No. 233/3 e Application Bulletin - Metrohm

No. 233/3 eApplication BulletinOf interest to: General analytical laboratories A 1, 3, 4, 12Detergents, surfactants and cosmeticsPharmaceutical industryTitrimetric/potentiometric determination of anionicand cationic surfactantsSummarySurfactant analysis plays a major role worldwide. Whether in formulations or startingproducts – the content should be determined as quickly, precisely and accuratelyas possible. The «two-phase titration» according to Epton employs chloroform,a solvent that is now rightly taboo. Further, in some cases the equivalencepoint is difficult to detect and the titration cannot be automated. This method alsorequires a great deal of analytical experience.The surfactant ISE provides help here in many cases and also benefits the environment.It has been specially developed for surfactant determinations using potentiometricindication and has already been tested in thousands of titrations incombination with a sample changer.Anionic surfactants can be titrated with cationic surfactants and vice versa. ThisBulletin describes a wide range of substances that can be determined and lists therelevant working conditions and parameters. Moreover, a titrant for anionic surfactantsis introduced that provides larger and steeper potential jumps and hence improvesthe precision of the determinations when used in conjunction with the surfactantISE and the titrator.Theory behind the electrodeThe surfactant ISE electrode is a PVC liquid membrane electrode with a membranecomposition (ionophore/plasticizer) that has been specially optimized for thedetermination of ionic surfactants. The electrode potential is due to a specific interactionbetween the ion carrier incorporated in the PVC membrane and the analyteions (surfactants) in the analysis solution. This interaction leads in an equilibriumreaction to a potential crossing of the analyte ions from the analysis solution intothe membrane and – as a result of this – to the formation of an electrical potentialdifference at the phase boundary analysis solution/membrane, which can bemeasured at zero current (potentiometrically) against a reference electrode. Theextent of ion transfer from the analysis solution into the membrane depends on theconcentration. The relation between the analyte ion concentration and the electricalpotential is described by the Nernst equation:E = E 0 + s * lg a MIn the equation, «s» represents the electrode slope. In the ideal case it is ca. 59mV per concentration decade for monovalent anions and cations at 25 °C. In practice,lower slopes are frequently observed.

Application Bulletin No. 233/3 eDetermination of anionic and cationic surfactants Page 2Owing to a variety of circumstances (membrane composition, special properties ofthe surfactants such as boundary activity, substantivity [= tendency to absorb onsurfaces], formation of micelles), Nernstian behaviour cannot always be assumedfor the «High Sense» electrode. In practice, this means that• the electrode is not suitable for direct potentiometric concentration determinationsand• the titration should always be evaluated using the inflection point of the S-shaped titration curve. Equivalence point titrations are usually not to be recommended.Instruments and accessories• For example: Titrinos 702 or 716 or 736 or 751 or Titroprocessor 726, 727 TitrationStand, 722 Propeller Rod Stirrer as well as eventual a printer with appropriateprinter cable• Exchange unit 6.3014.223• Ionic surfactant electrode 6.0507.120 and reference electrode 6.0726.100 (KCl,c = 3 mol/L) with appropriate electrode cablesReagents• Buffer solution pH = 2.0 citrate/HCl, e.g. Merck Titrisol no. 109882• Buffer solution pH = 3.0 citrate/HCl, e.g. Merck Titrisol no. 109883• Buffer solution pH = 6.0 citrate/NaOH, e.g. Merck Titrisol no. 109886• Buffer solution pH = 7.0 phosphate, e.g. Metrohm no. 6.2305.020• Buffer solution pH = 10.0 boric acid /KCl/NaOH, e.g. Merck Titrisolno. 109890• Hydrochloric acid c(HCl) = 2 mol/L and c(HCl) = 0.1 mol/L as well as sodiumhydroxide c(NaOH) = 2 mol/L (to adjust to other pH values)• Methanol puriss p.a. and distilled or demineralized water• TitrantsGeneralThe selection of the titrant is very important. The lower the solubility of the compoundsformed (the greater their oleophilic properties), the larger and steeperthe potential jump of the titration curve. Advantage should be taken of this effectwhen formulations are titrated. (Additives can lead to considerable flattening ofthe potential jumps.)Work is preferably performed with titrants of concentration 0.004 mol/L; only inexceptional cases is a concentration of 0.01 mol/L used.The weight of titrant required is given by:Weight [in g] = M r * 0.004 (or * 0.01) to 1 literWeigh in the required amount (plus slight excess*) exactly and dissolve in dist.water, warming gently if necessary. Make up to 1 liter with dist. water at 20 °C.The titer is determined against an anionic or cationic surfactant. As long as thesolution in the reagent bottle and the Exchange Unit has not stabilised, the titeris also not stable (surfactants are substantive, i.e. they tend to adhere to surfaces;this applies particularly to cationic surfactants). Consequently, use alwaysthe same reagent bottles and Exchange Units and allow the solution tostand for 1 day before determining the titer.*) By their nature, the substances do not contain 100 % active substance. The major by-product isusually water. The water mass fraction can amount to up to 8 % and is very difficult to remove.It is essential to take these circumstances into account when weighing in the titrant.

