Amphoterics acid-base titration

The amphoteric nature of wool was demonstrated in the early studies of Speakman and Hirst (1933), Elod (1933), and in particular by the complete acid-base titration curve obtained by Speakman and Stott (1934). Even earlier attempts had been made to determine the isoelectric point of wool by the methods indicated in Table XXIII. Some variation in the isoelectric point is to be expected because the pH at which the net charge, including bound ions, is zero depends on the nature and concentrations of ions in the aqueous environment. For example, Sookne and Harris (1939) have shown that the early electrophoretic value of Harris (1932) was affected by the absorption of phthalate ions from the buffer solutions. With acetate buffer they obtained values of 4.2 and 4.5 for powdered wool and cortical cells, respectively. The isoelectric points listed in Table XXIII are... 

D. Acid-base titration. All amphoterics can be titrated through two end-points, either potentiometrically or with indicators, in ethanol or propan-2-ol. This is not very useful, because other weak acids and bases, which are almost always present, also titrate. The method described in section 7.2.3 may be generally applicable. The following is an alternative. 

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. 

Potentiometric acid-base titration. With reservations about the effects of other weak acids and bases, amphoterics can be determined in any mixture with quats by potentiometric acid-base titration in ethanol or propan-2-ol. WW amphoterics consume two mols of acid or base per mol. WS and SW amphoterics consume one mol per mol. Mixtures can be... 

Acid-base titration. This is good for weakly basic cationics, and can be satisfactory for amphoterics provided that other weak acids and bases are not present at too high a concentration, or if the titration can be done in such a way as to nullify their effects (cf. [2] for carboxybetaines). 

That approach is useless for SW amphoterics BEC titration measures the total active and SDS titration in acid solution measures nothing. Acid-base titration measures both the acid group of the carboxylate and the basic group of the amphoteric. If the sample is dried and dissolved in glacial acetic acid, the amine function of the SW amphoteric can be titrated with perchloric acid in glacial acetic acid, but it is likely that other weak bases will be present. Extraction of the carboxylic acid is the simplest way. 

Owing to the amphoteric nature of metal oxide surface sites, acid-base titrations are commonly performed. Since siuface sites can accept up to two protons, titration plots of metal oxide powders are qualitatively similar to titration plots of diprotic weak acids (or weak bases) in solution. As with diprotic weak acids, the quantitative shape of metal oxide powder titration plots is determined by the two proton mass action constants, and K 2- e take the same proton mass action constant values used in Section II.C (p. al P- a2 8.0) a titration plot similar to the one shown... 

Potentiometric titration with HCl gives a characteristic value proportional to the content of amphoteric surfactant (1,10,11). In some cases, solvent systems have been optimized so that acid-base titration is suitable for assay of the product, as described below for characterization of alkylbetaines (12). 

The analysis of solids after dispersion in liquids offers a number of advantages because of the availability of a number of titration techniques, which augment the functional adsorption on dry surfaces. For acid-base potentiometrlc titration of Insoluble oxides in aqueous media the principle has already been explained in sec. I.5.6e and we shall return to it in sec. 3.7a. For amphoteric oxides the (pristine) point of zero charge can be measured it is determined by the difference between pK3,.,(jand pEjigg with some theoretical analysis these two constants can also be established individually. 

Plausibly there is a correlation of the solubility of the stable oxide, hydroxide, or oxyhydroxide of a cation with the stability of the first hydrolysis product (Figure 6. 7). Many multivalent hydrous oxides are amphoteric because of the acid-base equilibria involved in the hydrolysis reactions of aquo metal ions. Alkalimetric or acidimetric titration curves for hydrous metal oxides provide a quantitative explanation for the manner in which the chaige of the hydrous oxide depends on the pH of the medium. The amphoteric behavior of solid metal hydroxides becomes evident from such titration curves. From an operational point of view, such hydrous oxides can be compared with amphoteric polyelectrolytes and can be considered hydrated solid electrolytes, fn -... 

Weak bases (amines and amine oxides) must be titrated in a medium containing not less than 0.01 mol/1 and betaines in a medium containing not less than 0.1 mol/1 H. Amphoterics must be titrated in a medium containing not less than 0.01 mol/1 (cationic function) or not less than 0.01 mol/1 OH (anionic function). These conditions are met by adding 10 ml 1.0 mol/1 hydrochloric acid to solutions of betaines and 10 ml 0.1 mol/1 hydrochloric acid or sodium hydroxide, as appropriate, in the other cases. 

Acid-base potentiometric titration is applicable to amphoterics. Typically, either an excess of acid or of base is added, and the titration is conducted with base or acid, respectively. The first inflection is due to the excess acid or base and subsequent inflections are due to end points associated with the surfactant. Hydrolysis and consequent blurring of the end point can be minimized by using a highly alcoholic solvent (114). Since other acidic or basic components will interfere, this approach is only applicable to concentrated surfactant solutions, not to formulations. In fact, impurities in the concentrated surfactant will interfere, which sometimes limits the usefulness of titration for assay. For determination of amphoterics in formulations, ion-pair titration is often used. 

Since they are amphoteric, these substances can be titrated with either a strong acid or a strong base. Many amino acids are too weak to be titrated in aqueous solutions, but some will give adequate end points, especially if a pH meter is used to construct a titration curve. 

The titration must be done with alcoholic acid or alkali, and is best done potentiometrically with an autotitrator. Next best is manual potentiomet-ric titration, but it is satisfactory to use indicators, methyl orange or bromophenol blue for the low-pH end-point and phenolphthalein for the high-pH end-point. The information obtained is often a useful crosscheck on the mass balance in the analysis of an unknown, but it can also help in the analysis of WW amphoterics, which contain appreciable amounts of non-surfactant weak acids and bases. 

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. 

All amphoterics, including betaines, can be titrated between end-points at high and low pH. Unfortunately the raw materials contain other weak acids and bases that also titrate. The following method [19, 20] avoids this difficulty. 

Mixtures of weak bases with WS amphoterics (carboxybetaines) can be analysed by the method described in section 7.2.4 [2]. The betaine is determined by potentiometric titration of an initially acid solution in methyl isobutyl ketone with alkali, and the weak base by potentiometric titration of an initially alkaline solution in 50% aqueous propan-2-ol with acid. This procedure distinguishes between tertiary amines and amine oxides (section 7.8). 

Hence the pKa values of a new natural product may be recorded e.g. as pK (proton gained) 3 3, pKg (protons lost) 7 4 and 9 3 and the results may remain in this state for some time until each pKg can be assigned to its ionizing group. Thus the above figure of 3 3, obtained by titration with acid, would be that of a weak base if the substance were an ordinary amphoteric substance, but it would be that of a strong acid if the substance were a zwitterion. 

All four of these characteristics must be displayed by a particular substance for classification as an acid or a base. However, in order to make the definitions as concise as possible, the first and second properties, neutralization and titration with indicators, are chosen for specific mention. Acids are substances which, like hydrogen chloride, neutralize sodium hydroxide or any other base. Bases are substances which, like sodium hydroxide, neutralize hydro gen chloride or any other acid. Many substances are capable of acting in either way and are called amphoteric. 

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