Determination by two-phase titration

Ethoxylates with six or more ethylene oxide units form pseudocationic complexes with many metals, and the tetraphenylborates of these are insoluble or sparingly soluble in water. Several volumetric methods for the determination of ethoxylates are based on this. Some of them are two-phase procedures. 

Tsubouchi and Tanaka [3] described the following method, in which the sodium-nonionic tetraphenylborate is extracted into an organic solvent and titrated with a quaternary surfactant, which displaces the sodium-nonionic complex. Anionics do not interfere chemically, but cause emulsification problems and are better removed. Potassium, ammonium, calcium, chloride and sulphate ions do not interfere because the complex is extracted. The aqueous layer remains colourless throughout the titration. 

The buffer solution used at step 4 is made from a solution containing 

mol/l sodium dihydrogen phosphate and 0.05 mol/1 sodium borate by adding 6 M sodium hydroxide until the pH is 9.0, although the actual pH is not critical between pH 8.5 and 9.5. 

Place 10 ml of a solution of the sample containing 0.2-2.0 mg nonionic and less than 10 mol/1 anionic surfactant in a separating funnel. 

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Table IV. Partition Coefficients of Ion Pairs Determined by Two-Phase Titrations ...

The two-phase titration can be also successfully used for the determination of cationic surfactants using standardized solutions of anionic surfactants. In this case the mechanism of titration is the reverse, i.e. the chloroform layer undergoes color change from blue to pink. More often, and especially at higher concentrations, cationic surfactants are determined by two-phase titration using sodium tetraphenylborate titrant and bromophenol blue indicator [32], Tetraphenylborate ion is also used in the determination of potassium, rubidium and cesium, so the ions of these metals, if present, interfere with the determination of quaternary ammonium surfactants by this method. 

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. 

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. 

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. 

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. 

Adsorption measurements The adsorption of various cationic surfactants was determined in aqueous suspensions and the specific excess amount was calculated using the relation nf" = V° JS.c/m, where V° is the volume of the liquid phase, m is the mass of the solid, Ac is the difference between the initial and equilibrium concentrations (Ac = Co — c q). The equilibrium concentration of surfactants was determined by two-phase titration method [33]. 

The ether sulfate can be determined by two-phase titration. However, interference from sulfated impurities (sulfated alcohol/alkylphenol and sulfated PEG) in the titration requires that a preliminary separation step be performed first for high accnracy. This analysis may also be performed by HPLC, as described in Chapter 7. 

ISO 6843 specifies a variation of the extraction method for determination of PEG in nonionic surfactants. In this case, an aqueous salt solution of the surfactant is extracted with 9 1 ethyl acetate/l-butanol. Almost all of the ether sulfate goes into the organic phase, leaving behind PEG, PEG sulfate, and sulfate ion, as well as a small amount of the ether sulfate. Alkyl- or alkylphenol ethoxylate and alkyl- or alkylphenol sulfate are expected to partition primarily in the organic phase. The amount of anionic surfactant remaining in the aqueous phase is determined by two-phase titration, as is the equivalent weight of the extracted material (71). 

Selective extraction and titration. The surfactant mixture is freeze-dried, mixed with sand, and subjected to Soxhlet extraction with isopropanol. The ester is soluble in isopropanol, while the acid salt is not. The surfactant concentration of each fraction is determined by two-phase titration (122). 

Konig and Walldorf recommend determining surfactants after evaporating the sample to dryness and obtaining a 95% ethanol extract of the residue (35,36). Qualitative identification is made by TLC examination of the original extract, as well as by IR analysis. The extract is further partitioned prior to IR analysis The residue is dissolved in water, and an acid ether extract is obtained. The aqueous phase is neutralized and evaporated to dryness. The residue from both phases is analyzed by IR spectroscopy. Anionic surfactants and soap are determined by two-phase titration. Nonionics are separated by ion exchange techniques from the other components and identified by IR or TLC on silica gel impregnated with oxalic acid. PEG is separated from nonionic surfactant by partition between water and butanol, and its identity confirmed by TLC. 

