Potentiometric titration with benzethonium chloride

Construction of electrodes and experimental procedure were fully described in section 3.6. It is not recommended for fatty ester a-sulphonates or carboxylates. Whilst a-olefin sulphonates give good curves, it is uncertain which species is titrated. The method is satisfactory for many formulated products, but should be evaluated for individual applications. It is the method of choice when it is applicable. It is most conveniently done by autotitrator. 

Indicating electrode any of the electrode types described in section 3.6.2, and others, may be used. ASTM Method D 4251-89 [1] recommends the Orion 93-07 electrode, but an earlier version, D 4251-83, recommended the HNU ISE 20-31-00 nitrate electrode. Roth Scientific s RS 5000 electrode is also satisfactory. The Orion 93-05 fluoroborate electrode, which responds to cationics, can also be used. 

Reference electrode calomel or silver-silver chloride. 

Weigh a sample containing about 3g of anionic active. Dissolve in water, dilute to 250 ml and mix. 

Insert the electrodes and a magnetic stirrer bar or a propeller-type stirrer. Add enough water to ensure that the electrode tips are immersed. 

---

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. 

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. 

Soaps, sarcosinates and alkylether carboxylates (ethoxycarboxylates, polyethylene glycol monoesters) give poor curves in potentiometric titrations with benzethonium chloride, and ISO 2271 in alkaline solution does not always work very well for these compounds, particularly those of shorter chain length. The bromophenol blue method works well provided that the solution is distinctly alkaline it is necessary to add 5 ml 0.1 M sodium hydroxide before titrating. 

These materials can be titrated with benzethonium chloride, either by ISO 2271 or potentiometrically. 

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. 

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. 

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. 

B. Titration with benzethonium chloride in alkaline solution. The solution must be at least 0.1 M in hydroxide ion. Potentiometric titration is preferred, and the bromophenol blue method is probably better than the mixed indicator procedure. The present author s limited experience suggests that WW amphoterics are titrated rather more successfully as cationics than as anionics. 

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. 

Liu (2007) introduced another method called soap extraction to quantify acid number. Because the anionic surfactant can be accurately determined by potentiometric titration (see Appendix A in Liu, 2007) with benzethonium chloride (hyamine 1622), it is reasonable to use this method to find the natural soap amount. Because this potentiometric titration is for the aqueous phase, the soap should be extracted into the aqueous phase as the first step. As an anionic surfactant, the natural soap may stay in the oleic phase and form Winsor type 11 microemulsion when the electrolyte strength is high. To extract the soap into the aqueous phase, NaOH is used to keep the pH high with low electrolyte strength. Also, isopropyl alcohol is added to make the system hydrophilic so that soap will partition into the aqueous phase. 

Add to a 150-mL beaker a sample consisting of about 0.15 mmoles anionic surfactant in a volume of about 50 mL H2O. Titrate potentiometrically with benzethonium chloride solution at a rate of 0.5 mL/min until well past the inflection point of the curve. An automatic titrator with 5-mL buret assembly is used, equipped with a nitrate-selective electrode (HNU Systems model lSE-20-31-00, Orion model 93-07, or equivalent) and a Ag/AgCl reference electrode with ground-glass sleeve. 

Titrate potentiometrically with 0.04 M benzethonium chloride with continuous stirring. 

Titrate potentiometrically with 0.004 M benzethonium chloride to just past the end-point. Add 10 ml 2M sodium hydroxide and continue titrating with 0.004 M benzethonium chloride until the end-point is again reached. 

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. 

---