Application Bulletin No. 233/3 eDetermination of anionic and cationic surfactants Page 3Titrants for anionic surfactantsa) TEGO ® trant A 100: 1,3-didecyl-2-methylimidazolium chloride (DDMICl);M r = 399.7 g/mol; Metrohm no. 6.2317.000 (6 g) or 6.2317.010 (60 g)b) Hexadecylpyridinium chloride (HDPCl), also called cetylpyridinium chloride(CPCl); M r = 358.01 g/mol; Fluka no. 52 349c) Hyamine ® 1622 (benzethonium chloride); M r = 448.10 g/mol; Merck no.112058Titrants for cationic surfactantsa) Sodium dodecylsulfate (sodium lauryl sulfate LAS); M r = 288.38 g/mol;Merck no. 112533 or Fluka no. 71 727; stabilized against bacterial attack by0.5 % formaldehyde.Note: Extensive investigations over a considerable period of time haveshown that the content of the Merck product is 99.2 % (rest: water, sodiumsulfate and lauryl alcohol).b) Dioctylsodium sulfosuccinate (DOS) (bis(2-ethylhexyl)-sodium sulfosuccinate);M r = 444.57 g/mol; Fluka no. 86 139Note: May saponify at higher pH values (shelf life still unknown)!c) For certain applications: sodium tetraphenylborate; M r = 342.23 g/mol;Merck no. 106669Note: Caution! Reacts with potassium and ammonium ions and with certainamines. Use sodium chloride (c(NaCl) = 1 mol/L) as bridge electrolyte(6.0726.100 reference electrode). Titer may not be stable.Preparation, maintenance and storage of the surfactant electrode• The electrode is stored dry.• The electrode is best conditioned by two to three titrations whose results shouldbe ignored.• Adherent deposits are removed with a soft paper towel, moistened in methanol.In sample changer operation, the electrodes are dipped briefly in methanol,while stirring.• The electrode is not resistant towards almost any organic solvents (PVC membrane).Chloroform, hydrocarbons, acetone, MIBK, tetrahydofuran, etc. destroythe electrode. High proportions of methanol (30...40 %) or ethanol (20 %) in thesolvent shorten the lifetime of the electrode.• Several thousand titrations can be performed with the electrode under normalconditions. Evidence of a decrease in the responding behaviour of the electrodeis shown in flatter titration curves and a shortened potential range. For a shorttime, such an electrode can be regenerated by submerging it for 30 min. in asodium lauryl sulfate solution (0.004 mol/L). If this does not help, the electrodemust be replaced.

Application Bulletin No. 233/3 eDetermination of anionic and cationic surfactants Page 4pH ranges for some detergentsAnionic surfactantsTitration recommendations for anionic surfactantsSulfatesSulfonatesCarboxylates (soaps)Sulfosuccinates(SO3)Sulfosuccinates(SO3 + COOH)0 1 2 3 4 5 6 7 8 9 10 11 12 13 14pH value of the titration solutionTitration not possibleTitration possibleRecommended for titrationTir• In the case of samples that contain both sulfosuccinates and betaines, the sulfosuccinatesare titrated at pH = 3.0.• For the determination of sulfosuccinates, the following pH values are selected:– sulfonate group at pH = 2.0– sulfonate and carboxylate group at pH = 10.0.Cationic surfactantsTitration recommendations for cationic surfactantsQuatsEster quatsFatty aminesFatty amines /Adducts of EO0 1 2 3 4 5 6 7 8 9 10 11 12 13 14pH value of the titration solutionTitration not possibleTitration possibleRecommended for titrationTir