Wang has determined citric acid and procaine in Xiaozhi injectable (an anti-hemorrhoid agent) by two-phase titration [81]. With constant shaking, a mixture of 1 1 1 2 Xiaozhi solution - water - chloroform -ethanol was titrated against 0.1 M sodium hydroxide to the phenolphthalein end point, thus obtaining the citric acid content. Dilute nitric acid was then added to the solution until the pink color disappeared, and the solution titrated against 0.1 M silver nitrate (with 5 drops of... 

An oil s natural soap amount cannot be determined just by nonaqueous phase titration. Oils with high acid number by nonaqueous phase titration usually have high soap content however, this is not always true (Liu, 2007). Because those acids that cannot generate soap will not be detected by the potentiometric titration, the acid numbers obtained by the soap extraction are less than the acid numbers determined by nonaqueous phase titration, as expected. There is no general ratio between those two acid numbers. Figure 10.10 compares the acid numbers measured from the two methods. The data in this figure show that the acid number from the soap extraction was about one half of the value from the nonaqueous phase titration. 

High surfactant concentration systems The petroleum sulfonate and alcohol of given quantities were dissolved in equal volumes of oil and brine. The system was shaken vigorously and was rotated on a slow rotor for several days and was then allowed to stand until the phase volumes did not show any change with time. The surfactant concentration was determined by two-phase dye-titration or UV-absorbance measurements. 

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. 

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. 

Possible surface-active impurities are soap in the taurine derivatives and fatty amine in sulphosuccinamates. They are easily determined by extraction with petroleum ether or by two-phase titration in acid and alkaline solution. Soap may also be determined by potentiometric acid-base titration. 

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. 

ASTM standard D3673 specifies methods for gross characterization of a-olefin sulfonates assay by two-phase titration, water by azeotropic distillation, unsulfonated material by extraction, sulfate and chloride by titration, alkalinity by sequential titrations. pH is determined on a 1% aqueous solution and color is according to the platinum-cobalt scale (9). Internal olefin sulfonates may be analyzed by the same methods used for a-olefin sulfonates (81). 

These compounds can be determined by either acid-base titration as carboxylic acids or by ion-pair titration. In the case of ion-pair titration, sarcosinates, like other carboxylic acids, can be titrated by two-phase titration at high pH with a cationic titrant (137). In addition, their titration has been demonstrated at low pH either by two-phase titration with an anionic surfactant (138) or one-phase titration with tetraphenylborate using a surfactant-selective electrode (139). These procedures are described in Chapter 16. Both acid-base titration and titration with a cationic surfactant will measure free fatty acid along with the acylsarcosine content. While titration at low pH with anionic titrants is presumably not subject to interference by free fatty acid, this has not been confirmed in the literature. 

The compendial method for assay of benzalkonium chloride is based upon reversal of the quaternization reaction. The compound is reacted with Nal, the benzyl iodide and tertiary amine are removed by extraction, and the iodide consumed is determined by titration with KIO3 (9). The alkyl chain length distribution is determined by HPLC. Other tests for characterization of benzalkonium salts are similar to those performed on alkyl quaternaries. These salts are also readily determined by two-phase or potentiometric titration with an anionic surfactant. 

Procedure Determination of Cationic Surfactants by Two-Phase Titration with a Hydrophobic Indicator (90)... 

Tsubouchi, M., J. H. Mallory, Differential determination of cationic and anionic surfactants in mixtures by two-phase titration. Analyst, 1983,108, 636-639. 

Poloxamers are used primarily in aqueous solution and may be quantified in the aqueous phase by the use of compleximetric methods. However, a major limitation is that these techniques are essentially only capable of quantifying alkylene oxide groups and are by no means selective for poloxamers. The basis of these methods is the formation of a complex between a metal ion and the oxygen atoms that form the ether linkages. Reaction of this complex with an anion leads to the formation of a salt that, after precipitation or extraction, may be used for quantitation. A method reported to be rapid, simple, and consistently reproducible [18] involves a two-phase titration, which eliminates interferences from anionic surfactants. The poloxamer is complexed with potassium ions in an alkaline aqueous solution and extracted into dichloromethane as an ion pair with the titrant, tet-rakis (4-fluorophenyl) borate. The end point is defined by a color change resulting from the complexation of the indicator, Victoria Blue B, with excess titrant. The Wickbold [19] method, widely used to determine nonionic surfactants, has been applied to poloxamer type surfactants 120]. Essentially the method involves the formation in the presence of barium ions of a complex be-... 

