Tetraphenylborate ion

TPhBE tetraphenylborate ion-selective electrode (reference electrode)... 

Two aqueous phases separated by a liquid membrane, EM, of nitrobenzene, NB, were layered in a glass tube, which was equipped with Pt counterelectrodes in W1 and W2 and reference electrodes in three phases as in Eq. (1). Reference electrodes set in W1 and W2 were Ag/AgCl electrodes, SSE, and those in LM were two tetraphenylborate ion selective electrodes [26,27], TPhBE, of liquid membrane type. The membrane current, /wi-w2 was applied using two Pt electrodes. The membrane potential, AFwi-wi recorded as the potential of SSE in W2 vs. that in W1. When a constant current of 25 /aA cm was applied from W1 to W2 in the cell given as Eq. (1), the oscillation of membrane potential was observed as shown in curve 1 of Fig. 1. The oscillation of AFwi-wi continued for 40 to 60 min, and finally settled at ca. —0.40 V. 

Here, TPA and TPhB denote tetrapentylammonium ion and tetraphenylborate ion, respectively. 

Shoukry et al. have prepared plastic membrane ion-selective electrodes for the determination of procaine and other anaesthetic compounds [69]. The electrode selective for procaine was prepared with the use of a membrane containing 15% of the procaine tetraphenylborate ion pair with 40% of dioctyl phthalate and 45% of poly vinyl chloride (PVC). The membrane was attached as a disc (12 mm diameter, 0.3 mm thick) to the polished PVC cap of the electrode tube, which contained an internal solution of 0.1 M sodium chloride made 1 mM in the same drug, and in contact with a Ag-AgCl wire. Linear response ranges were determined to be 20.0 pM to 16 mM for procaine over the pH range of 3.1 to 7.9. The electrodes could also be used in the potentiometric titration of the drug with 0.01 M sodium tetraphenylborate. 

Satake et al. reported the use of a coated wire electrode sensitive to procaine and other local anesthetic cations, and their application to potentiometric determination [73]. Electrodes were constructed from a copper wire (0.8 mm diameter), coated with a PVC membrane comprising a mixture of the drug-tetraphenylborate ion-pair, dioctyl phthalate, polyvinyl chloride, and tetrahydrofuran. Potential measurement was made with respect to a Ag-AgCl reference electrode. The electrodes showed linear responses with a Nemstian slope for procaine over the concentration range investigated. The method was used for analyses of the drug in pharmaceutical preparations. 

The tetraphenylborate ion-doped polypyrrole electrode was sensitive to zinc ions [464, 465], and its sensitivity was dependent on the polymer macrostructure. 

Wachter, W., Buchner, R., and Hefter, G. Hydration of tetraphenylphosphonium and tetraphenylborate ions by dielectric relaxation spectroscopy. 7. Phys. Chem. B. 2006, 110,5147-5154. 

Sodium tetraphenylborate, (C5H5>4B Na, is an important example of an organic precipitating reagent that forms salt-like precipitates. In cold mineral acid solutions, it is a near-specific precipitating agent for potassium and ammonium ions. The precipitates have stoichiometric composition and contain one mole of potassium or ammonium ion for each mole of tetraphenylborate ion these ionic compounds are easily filtered and can be brought to constant mass at 105°C to 120°C. Only mercury(Il), rubidium, and cesium interfere and must be removed by prior treatment. 

The two-phase titration can be also successfully used for the determination of cationic surfactants using standardized solutions of anionic surfactants. In this case the mechanism of titration is the reverse, i.e. the chloroform layer undergoes color change from blue to pink. More often, and especially at higher concentrations, cationic surfactants are determined by two-phase titration using sodium tetraphenylborate titrant and bromophenol blue indicator [32], Tetraphenylborate ion is also used in the determination of potassium, rubidium and cesium, so the ions of these metals, if present, interfere with the determination of quaternary ammonium surfactants by this method. 

In order to split the experimentally available Ahydr fE° values dealing with entire electrolytes into the ionic contribution a value must be estimated for just one ion. Conventional values are obtained on setting Ahydr f° (H+) " =0 at all temperatures. The absolute value Ahydrf / (H+, aq)=-1103 7 kJ mor at 298.15 K results (Marcus 1987) according to the TPTB assumption, equating the standard enthalpies of hydration of the tetraphenyphosphonium and tetraphenylborate ions ... 

Potassium, rubidium and caesium ions together with tetraphenylborate ions form a precipitate which does not dissolve readily in water. According to W. Geilmann and W. Gebauhr the solubility products are ... 

An alternative is to use a pre-prepared stable ate complex, so that no Lewis base needs to be added to the mixture. One simple example is the tetraphenylborate ion, as in NaBPh4, which can acts as a phenyl transfer agent. A wide range of palladium catalysts have been used. While complexes with simple phosphines, such as triphenylphosphine, work well in many cases, the use of more sophisticated ligands is required in more difficult cases including the coupling of unactivated aryl chlorides. ... 