Application Bulletin No. 233/3 eDetermination of anionic and cationic surfactants Page 5• For the determination of the degree of quarternization, an additional titration isperformed at pH = 3.0 to obtain the sum of the quaternary ammonium compoundsand the tertiary starting amine.• The ester quats contained in softeners are usually titrated at pH = 2.0. However,some ester quats exhibit their greatest stability at pH = 5 or pH = 7 andshould thus be determined at these pH values. Here, it should be noted that thepH has to be adjusted immediately following the sample weighing. Slightly alkalineconditions can very quickly lead to ester cleavage resulting in the ester losingits surfactant properties. This would lead to erroneous results.• Betaines and amphoteric surfactants cannot be titrated. However, when presentin the protonated form (at pH = 0...1), they can interfere with the determinationof other surfactants.AnalysisThe dynamic titration (DET) is the most suitable as it allows the fastest titrationsand offers the best reproducibilities. However, titrations can naturally also be performedin the MET or GET mode.An amount of sample that corresponds to a titrant consumption of at least 10 mL isweighed into a beaker and ca. 50 mL dist. water are added. Following addition of10 mL buffer solution and 5 mL methanol, the titration is started (examples in theappendix). In the case of concentrates and raw materials, for reasons of accuracya dilution should first be performed. In order to prevent formation of micelles, about10...50 % methanol are added. An aliquot of the initial dilution is then used for thetitration (take into account the methanol fraction of the initial dilution).Anionic surfactants• Anionic surfactants are normally titrated with a cationic surfactant at pH = 3.0.For special cases, see the corresponding graph.• True, soluble soaps (sodium or potassium salts of higher fatty acids) must be titratedwith TEGO ® trant A 100 at pH values > 10. Other cationic titrants producepoor titration curves that are often impossible to evaluate. In mixtures, anionicsurfactants and soaps are determined as a sum. With decreasing pH value, thefraction of the soaps determined by titration becomes increasingly smaller.Complete separation is achieved by acidifying the sample to pH = 2.0 (allow thesample to stand 13...30 min for the reaction to run to completion). In the titrationwith TEGO ® trant A 100 only the anionic surfactants are then determined. ThepH value for the sum titration depends on the sample used and must be determinedby preliminary titrations (pH = 10...13). Two potential jumps result withthe second being used to determine the sum of the soaps and anionic surfactants.Cationic surfactants• Cationic surfactants are normally titrated with an anionic surfactant at pH =10.0. For special cases, see the corresponding graph.• Samples that contain amine hydrochlorides are titrated at pH = 3.0 if this classof substances needs to be detected. On the other hand, if the content withoutamine hydrochlorides is required, work is performed at pH = 10.0.

Application Bulletin No. 233/3 eDetermination of anionic and cationic surfactants Page 6Calculation1 mL anionic surfactant 0.004 mol/L = 1.5988 mg DDMICl= 1.4320 mg HDPCl= 1.7924 mg Hyamine ® 1622= 0.0560 mg surfactant-N1 mL cationic surfactant 0.004 mol/L = 1.1535 mg sodium dodecylsulfate= 0.1280 mg surfactant-SComments• If the electrode is left in the titrated sample solution for too long, in the next titrationit will either not respond at all or wrongly. In such cases, rinse with methanoland place in 0.004 mol/L sodium dodecylsulfate solution for a few minutes.• It is always advisable to immerse the electrode in the sample solution for 20...40s before each titration to allow adjustment to the sample matrix.• You can extend the lifetime of the electrode by storing it «dry».• The titrant TEGO ® trant A 100 provides much steeper titration curves and largerpotential jumps than other titrants. These advantages are particularly apparentwith surfactants and soaps that produce only very weak potential jumps that aredifficult to evaluate, for instance with Hyamine ® 1622. In contrast to Hyamine ®1622, with TEGO ® trant A 100 lower detergent concentrations can thus be determined.However, exercise caution in this connection as the values may betoo low.• If sufficient sample is available, the sample weight should be chosen to producea titrant consumption of at least 10 mL. Only this guarantees determination ofthe total amount of surfactants. While lower sample weights give better titrationcurves, they sometimes lead to values that are too low.• Addition of methanol is absolutely necessary, particularly in finished formulations(e.g. shampoos, shower gels, softeners). In some cases it may even benecessary to increase the methanol content drastically in order to obtain preciseresults. Possible micelle and foam formation can be prevented in this way (seealso examples in the appendix).• The determination limit for ionic surfactants in waste waters, for example, lies atapprox. 3 - 5 mg/L. For this purpose the titrant must be diluted to 0.002 mol/L.Measure 100 mL of the sample solution and ajust its optimum pH value withNaOH or HCl. Do not add methanol!• This electrode is not suitable for fatty or oily products (e.g. drilling and cuttingoils, cooling lubricants, cleaning baths, special household cleaning agents andfurniture polishes, shower gels and bath salts). Here a Surfactrode must beused. Please see Application Bulletin No. 269.