The chemical properties of the alkylarylsulfonates are used in its analytical determination. As anions, LAS and other anionic surfactants react with large cations to salts, which are soluble in organic solvents (e.g., CHC13). By analysis it can be seen that cations such as Hyamine 1622 (25) and methylene blue, which rearrange with LAS to complex (26), are widely spread. These reactions are the basis for the so-called two-phase titration, an extensively used method... 

A number of proven methods is available for the analysis of commercial alkanesulfonates. The total sulfonate content and the monosulfonate content can be determined quite accurately by extractive methods and different forms of two-phase titration. 

ISO 2271 1989, Surface active agents Determination of anionic-active matter by manual or mechanical direct two-phase titration procedure. 

Active matter (anionic surfactant) in AOS consists of alkene- and hydroxy-alkanemonosulfonates, as well as small amounts of disulfonates. Active matter (AM) content is usually expressed as milliequivalents per 100 grams, or as weight percent. Three methods are available for the determination of AM in AOS calculation by difference, the two-phase titration such as methylene blue-active substances (MBAS) and by potentiometric titration with cationic. The calculation method has a number of inherent error factors. The two-phase titration methods may not be completely quantitative and can yield values differing by several percent from those obtained from the total sulfur content. These methods employ trichloromethane, the effects from which the analyst must be protected. The best method for routine use is probably the potentiometric titration method but this requires the availability of more expensive equipment. 

Analytical methods for qualitative and quantitative determination of all kinds of surfactants have long been established. There can be found either wet chemical methods—like titration—or methods of instrumental analytics. A classical method for anionic surfactants is the so-called two-phase-titration [63]. Using this method nearly every molecule bearing an anionic group is detected. The correct determination of sulfosuccinates is limited to the interferences of the other ingredients and byproducts present. With few exceptions all sulfosuccinates react with special indicators, e.g., hyamin 1622 (Rohm and Haas), to form an anionic-cationic complex and hence are to be analyzed by active content titration. 

BASIS OF MANUAL PHOTOMETRIC TITRATION. The determination of anionic surfactants by a photometric titration employs a cationic indicator to form a coloured complex with the surfactant which is insoluble in water but readily soluble in chlorinated solvents (1 ). The end point of the titration occurs when there is a loss of colour from the organic phase. A considerable improvement in this technique is achieved by the use of a mixture of anionic and cationic dyes (4 ), for example disulphine blue and dimidium bromide (Herring s indicator (3)). The sequence of colour changes which occurs during the two phase titration of an anionic surfactant (AS) with a cationic titrant (CT) using a mixed indicator consisting of an anionic indicator (AD) and cationic indicator (CD) is summarised in Scheme 1 ... 

The sulfonate content was determined either by the well-known technique of two-phase titration with hyamine or by liquid chromatography (HPCL). Nonionic surfactants were analyzed by HPLC (16) in the reverse or normal phase mode depending on whether the aim was to determine their content in effluents or to compare their ethylene oxide distribution. 

The use of distribution coefficients for the QSAR treatment of ionizable compounds has been extended to consideration of ion-pair partitioning into biolipid phases. Two experimental methods for determining ion-pair partition coefficients are described. One is a single-phase titration in water-saturated octanol, in which case (for acids) log Pj = log P + pKa - pKa. The other is a two-phase titration (octanol/water) from which the ratio (P + 1)/(Pj + 1) can be calculated. An example outcome is that the uncoupling activity of phenols can be represented by an equation in log instead of log D and pKa. 

These new methods for determining the partition coefficients of ionized species are still e3q>erimenta1, but they are presented in a spirit that they may stimulate thinking and further refinement. Single-phase titrations with HCl in octanol have only recently been run. A possible concentration dependency of pKa in the single-phase titrations has been suggested by a referee and will be looked for. Further refinement of the two-phase titrations may incorporate ion-pair association constants. 

Veinbergs et al. have reported a comparative study of the methods used for the determination of procaine in Celnovocaine by titrimetry and photometry [79]. Tsubouchi et al. have reported the application of singlephase endpoint change systems in two-phase titrations to the analysis of procaine and some other amines [80]. 

Several methods exist to determine the level of active matter in raw materials and finished products. Quaternaries can be assayed by standard two-phase titration. 

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