Titration with sodium tetraphenylborate

Provided the solution is at least 0.1 M in hydrogen ion, betaines can be titrated potentiometrically with NaTPB. Vytfas et al. [18] successfully titrated cocodimethylbetaine, cocoamidopropylbetaine and an imidazoline derivative with NaTPB using the coated aluminium wire described in section 3.6.3. The potential changes at the end-point were not very large, and the method would probably be useful mainly for fairly pure samples such as raw materials rather than for products. 

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. 

Buschmann [22] has reported an unusual titration method for betaines, which is more fully discussed in section 7.4. 

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Regarding titrants, cationic substances (for example, protonized alkaloids, compounds containing quaternary nitrogen, etc.) are usually titrated with sodium tetraphenylborate, the exact concentration of which is determined titrimetrically against a standard substance such as thallium(I) nitrate or pure copper(II) or nickel(II) salts in the presence of 1,10-phenanthroline. For titrations of anionics, substituted quaternary ammonium or pyridinium salts are applied. The procedures are simple and represent an ecologic alternative to so-called two-phase titrations. ... 

A number of authors have described the potentiometric titration with sodium tetraphenylborate (TPB), NaB(C6H5)4, of the pseudocationic... 

Single-phase titration with sodium tetraphenylborate... 

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. 

In another version of the two-phase titration, also suffering from interference by anionic surfactants, the cationic is first extracted from water into chloroform with excess methyl orange. The two phases are left in the separatory funnel and titrated with sodium tetraphenylborate solution, with shaking, until the yellow color is expelled from the organic phase (89). 

FIG. 1 Potentiometric titration with sodium tetraphenylborate of a mixture of a cationic (cetylpy-ridinium chloride) and an amphoteric (cocoamidopropylbetaine) surfactant, followed with a specially made PVC-membrane electrode. The first inflection is due to titration of the cationic and the second to titration of the amphoteric surfactant. (Reprinted with permission from Ref. 81. Copyright 1992 by Karl Hanser Verlag.)... 

In their applications of A.C. oscillopolarographic titration for pharmaceutical analysis, Huang et al. reported a method for the titration of procaine hydrochloride with sodium tetraphenylborate [62]. Procaine hydrochloride was mixed with sodium tetraphenylborate in acetate buffer (pH 4.6). The precipitate was filtered off, and the unconsumed tetraphenylborate titrated with thallium sulfate by A.C. oscillo-polarography. The recovery was found to be 99.9 to 100.0%, and the coefficient of variation (n = 10) was 0.19%. The method could also be used to identify outdated samples of procaine hydrochloride injection solution, as its loss of water solubility is indicated by an incision in the titration curve. 

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. 

After precipitation of the halogen ions with AgN03, (>otassium is precipitated with sodium tetraphenylborate as described. The excess of tetraphenylborate is then determined by AgNO 3 titration, using a silver electrode for endpoint detection. The method has been applied to seawater samples as small as 1 mL as well as for interstitial waters, with a precision of better than 1 %. The procedure saves the tedious filtration and drying process but offers a somewhat lower precision than the gravimetric method. A detailed description is presented by Marquis and Lebel (1981). 

Quaternary amines may be assayed by titration with an anionic surfactant by the potentio-metric or two-phase procedures described in Chapter 16. The ISO procedure specifies titration with sodium dodecyl sulfate, which is available in high purity (7). Sodium tetraphenylborate may also be used if potassium and ammonium ions are absent. Other approaches to assay are based on acid-base titration or quantification of the anion associated with the quaternary. 

Procedure Titration of Ethoxylated Surfactant with Sodium Tetraphenylborate (62)... 