Application Bulletin No. 233/3 eDetermination of anionic and cationic surfactants Page 7Practical examples1. Titration parametersTo achieve optimum evaluation of the titration curves, we recommend to work withthree different titrator settings: one for steep, one for intermediate and one for flattitration curves.Titration curveCommand steep intermediate flatMEAS 1 30 s * 30 s * 30 s *QUANTITY U U UDRIFT/min 30 mV 50 mV 50 mVM.DELAY 32 s 26 s 26 sDYNT 30 s 30 s 30 sMPT.DENSITY 5 3 2DOS.RATE/min 30 mL 30 mL 30 mLf.VRESOL 0.1 % 0.1 % 0.1 %N.EP’s 8 8 8VOLUME 20 mL 20 mL 20 mL*) The waiting time of 30 s is needed to allow the electrode to adjust to the titration solution.2. Analysis of raw materials2.1 Determination of anionic surfactants2.1.1 Fatty alcohol ether sulfates5 % methanol, 10 mL buffer solution pH = 3.0; minimum consumption of TEGO ® -trant 0.004 mol/L: 10 mL; parameters «intermediate titration curve».2.1.2 Fatty alcohol sulfates5 % methanol, 10 mL buffer solution pH = 3.0; minimum consumption of TEGO ® -trant 0.004 mol/L: 10 mL (if based on coconut fat: 12 mL); parameters «intermediatetitration curve».2.1.3 Sulfosuccinate monoesters5 % methanol, 10 mL buffer solution pH = 1.0 or pH = 2.0 if only the sulfonategroup needs to be determined, 10 mL buffer solution if the sulfonate and carboxylategroups need to be determined; minimum consumption of TEGO ® trant 0.004mol/L: 10 mL; parameters «intermediate» or «flat titration curve» (depending on thenumber of POE units); take into consideration the hydrolysis stability of the estergroup!2.1.4 Sulfosuccinate diesters5 % methanol, 10 mL buffer solution pH = 3.0; minimum consumption of TEGO ® -trant 0.004 mol/L: 10 mL; parameters «steep titration curve»; take into considerationthe hydrolysis stability of the ester group!2.1.5 α-olefin sulfonates5 % methanol, 10 mL buffer solution pH = 3.0; minimum consumption of TEGO ® -trant 0.004 mol/L: 8 mL; parameters «intermediate titration curve»; values may betoo high in comparison with the two-phase titration*.