According to Parker [6] the standard Gibbs energy for transfer of the tetraphenyl-arsonium ion (TPAs ) and of the tetraphenylborate ion (TPB"") are equal for any pair of solvents. The standard Gibbs energy for transfer of the TP As and TPB ions can be determined from the distribution coefficients between any pair of immiscible solvents. If the distribution coefficient for the TP As A salt is found for any arbitrary ion A , then its standard Gibbs transfer energy is... 

Geske (10) applied his theoretical observations to an experimental study of the electro-oxidation of tetraphenylborate ion and found an apparent increase in the number of faradays per mole with decreasing initial concentrations of electro-active material. By a consideration of the data obtained from current-time curves and solution analysis, it was possible to support a reaction scheme in which some of the tetraphenylborate ion is removed by reaction with hydrogen ions generated by interaction of one of the primary electrolysis products with the solvent. Reasonable agreement was found between the experimental findings and the predictions of the theoretical treatment given above. 

An interesting study involving the controlled-potential oxidation of tetraphenylborate ion was carried out by Geske (271, 272) who showed how coulometric data can be used to elucidate the mechanisms of secondary reactions following the primary electrode process. Rechnitz and Laitinen (273) also used controlled-potential coulometry to confirm the mechanism of the molybdenum-catalysed reduction of perchlorate ion. An attempt by Kennedy (274) to study the kinetics of the reduction of phosphotimgstic acids was unsuccessful owing to interference by secondary reactions. 

It is unusual to perform an assay on AE, since the commercial products are reasonably pure. Assay is not a regular part of the certificate-of-analysis quality check. Simple determination of AE in aqueous solution may be performed by potentiometric titration with tetraphenylborate ion as described in Chapter 16. This titration is less effective for AE with less than 10 moles of ethylene oxide. In this case, the product may be titrated after conversion to its sulfated derivative, also described in Chapter 16, although this is rarely done. Titration by the HI cleavage method (Section II of this chapter) may also be used for assay. AE can be determined by all of the chromatographic techniques described in later chapters. These are generally unsuitable for assay because of the 1—5% uncertainty associated with chromatographic determinations. 

APE may be titrated in aqueous solution with tetraphenylborate ion, as described in Chapter 16. As with other ethoxylates, this titration is incomplete for products with less than about 10 moles of ethylene oxide. Chapter 16 describes how APE of lower degree of ethox-ylation may be converted to the corresponding sulfate and titrated as an anionic surfactant. APE is sometimes assayed simply by UV spectrophotometry. APE is easily determined by HPLC and indeed by all of the chromatographic methods described in this volume. 

There are no truly satisfactory methods for assay of these products. They can be determined by most chromatographic methods, as discussed in later chapters. An approximation of active agent content may be made by determining the main impurities, namely free fatty acid, PEG, and water, and subtracting from 100%. Concentration of acid ethoxylates in solution can be determined by potentiometric titration with tetraphenylborate ion as described in Chapter 16. Free PEG is also determined by this titration. 

A widely accepted titrant for two-phase titration of cationics is tetraphenylborate ion (88). The titration depends upon the surfactants maintaining their ionic character, and therefore the pH is critical. All common cationic surfactants can be titrated at pH 3, using methyl orange as indicator. At pH 10, all quaternaries can be titrated, but other amines are not sufficiently ionized. At pH 13, only certain quaternaries can be determined, but these include the commercially most abundant products alkyltrimethyl- and dialkyldimethylam-monium and benzyltrialkylammonium salts. Bromophenol blue is a suitable indicator for the titrations at pH 10 and 13. 

Some specialty cationics which are not sufficiently hydrophobic for direct potentiometric titration with tetraphenylborate can be determined indirectly. An excess of sodium tetraphenylborate is added to precipitate the surfactant, which is removed by filtration. Excess tetraphenylborate ion in the filtrate is then determined by titration with thallium(I) nitrate (110). This last procedure was used for determination of the amount of a cationic surfactant fixed to cotton fibers in a textile dying bath. 

These experimental electrodes with surfactants incorporated into the membranes can be used to monitor the titration of nonionics (as their Ba complexes) with tetraphenylborate (133). Ivanov and Pravshin report that an electrode suitable for determination of nonionics can be made from a membrane of plasticized PVC containing either cetylpyrid-inium dodecylsulfonate or the complex of barium and tetraphenylborate ions with a nonionic (134). These electrodes also respond to ionic surfactants and respond much better to tetraphenylborate than to nonionic surfactants. Thus, they are most useful for monitoring the end point of a titration of the barium complexes of nonionics with tetraphenylborate. Ionic surfactants interfere in the titration. 

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