In another version of this procedure the nonionic surfactant was first extracted batch-wise with sodium tetraphenylborate into 1,2-dichloroethane. The tetraphenylborate in the isolated organic phase was then titrated with a cationic surfactant, using Victoria Blue B as indicator (70). This titration can also be performed to an electrochemically detected end point. In this version, an excess of anionic surfactant is added to the cationic complex formed by the ethoxylated nonionic surfactant and potassium ion. The ion pair is extracted into dichloroethane, separated from the initial aqueous phase, then titrated with cationic surfactant in the presence of additional water. The ion pair of the anionic surfactant and Fe(II)(l,10-phenanthroline)3 is added as indicator. The end point of the titration is indicated when the last of the anionic surfactant is complexed by the cationic titrant, causing the iron-phenanthroline cation to migrate to the aqueous phase, where it is detected as a change in potential at a platinum electrode (71). 

Gallegos, R. D., Titrations of nonionic surfactants with sodium tetraphenylborate using the Orion surfactant electrode, A a/y.tr, 1993,118,1137-1141. 

A sensitive potentiometric surfactant sensor was prepared based on the highly lipophilic l,3-didecyl-2-methyl-imidazolium cation in the form of its tetraphe-nylborate associate. The sensor responded fast and showed a Nemstian response for the following surfactants under investigation CPC, CTAB, and hyamine with slope values of 59.8, 58.6, and 56.8 mV decade respectively. The sensor served as an end-point detector in ion-pair surfactant potentiometric titrations using sodium tetraphenylborate as titrant. Several technical grade cationic surfactants and a few commercial disinfectant products were also titrated, and the results were compared with those obtained from a two-phase standard titration method. The results, compared to those obtained using a commercial surfactant electrode with the standard two-phase titration method, exhibited satisfactory mutual agreement. 

A method was described for the determination of nonionic surfactants containing poly(oxyethylene) chains with sodium tetraphenylborate, based on the precipitatiOTi of ternary compounds in the presence of bivalent metal ions (barium salts). Titrations were monitored potentiometricaUy with a simple PVC membrane-coated aluminum wire electrode plasticized with 2,4-dinitrophenyloctyl ether. " ... 

A commercially available fluoroborate ion-selective indicator electrode was used for the potentiometric titration of surfactants and soaps.The titrants were HDTAC, hexadecylpyridinium chloride (HDPC), and diisobutylphenoxy-ethoxyethyI-(dimethyI)benzyIammonium chloride (DIPEBC). Similarly, Benoit et al. " titrated CTAB with sodium tetraphenylborate potentiometrically using a commercial CI04 selective electrode. The method proved to be precise exhibiting a relative standard deviation of 1.1 %. 

Vytras K (1977) Titration of organic cations with sodium tetraphenylborate indicated by K ion selective electrode Crytur. Collect Czech Chem C 42 3168-3174... 

Vytras K, Remes M, Kubesova-Svobodova H (1981) Coated-wire organic ion-selective electrodes in titrations based on ion-pair formation determination of arenediazonium salts with sodium tetraphenylborate. Anal Chim Acta 124 91-98... 

Vytfas K, Dvofakova V, Zeman I (1989) Titrations of non-ionic surfactants with sodium tetraphenylborate using simple potentiometric sensors. Analyst 114 1435-1441... 

Discussion. Potassium may be precipitated with excess of sodium tetraphenyl-borate solution as potassium tetraphenylborate. The excess of reagent is determined by titration with mercury(II) nitrate solution. The indicator consists of a mixture of iron(III) nitrate and dilute sodium thiocyanate solution. The end-point is revealed by the decolorisation of the iron(III)-thiocyanate complex due to the formation of the colourless mercury(II) thiocyanate. The reaction between mercury( II) nitrate and sodium tetraphenylborate under the experimental conditions used is not quite stoichiometric hence it is necessary to determine the volume in mL of Hg(N03)2 solution equivalent to 1 mL of a NaB(C6H5)4 solution. Halides must be absent. 

