Two-phase titration with benzethonium chloride

This is the most widely used method for total anion-active content, and was fully described in section 3.5. It can be used for raw materials, most formulated products, and without extraction for fractions from ion-exchange separations. 

Weigh a sample containing about 1.5 g of anionic active. Dissolve in water, dilute to 11 and mix. 

Pipette 20 or 25 ml into a 100 ml stoppered measuring cylinder, a stoppered fla k or the mechanical apparatus described in ISO 2271. 

Add 10 ml water, 15 ml chloroform and 10 ml acid mixed indicator (disulphine blue VN and dimidium bromide). 

Titrate with 0.004 M benzethonium chloride with thorough shaking or stirring after each addition until the pink colour is just discharged from the chloroform layer. If the chloroform layer becomes blue the end-point has been overshot. 

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When performing the two-phase titration with benzethonium chloride in alkaline medium and in the presence of phenolphthalein as indicator, not only the monosulfonates but also the di- and polysulfonates are determined [19]. The content of di- and polysulfonates is the difference between two titration results giving the amounts of total sulfonates and monosulfonates. Identifying the endpoint requires some experience. 

The ester cannot be determined by saponification because this slowly removes the sulphonate group as well as hydrolysing the ester group. It can be determined by acid hydrolysis followed by measurement of the a-sulphonated carboxylate salt produced, either by potentiometric titration of the weak acid or by two-phase titration with benzethonium chloride. Both procedures measure the a-sulphonated ester plus the unsulphonated ester. If the latter is present at a significant level, it can be determined (section 5.11.2) and corrected for. 

Mono- and dialkylsulphosuccinate may be determined by two-phase titration with benzethonium chloride. Materials made by the sodium... 

Fatty acids and acyl sarcosines may be extracted with petroleum or diethyl ether from an acidified aqueous solution if no other surfactants are present, or with petroleum ether from acidified 50% ethanol if they are. The extract is evaporated and the residue weighed. Alkylether carboxylic acids cannot be quantitatively extracted with petroleum ether. They can be extracted from aqueous solution with chloroform, but they are best determined by two-phase titration with benzethonium chloride in akaline solution (bromophenol blue method) or direct potentiometric acid-base titration. 

The free fatty acid can be directly determined by two-phase titration with benzethonium chloride, in which case the presence of basic catalyst is irrelevant. Bares and Zajic [17] used this approach, with bromocresol green as indicator, and taking as the end-point the final disappearance of colour from the aqueous layer. A blank test was necessary. They showed that quantitative titration only occurred when the pH was at least 11.3. The bromophenol blue method would serve equally well. 

Assay is conventionally performed by two-phase or one-phase titration with benzethonium chloride, as described in Chapter 16. Assay may also be performed by determination of... 

These materials are readily determined by two-phase titration (ISO 2271) or potentiometric titration with benzethonium chloride. On acid hydrolysis they yield the corresponding alcohol, a sulphate ion and a hydrogen ion, and this affords three additional approaches—determination of the increase in acidity, of the amount of fatty alcohol or ethoxylated alcohol liberated, and of the sulphate ion. The experimental procedure may be varied within limits for example ISO 2870 [5] and ASTM D 1570-89 [6] differ with respect to the identity of the acid used, duration of boiling, choice of indicator and other details. The following procedures are similar to both of those standard methods. The choice of indicator is immaterial for these particular compounds, but is of crucial importance in some other cases. 

Titration with benzethonium chloride in acid solution measures the sulphonated ester plus the sulphonated carboxylic acid, only the sulphon-ate group of the latter being titrated. In alkaline solution the titration measures the sulphonated ester plus twice the sulphonated carboxylate, both the sulphonate and the carboxylate group being titrated. Alpha-sulphonated esters give poor potentiometric titration curves, and two-phase titration is strongly preferred. 

All may be determined by two-phase or potentiometric titration with benzethonium chloride. Titration of sulphosuccinamates in acid solution measures the sulphonate group alone (but see section 5.13.3), and titration in alkaline medium measures the carboxylate group as well (cf. section 5.11.3). Sulphosuccinamates, possessing a carboxylate group, can also be determined by potentiometric acid-base titration. 

SW sulphobetaines are more like anionics than cationics, and can be titrated with benzethonium chloride in alkaline solution, by either two-phase or potentiometric procedures. They cannot be titrated with NaTPB. 

A modification of two-phase titration calls for preparing only one titration system for both cationics and anionics the benzethonium chloride, disulfine blue/dimidium bromide method which is standard for anionics. For the determination of cationics, an excess of standard sodium dodecylsulfate is added, and the excess determined by titration with benzethonium chloride (31). 

The two-phase titration is based on the reaction of anionic surfactants with cations—normally large cationic surfactants—to form an ion pair. The preferred cationic is benzethonium chloride (Hyamine 1622, 1) because of the purity of the commercially available product. On neutralization of the ionic charges, the ion pair has nonpolar character and can be extracted continuously into the organic phase, e.g., chloroform, as it is formed. The reaction is monitored by addition of a water-soluble cationic dye, dimidium bromide (2), and a water-soluble anionic dye, disulfine blue (3). The cationic dye forms an extractable... 

If desired the sulphatosulphonate can be acid-hydrolysed in the same way as an alkyl sulphate and determined by two-phase titration of the resulting hydroxyalkane sulphonate with benzethonium chloride. Any true disulphonates present are not measured. 

Mixtures of amphoterics are not likely to be encountered, but a basic approach is outlined which will succeed in many cases. Acid-base titration is unlikely to be useful because of the presence of other weak acids and bases. The procedure therefore relies on titration with SDS and/or benzethonium chloride (BEC) and will not always be completely successful. In all cases, if two-phase titration fails, potentiometric titration is still likely to succeed. Acid solutions must be at least 0.1 M in H and alkaline solutions at least 0.1 M in OH . It may be possible to replace SDS with NaTPB in at least some cases. Sulphobetaines cannot be determined in the presence of other amphoterics without separation. 

An elegant form of the two-phase titration method for the determination of an anionic surfactant is the following. Benzethonium chloride (Hyamin 1622, Rohm and Haas) is used as a cationic surfactant, while a mixture of diimidium bromide (cationic) and disulfin blue VN (anionic) is used as the indicator. The anionic surfactant forms a pink complex with the cationic diimidium bromide, which dissolves in the chloroform. Upon titration with Hyamin 1622, a stronger complex is formed with the anionic surfactant. Hyamin 1622 thus displaces the diimidium bromide and the organic phase is decolorized, while the aqueous phase turns red. Excess Hyamin 1622 forms a blue salt with the anionic disulfin blue VN, which then dissolves in the organic phase. 

Active agent content of anionics is usually determined by titration with a cationic surfactant such as benzethonium chloride. Two-phase titration is suitable for assay of alkylaryl sulfonates alkyl sulfates and hydroxysulfates, alkanesulfonates, fatty alcohol ethoxysul-fates and alkyIphenolethoxysulfates and dialkyIsulfosuccinates (4,5). The results on ma-... 

The total of a-sulfo fatty acid methyl ester and a-sulfo fatty acid can be determined by the standard two-phase titration method (Chapter 16) with benzethonium chloride titrant and either methylene blue or mixed indicator. The titration is carried out at acid pH so that the carboxylic acid groups of unesterified material and free fatty acid are not measured (120,121). 

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... 

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