Amine oxides titration

Although consumption of the hydroperoxide is normally complete, the absence of this peroxide in the reaction mixture should be established by testing with moist starch-iodide paper or by iodometric titration.2 The amine oxide content may be determined by titration with standard aqueous hydrochloric acid after any amine present has been consumed by reaction with methyl iodide for 1 hour at room temperature.3 From this volumetric analysis the submitters determined the yield of amine oxide to be 86%. The checkers found that the reaction could be followed by measuring the n.m.r. spectra in -butyl alcohol solution where the n.m.r. N-methyl signals of the amine (at S 2.03) and the amine oxide (at S 2.98) are readily observed. 

The first three tertiary amines in the aliphatic series were studied by Strecker and Baltes. The authors studied tri-n-butylamine, tri-n-hexylamine, and tri-n-heptyl-amine. Each amine was ozonated at dry ice temperature and treated with picric acid. The tri-n-butylamine oxide picrate readily deposited as crystals on standing. Although the latter two amine oxides formed oily products at first, on long standing in the refrigerator they gave crystals of the amine oxide picrates. The purified amine oxide picrates were analyzed by direct titration with perchloric acid. 

Analysis. The amine oxide picrates were determined according to the procedure of Clark and Wang. A sample of 0.2 to 0.4 meq. of the picrate was dissolved in 10 ml. of glacial acetic acid. About 3 drops of methyl violet indicator were used. The color change from deep to light blue was used as the end point. The solutions were titrated with standard O.IV perchloric acid in glacial acetic acid with a microburet. 

One of the earliest separations in gas liquid chromatography was that of James et al. who used a mixture of hendecanol and liquid paraffin on celite using ammonia and the methyl amines as eluents in the order of their melting points. Other stationary phases used for this and for other similar separations include triethanolamine, a mixture of w-octadecane and n-hendecanol, and polyethylene oxide. Titration cell, the first detector designed specifically for gas chromatography, was used in these early studies of the separation of ammonia and ethylamines. More recently thermal conductivity cells have been used for the detection of these compounds. 

Analytical methods include thin-layer chromatography (69), gas chromatography (70), and specific methods for determining amine oxides in detergents (71) and foods (72). Nuclear magnetic resonance (73—75) and mass spectrometry (76) have also been used. A frequendy used procedure for industrial amine oxides (77) involves titration with hydrochloric acid before and after conversion of the amine to the quaternary ammonium salt by reaction with methyl iodide. A simple, rapid quaUty control procedure has been developed for the determination of amine oxide and unreacted tertiary amine (78). 

Weak bases (amines and amine oxides) must be titrated in a medium containing not less than 0.01 mol/1 and betaines in a medium containing not less than 0.1 mol/1 H. Amphoterics must be titrated in a medium containing not less than 0.01 mol/1 (cationic function) or not less than 0.01 mol/1 OH (anionic function). These conditions are met by adding 10 ml 1.0 mol/1 hydrochloric acid to solutions of betaines and 10 ml 0.1 mol/1 hydrochloric acid or sodium hydroxide, as appropriate, in the other cases. 

As in previous chapters, this chapter deals with the analysis of cationics and amphoterics either alone, as raw materials or as fractions isolated by ion exchange or otherwise, or in formulated products. Fractions isolated by ion exchange are likely to contain other materials, analogously with anionics. Amines, ethoxylated amines and amine oxides are included in this chapter because they are bases and capable of a cationic function. They are retained as cations by ion-exchange columns and do not appear in the nonionic fraction of separated mixtures, they can be titrated with acids and, in acid solution, with sodium dodecyl sulphate, provided the ethylene oxide chains of ethoxylates are not too long. 

Note 1. At pH 3.0, all quats and species with weakly basic nitrogen are titrated. The latter category includes amines, and although Cross did not study them, ethoxylated amines and amine oxides. 