Application Bulletin No. 233/3 eDetermination of anionic and cationic surfactants Page 82.1.6 Secondary alkanesulfonates5 % methanol, 10 mL buffer solution pH = 3.0; minimum consumption of TEGO ® -trant 0.004 mol/L: 10 mL; parameters «intermediate titration curve»; values may betoo high in comparison with the two-phase titration*.2.1.7 Linear alkylbenzene sulfonates5 % methanol, 10 mL buffer solution pH = 3.0; minimum consumption of TEGO ® -trant 0.004 mol/L: 10 mL; parameters «steep titration curve».2.1.8 Isethionates10 % methanol, 10 mL buffer solution pH = 3.0; minimum consumption of TEGO ® -trant 0.004 mol/L: 10 mL; parameters «intermediate titration curve».Dissolve coconut fat isethionates in 5 mL each of methanol and dist. water withgentle warming (allow to stand 3...5 min); then add 90 mL dist. water and 2 mLc(HCl) = 0.1 mol/L and titrate immediately.*) With the potentiometric titration, it is easily recognized that titration is performed according to decreasingoleophilic character. In particular, the differentiated titration curve indicates different inflectionpoints with several products (e.g. C-12 – C-10 – C-8). This could be one of the reasons whysomewhat higher values (up to 3 %) are found in comparison with the two-phase titration.2.2 Determination of cationic surfactants2.2.1 Benzalkonium halides10 % methanol, 10 mL buffer solution pH = 3.0 or pH = 10.0; minimum consumptionof sodium lauryl sulfate 0.004 mol/L: 6 mL, with bis(2-ethylhexyl)-sodium sulfosuccinate0.004 mol/L: 4 mL; parameters «intermediate titration curve»; take intoconsideration the substantivity of the cationic surfactants!2.2.2 Quaternary ammonium compounds10 % methanol, 10 mL buffer solution pH = 10.0; minimum consumption of sodiumlauryl sulfate 0.004 mol/L: 10 mL; parameters «intermediate titration curve»; takeinto consideration the substantivity of the cationic surfactants!3. Analysis of formulations3.1. Determination of anionic surfactants in formulations3.1.1 Rinse-off formulations without sulfosuccinatesWeigh in 100...300 mg sample, add 10 mL buffer solution pH = 5.0 and 40 mL dist.water and dissolve sample, add 5 mL methanol and titrate with TEGO ® trant 0.004mol/L; minimum consumption of titrant: 10 mL; depending on the formulation, parametersfor «intermediate» or «flat titration curve»; perform at least three determinations(turbidities caused by pearl lustre concentrates do not interfere).3.1.2 Rinse-off formulations with sulfosuccinatesWeigh in 100...300 mg sample, add 10 mL buffer solution pH = 3.0 and 40 mL dist.water and dissolve sample, add 5 mL methanol and titrate with TEGO ® trant 0.004mol/L; minimum consumption of titrant: 10 mL; depending on the formulation, parametersfor «intermediate» or «flat titration curve»; perform at least three determinations(turbidities caused by pearl lustre concentrates do not interfere).3.1.3 Mouth wash solutions on a lauryl sulfate basisTo 5...10 g sample add 10 mL buffer solution pH = 3.0 and 30 mL dist. water, thenadd 5 mL methanol and titrate with TEGO ® trant 0.004 mol/L; minimum consumptionof titrant: 10 mL; parameters «intermediate titration curve».3.1.4 Toothpastes on a lauryl sulfate basisTo 1...2 g sample add 10 mL buffer solution pH = 3.0 and 40 mL dist. water, thenhomogenize with ULTRA-TURRAX ® high frequency mixer; rinse ULTRA-TURRAX ®with 10 mL methanol and titrate sample solution with TEGO ® trant 0.004 mol/L;minimum consumption of titrant: 10 mL; parameters «intermediate titration curve».