Pipette 25.0 mL of the potassium ion solution (about 10 mg K + ) into a 50 mL graduated flask, add 0.5 mL 1M nitric acid and mix. Introduce 20.0 mL of the sodium tetraphenylborate solution, dilute to the mark, mix, then pour the mixture into a 150mL flask provided with a ground stopper. Shake the stoppered flask for 5 minutes on a mechanical shaker to coagulate the precipitate, then filter most of the solution through a dry Whatman No. 40 filter paper into a dry beaker. Transfer 25.0 mL of the filtrate into a 250 mL conical flask and add 75 mL of water, 1.0 mL of iron(III) nitrate solution, and 1.0 mL of sodium thiocyanate solution. Titrate with the mercury(II) nitrate solution as described above. 

Marquis and Lebel [534] precipitated potassium from seawater or marine sediment pore water using sodium tetraphenylborate, after first removing halogen ions with silver nitrate. Excess tetraphenylborate was then determined by silver nitrate titration using a silver electrode for endpoint detection. The content of potassium in the sample was obtained from the difference between the amount of tetraphenyl boron measured and the amount initially added. 

Bulk chlorpromazine can be dissolved in 15 mL of H2O (or injectable solutions diluted to 15 mL), to which is added 10 mL of 2.5% aqueous acetic acid. This solution is titrated with 0.05M sodium tetraphenylborate reagent at a constant potential of 0.12V (dropping mercury electrode and external S.C.E.), and the polarogram recorded [150]. 

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. 

Various aromatic amines, phenols, and compounds containing active methylene groups can be titrated with arenediazonium salts, from which 4-bromo-l-naphthale-nediazonium chloride seems to be the most widely applicable titrant. Compounds that react slowly with arenediazonium salts can be determined by back-titration when the excess of arenediazonium salt is back-titrated with either sodium tetraphenylborate or 2,4-diaminotoluene. Indirect determination is useful for secondary amines, which react with arenediazonium ions to form triazenes. The determination of diazonium salts of ampholytic character is based on the reaction of these salts with l-phenyl-3-methyl-5-pyrazolone, the excess of which is titrated with 4-bromo-l-naphthalenediazonium chloride solution. 

Tsubouchi and Tanaka [3] described the following method, in which the sodium-nonionic tetraphenylborate is extracted into an organic solvent and titrated with a quaternary surfactant, which displaces the sodium-nonionic complex. Anionics do not interfere chemically, but cause emulsification problems and are better removed. Potassium, ammonium, calcium, chloride and sulphate ions do not interfere because the complex is extracted. The aqueous layer remains colourless throughout the titration. 

A sample containing 5-50 mg ethoxylate in about 90 mL water is mixed with 10 mL of 0.1 M barium chloride solution and titrated with 0.01 M aqueous sodium tetraphenylborate solution (prepared with 10 g polyvinyl alcohol and 10 mL 0.1 M NaOH per liter of titrant). The titration is made potentiometrically, using the Metrohm surfactant indicating electrode and a silver/silver chloride reference electrode (NaCl filling solution). The electrode must be rinsed free of precipitate after every few titrations using methanol. An electrode... 

A sample of 15 mL volume, containing 5 x 10 to 5 x 10 M of cationic surfactant, is placed in a 200-mL conical flask with 5 mL of phosphate buffer solution, 1 drop of indicator solution, and 0.5-1.5 mL 1,2-dichloroethane. Titrate with 5 x 10 M sodium tetraphenylborate solution from a 50-mL buret, shaking after each addition of titrant as the end point is reached. The 1,2-dichloroethane phase is initially blue, turns green in the vicinity of the end point, and becomes yellow when one drop of excess titrant is added. 

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. 

A mixture consisting of 25 mL of an aqueous (0.2 to 1.0 mM) solution of chlorpromazine hydrochloride and 5 mL of 0.1 M acetate buffer (pH 3.3) was titrated to a potentiometric endpoint with 0.0 IM sodium tetraphenyl-borate [77]. The titrant was added at a rate of 0.36 mL/min with continuous stirring, and the temperature of the medium was maintained at 22 2°C. The end point was detected by a tetraphenylborate-selective electrode. 

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