The determination of amine oxides is straightforward they can be titrated in acid solution with sodium dodecyl sulphate or NaTPB, either poten-tiometrically or in a two-phase system, for which purposes the solution must be at least 0.1 M in hydrogen ion, and they can be titrated with acid. However, for quality-control purposes it is necessary to determine the free tertiary amine content. This can be done by potentiometric titration, either in a solvent which permits discrimination on the basis of base strength, or before and after removal of the amine oxide. Methods are described for both of these. 

In the first method [31] the sample is titrated with hydrochloric acid in 50% propan-2-ol. In water or propan-2-ol alone only one point of inflection is observed, but in the mixture the tertiary amine is the stronger base and is titrated first, well separated from the amine oxide. 

The titration curve shows three points of inflection. The first represents the titration of the sodium hydroxide, the second the titration of the tertiary amine and the third the titration of the amine oxide. 

Mixtures of weak bases with WS amphoterics (carboxybetaines) can be analysed by the method described in section 7.2.4 [2]. The betaine is determined by potentiometric titration of an initially acid solution in methyl isobutyl ketone with alkali, and the weak base by potentiometric titration of an initially alkaline solution in 50% aqueous propan-2-ol with acid. This procedure distinguishes between tertiary amines and amine oxides (section 7.8). 

Suppose we consider some very weakly basic compounds, such as ketimines, phosphines, and oxiranes. A very interesting method of dealing with oxiranes was developed by Durbetaki. The oxirane was reacted with hydro-bromic acid to form the bromohydrln. This type of reaction has long been known using hydrochloric acid, but in that medium the reaction takes approximately three hours. In glacial acetic acid, the reaction is enough to allow you to titrate directly at normal speed. You can get an end point potentiometrically or with an indicator. In fact, if you have a mixture of amine and oxirane, you can get two potentlometric breaks, the first for the amine and the second for the oxirane. Amides, phosphene oxides, triphenyl methanol, and amine oxides are very weak bases and cannot be titrated in glacial acetic acid under ordinary conditions. However, they can be titrated if one uses acetic anhydride as solvent, or if one uses a solvent that is mixed with acetic anhydride. Why does acetic anhydride work There are two reasons. First, it removes the last trace of water from the solution secondly, perchloric acid in the presence of acetic anhydride forms the ion CHsCO". Since this is an extremely reactive substance, one can titrate very weak bases. 

Titration in acetic acid, with and without chemical reaction (148) The total amine oxide and tertiary amine content of the product may be determined by direct potentiomet-ric titration with perchloric acid in acetic acid solvent. Another portion of the sample is reacted for 10 min with acetic anhydride, converting tertiary amine oxide to an amide and an aldehyde. The reaction product is titrated with perchloric acid in 2 1 acetic acid/acetic anhydride. Under these conditions, only tertiary amine is titrated. 

Stepwise titration in acetonitrile or MEK If the titration of an amine oxide is followed potentiometrically, two inflections are observed in certain solvents. The first corresponds to the titration of one-half of the equivalents of amine oxide the second is the sum of the remainder of the amine oxide plus any umeacted free amine. This procedme is limited to amine oxides containing an A-methyl group, since only these will give the double inflection (149). 

The titration is conducted on an automatic recording titrator equipped with a glass indicator electrode and platinum reference. A sample containing about 1.5 meq amine oxide is dissolved in 60 mL acetonitrile or methyl ethyl ketone and titrated with 0.1 M HCIO4 in acetonitrile. The amine oxide is calculated as corresponding to twice the milliequivalents determined at the first inflection the free amine corresponds to the difference between the first and second inflection, less the milliliters to reach the first inflection. 

Titration in isopropanol, with and without chemical reaction-. In this solvent, the two compounds give a single inflection point (150,151). The procedure consists of using isopropanol as solvent and alcoholic HCl as titrant. In one determination, methyl iodide is added to the mixture to react with the amine only the amine oxide is titrated. By making a second titration without added methyl iodide, the sum of amine plus amine oxide can be determined. 

Stepwise titration in isopropanollwater In 50 50 isopropanol/water, two inflections are seen in the potentiometric titration with HCl, corresponding to the titration of the free amine and of the amine oxide, respectively. Free hydroxide, if present, will appear as an inflection prior to the others. 

