Titration with sodium dodecyl sulphate

Both types of betaine can be titrated in a two-phase system by the method of ISO 2871-2, but the concentration of hydrogen ion in the solution must be at least 0.1 M. Even then the end-point is not very well defined. 

Rosen et aL [17] developed a variant of this procedure in which a small amount of ethanol was added to the solution. The method had to be calibrated for each individual type of betaine, by titrating pure material and adjusting the volume of ethanol until a result of 100% was obtained. Using 0.001 M sodium dodecanesulphonate as titrant they successfully titrated the two main types, 2-pyridinium fatty acids of different chain lengths, A -dodecyl-A -benzyl-A -methylglycine and nine commercial betaines. 

Potentiometric titration is much more satisfactory, using any of the electrodes described in section 3.6.2. Sample solutions and titrant can be up to 0.04 M. The solution must be at least 0.1 M in hydrogen ion. 

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The product is a tertiary base, which is cationic in acid solution and can still be titrated with sodium dodecyl sulphate. 

The method has become the international standard method, and it is described in ISO 2271 1989 [7], ISO 2871-1 1988 [8] and ISO 2871-2 1990 [9]. These methods are almost identical. ISO 2271 describes the titration of an anionic analyte with a standard solution of a cationic. ISO 2871-1 describes the determination of cationics with two alkyl chains by titration of a standard solution of sodium dodecyl sulphate with a solution of the cationic analyte. The cationic is dissolved in 20 ml of propan-2-ol and diluted to 11 with water to give an approximately 0.003 M solution. ISO 2871-2 describes the determination of cationics with one alkyl chain by titration with sodium dodecyl sulphate, i.e. the exact converse of 2271. The titration can be done in either direction without difficulty in most cases. 

This is an alternative to the procedure given in section 5.12.2. The mono-and dialkylsulphosuccinates are sorbed on a strongly basic anion exchanger chloride. The eluate contains the aspartate and free fatty alcohols. If desired, the fatty alcohols can be removed by extraction with petroleum ether from acid solution. The aspartate is determined by titration with sodium dodecyl sulphate. 

The methods described in this section and the next give a more or less complete analysis of the sample. The extractions in steps 1 and 2 can be done in the reverse order, in which case the first aqueous extract will contain the free ethanolamine plus the free fatty acid as soap, separation and determination of which is straightforward. The ester amine is determined by two-phase titration with sodium dodecyl sulphate. 

Two-phase titration with sodium dodecyl sulphate... 

This section includes methods for determining the total, primary, secondary and tertiary amine contents of fatty amines which are nominally primary, secondary or tertiary or mixtures. For determination, another approach is to titrate with sodium dodecyl sulphate. Either two-phase or potentiometric titration may be used. The solution must be distinctly acid, pH 3 or lower. Two-phase or potentiometric titration with NaTPB is also applicable. Procedures for all of these are given in section 7.1. The difficulty with these approaches is in deciding what value to use for the molecular weight. 

Ethoxylated amines with short ethylene oxide chains can be titrated with sodium dodecyl sulphate and NaTPB in acid solution, but the titrations become increasingly unsatisfactory as the chain length increases. The same is no doubt true of ethoxylated diamines. Commercial ethoxylated amines have typically 12-60 EO units, and the diamines have up to 25. 

Total active is determined by two-phase or potentiometric titration with sodium dodecyl sulphate (SDS), or with NaTPB at pH 3. This is the mandatory first step in all 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. 

Quats may be titrated potentiometrically with sodium dodecyl sulphate. The procedure is straightforward, being the converse of that described for anionics in section 5.2.2, and end-points are mostly sharp. Quat solutions and titrant may be up to 0.04 M, solubility permitting. Any of the electrode types described in section 3.6.2 is suitable, as are many others, with calomel or silver-silver chloride as reference. 

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. 

These colour migrations are reversed when a cationic surfactant is titrated with an anionic. The titrant for this variation is 0.004 M sodium dodecyl sulphate. 

Titrate with 0.004 M sodium dodecyl sulphate using acid mixed indicator (cf. ISO 2871-2). 

Sokolov et al. [26] have reported successful analysis of a mixture of pure sodium dodecyl sulphate and pure sodium dodecyl phosphate (mono- or diester not specified) by this method, and Ivanov et al. [27] analysed alkyl phosphates by titration of the column effluent with alkali to bromocresol green. 

Pipette 10 ml 0.004 M sodium dodecyl sulphate solution into a stoppered flask or cylinder or the mechanical apparatus shown in Figure 3.2. Add 10 ml water, 15 ml chloroform and 10 ml acid mixed indicator and titrate with the solution prepared at step 2, with thorough shaking or stirring after each addition. The chloroform is initially pink, becoming colourless or greyish at the end-point. A blue colour indicates that the end-point has been overshot. 

True sulphobetaines, with quaternary nitrogen, cannot be titrated with either sodium dodecyl sulphate or benzethonium chloride. They do not interact with ion exchangers, and so appear in the nonionic fraction of mixtures. They cannot be titrated with NaTPB by any of the procedures described earlier, but Buschmann [22] has reported successful titration of sulphobetaines by a technique which he calls electrochemically indicated amphimetry . 

One would expect diamines to be titratable with both sodium dodecyl sulphate and NaTPB in acid solution, requiring two molecules per molecule, but the present author has no experience of such titrations and is not aware of any reports of them in the literature. 

Figure 11.33 Ionization of p-nitrophenol in the presence of sodium dodecyl sulphate. The fractional ionization, a, was calculated from the absorbance of the p-nitrophenolate ion at 400 nm. The total p-nitrophenol concentration was 1 x 10 moll The solvent was 0.004 m sodium phosphate buffer, or 0.004 m glycylglycine buffer for the higher pH values, adjusted to the pH indicated and to a constant ionic strength of 0.1 m with NaCl. The solid lines are standard titration curves for the dissociation of a monobasic acid. The NaLS concentrations in g ml" were a, 0 b, 0.0144 c, 0.0288 d, 0.0576. From Herries et al [218].

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