylsulfate (NaDeS). These observations confirm that the enthalpy of transfer of NaDeS from water to the copolymer-surfactant micellar aggregates is mainly caused by the interactions of the hydrophobic tails and/or the polar heads with the hydrated sulfates of the copolymer. This feature makes the ITC technique a suitable tool for the determination of the enthalpy of micellization of nonionic surfactants. However, the use of the coated-wire electrode 11 of FIG. 1 permits potentiometric titrations of detergent matrices without the interference from inorganic ions.
Metal-cored wire is a tubular electrode that contains alloy powders, arc stabilizers and welding aids. The alloys are mixed with the core material, which is mostly calcium silicate. This allows the additives to be inserted into the steel at an ideal depth and achieve a physicochemical reaction. It avoids the reaction of added elements with air and slag, increases element yield, improves steel quality and reduces the cost of casting steel.
Coated-wire electrodes are used for potentiometric titrations of nonionic detergents that contain alkoxy groups capable of forming water-insoluble complexes with multivalent metal cations. These titrations are faster and less expensive than two-phase titrations and assay analyses with HPLC instruments.
According to Czechoslovakian patent No. 225,222 published in December, 1984, an ion-selective membrane was coated onto the conductive electrode by repeatedly dipping the conductive wire into a solution of PVC (85 mg), di-n-octyl phthalate and Reinecke acridine orange in the solvent tetrahydrofuran. The resulting membrane was thick enough to block most of the conductivity of the conductive wire.
In order to control the concentration of surfactants used in the production of commercial detergents and to ensure that such concentrations remain constant over time, it is normally necessary to perform quantitation tests on these compounds. Traditionally, such tests are accomplished using potentiometric titrations in which a conventional indicating electrode is matched with the type of surfactant being titrated. This process requires the addition of an aqueous barium chloride solution to the sample in order to precipitate a colored endpoint that can be identified by observation.
In an effort to reduce the need for this labor intensive procedure, a novel coated-wire sensing electrode has been developed. The invention is based on the use of a conductive wire that has been coated with a membrane composed of a mixture of polyvinyl chloride (PVC), di-n-octyl phthalate, and Reinecke acridine orange in the solvent tetrahydrofuran. After being dipped several times into this solution, the conductive wire is allowed to dry and the resulting membrane can be used as an indicating electrode in potentiometric titrations.
The coated-wire electrode 1 of FIG. 1 is capable of detecting anionic, cationic, and amphoteric surfactants that form water-insoluble complexes when titrated. The electrode is also selective in detecting surfactant molecules with ten or more carbon atoms in mixtures of surfactants and other lower molecular weight organic compounds or common inorganic ions. This is illustrated in Table VI which compares the results of a series of potentiometric titrations using the NEODOL and SLS titrants on a detergent matrix containing LAS and a mixture of NEODOL and HYM.
The observed enthalpy change with the binding of a surfactant to the polyelectrolyte is positive and endothermic, indicating that the aggregation of a surfactant with a polyelectrolyte is primarily driven by electrostatic interactions between the oppositely charged surfactant head and polyelectrolyte copolymer chain. Similarly, it is known that the aggregation of a polyelectrolyte with surfactant can be inhibited by varying the charge density of the polyelectrolyte, or by the presence of salt which acts to counteract the electrostatic interactions through charge screening.
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