Spike Solution Mix 250 mL 2-propanol, 250 mL water, 400 mg NaOH, and 2 g tri-n-butylamine. Protect from atmospheric CO2. Weigh out an aliquot of sample containing about 1 meq amine oxide and dissolve in 100 mL 50 50 2-propanol/water (carbonate-free). Add 10.00 mL spike solution. Titrate potentiometrically with 0.1 M HCl, dissolved in water or 2-propanoI. There will be three inflection points. Titrate a 10-mL aliquot of the spike solution separately. There will be two inflection points. Calculation ... 

Discussion Weak acids may be present in the sample which partially neutralize the amine. This interference is avoided by spiking with NaOH to convert the amine salt to the free amine. The anion from the acid will interfere with the third break of the titration. This interference is equivalent to the OH" consumed by the acid, and thus can be corrected by calculation. Commercial amine oxide samples contain only low levels of free amine, giving an inflection which is difficult to measure quantitatively. To produce a better titration curve, the titration vessel is spiked with a known quantity of amine. 

Titration with an anionic surfactant At low pH, long-chain amine oxides and the free amines can be determined by titration with an anionic surfactant or tetraphenylborate, using either the two-phase indicator method or the single-phase ion selective electrode method, as described in Chapter 16. The free amine can be separated from the aqueous sample solution by extraction at pH 10.5-12.0 with petroleum ether. The amine content is then determined by stripping off the petroleum ether, dissolving the residue in water, and titrating at pH 2 with an anionic. The amine oxide content is determined by difference (153). 

Wang, C. N., L. D. Metcalfe, Unreacted amines in long chain amine oxides by potentiometric titration, J. Am. Oil Chem. Soc., 1985,62,558-560. 

Metcalfe, L. D., Potentiometric titration of long chain amine oxides using alkyl halide to remove tertiary amine interference. Anal. Chem., 1962,34,1849. 

Walton, B., Potentiometric titration of long chain tertiary amine oxides and tertiary amines using a poly(vinyl chloride) membrane electrode. Analyst, 1994, /19,2202-2203. 

Long chain amine oxides are sufficiently basic that they may be titrated directly with HCl in isopropanol. Any residual tertiary amine will also be titrated under these conditions, but modern potentiometric titration apparatus allows differentiation of the amine and amine oxide in a single determination (Chapter 2). Alternatively, reaction with methyl iodide to form the quaternary amine from the tertiary amine will eliminate interference (78). Amine oxides which cannot be determined in isopropanol can generally be titrated potentiometri-cally with perchloric acid in acetic acid/acetic anhydride solvent. Acetic acid alone is not a suitable solvent for this titration (79,80). 

Amine oxides have cationic properties at low pH and so can be titrated with anionic reagents according to the procedures developed for cationic surfactants. Titrations with tetraphenylborate (81) and with dodecylsulfate (82) have been demonstrated, using a potentiometric end point. A turbidimetric end point has also been demonstrated (28). An am-perometric end point may be used in a two-phase system with dodecylsulfate titrant, if a suitable cationic indicator is added such as the iron(II) 1,10-phenanthroline complex (83). Long-chain amines, including any unreacted amine from synthesis of the amine oxide, will quantitatively interfere. 

A two-phase titration method for determining amine oxides in the presence of anionic surfactants calls for titration of the anionics first with benzethonium chloride at pH 9.5 using bromcresol green as indicator. A second aliquot of the sample is analyzed by adding exactly the amount of benzethonium chloride needed to complex the anionic surfactant. The amine oxide, with the ion pair of the anionic surfactant, is then extracted into chloroform at pH 9.5. The chloroform solution is mixed with water and titrated with alkyl-benzene sulfonate solution at pH 2 using methylene blue indicator (17). Another approach... 

Amine oxides can also be determined by redox titration. The compounds are reduced with stannous chloride to the corresponding amines, and the excess stannous chloride is determined by titration with ferric ammonium sulfate. Potassium indigo sulfate is a suitable indicator (84). Interferences are abundant, including atmospheric oxygen. 

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