Application Bulletin No. 233/3 eDetermination of anionic and cationic surfactants Page 93.1.5 Anionic surfactants (dioctyl sulfosuccinates) in glass/window cleaningagentTo 6...7 g sample add 10 mL buffer solution pH = 3.0 and 40 mL dist. water, thenadd 5 mL methanol and titrate with TEGO ® trant 0.004 mol/L; parameters «intermediatetitration curve».3.1.6 Anionic surfactants in WC cleaning agentTo 2.5...3.5 g sample add 10 mL buffer solution pH = 3.0 and 40 mL dist. water,then add 10 mL methanol and titrate with TEGO ® trant 0.004 mol/L; minimum consumptionof titrant: 10 mL; parameters «steep titration curve».3.1.7 Anionic surfactants in dishwashing concentrateTo ca. 3 g sample add 10 mL buffer solution pH = 5.0 and 40 mL dist. water, thenadd 10 mL methanol and titrate with TEGO ® trant 0.004 mol/L; minimum consumptionof titrant: 10 mL; parameters «steep titration curve».3.1.8 Anionic surfactants and soaps in all-purpose cleaning agentsTo ca. 3 g sample add 10 mL buffer solution pH = 5.0 (for the determination of theanionic surfactants) or 10 mL buffer solution pH = 10.0 (for the determination of theanionic surfactants and soaps) and 40 mL dist. water; then add 10 mL methanoland titrate with TEGO ® trant 0.004 mol/L; minimum consumption of titrant:8 mL; parameters «intermediate» to «flat titration curve».3.1.9 Sum of the anionic surfactants in a shampoo and shower gelWeigh ca. 200 mg sample exactly into a beaker and dissolve in 45 mL dist. waterand 3 mL methanol; add 5 mL buffer solution pH = 3.0 and titrate with TEGO ® trant0.004 mol/L; parameters «intermediate» (shower gel) to «steep titration curve»(shampoo); determination performed on the Titroprocessor.Calculation:mmol/100 g = E1 * C1 * CM * D1 * C5 / S0 ; 2; mmol/100 g% surfactant-S = E1 * C1 * CM * C2 * D1 / S0 ; 2; %E1 = mL TEGO ® trant consumed to reach the first EPC1 = 1000C2 = 3.2064 (M r (S) / 10)C5 = 100CM = 1.013 (titer)D1 = 0.004 (mol/L TEGO ® trant)S0 = sample weight in mg

Application Bulletin No. 233/3 eDetermination of anionic and cationic surfactants Page 10Titration curve for the determination of theanionic surfactants in a shampoo.Titration curve for the determination of theanionic surfactants in a shower gel.Results:The shampoo contains 29.41 mmol/100 g anionic surfactants and 0.94 % surfactant-S.The shower gel contains 31.88 mmol/100 g anionic surfactants and 1.02 % surfactant-S.3.2 Determination of cationic surfactants in formulations3.2.1 Hair conditionerTo 0.5...2 g sample add 10 mL buffer solution pH = 2.0 to pH = 10.0 (test andmatch to the particular formulation) and 40 mL dist. water and homogenize withULTRA-TURRAX ® ; rinse ULTRA-TURRAX ® with 10 mL methanol and titrate samplesolution with sodium lauryl sulfate 0.004 mol/L; minimum consumption of titrant:10 mL; parameters «flat titration curve»; take into consideration the substantivity ofthe cationic surfactants!3.2.2 Gargle solution on a benzalkonium basisTo 100...300 mg sample add 10 mL buffer solution pH = 3.0 or pH = 10.0 and 40mL dist. water, then add 10 mL methanol and titrate with bis(2-ethylhexyl)-sodiumsulfosuccinate 0.004 mol/L; minimum consumption of titrant: 10 mL; parameters«intermediate titration curve»; take into consideration the substantivity of the cationicsurfactants!3.2.3 Mouth/dental wash solutions on an amino fluoride basisTo 5...10 g sample add 10 mL buffer solution pH = 3.0 and 30 mL dist. water, thenadd 10 mL methanol and titrate with bis(2-ethylhexyl)-sodium sulfosuccinate 0.004mol/L; minimum consumption of titrant: 10 mL; parameters «flat titration curve»;take into consideration the substantivity of the cationic surfactants!

Application Bulletin No. 233/3 eDetermination of anionic and cationic surfactants Page 13• D. C. Cullum,Introduction to Surfactant Analysis,Blackie Academic & Professional, London, 1994, ISBN 0-7514-0025-4.• ASTM D 4251-89,Standard Test Method for Active Matter in Anionic Surfactants by PotentiometricTitration.• ASTM D 5070-90,Standard Test Method for Synthetic Quaternary Ammonium Salt in Fabric Softenersby Potentiometric Titration.• R. Schulz,Th. Goldschmidt AG, Zentralbereich Forschung/Analytik, Goldschmidtstrasse100, D-45127 Essen, Fax +49 201 173 19 97,Numerous personal communications regarding his work and lecture material.• R. Schulz, P. BruttelBestimmung ionischer Tenside in MundpflegeproduktenSÖFW-Journal 124/3, (1998) 138-146.• R. Schulz, P. BruttelAnalytik ionischer Tenside in HaarpflegeproduktenSÖFW-Journal 125/2, (1999) ........