Amphoterics titration with sodium

FIG. 1 Potentiometric titration with sodium tetraphenylborate of a mixture of a cationic (cetylpy-ridinium chloride) and an amphoteric (cocoamidopropylbetaine) surfactant, followed with a specially made PVC-membrane electrode. The first inflection is due to titration of the cationic and the second to titration of the amphoteric surfactant. (Reprinted with permission from Ref. 81. Copyright 1992 by Karl Hanser Verlag.)... 

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 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. 

Many amphoterics behave as cationic surfactants at acid pH, allowing them to be determined by the standard two-phase titration method. The mixed indicator method for the analysis of ionic surfactants, as described by Reid, Longman, and Heinerth, is adequate, but emulsions form in the vicinity of the end point so that precise analysis is difficult (115). Sometimes these problems can be minimized by reversing the titration. For example, a solution of sodium lauryl sulfate, buffer, and indicator is titrated with an aqueous sample containing a quaternary or amphoteric surfactant. The same end point is observed as described under the titration of anionics (94,95). Further improvement can be made by carefully adjusting the amount of ethanol added to the titration flask, depending on the individual surfactant being analyzed (116). 

The amphoteric character of the hydroxylated Ti02 surface has been demonstrated by Boehm and co-workers in a series of potentiometric titration experiments performed both with anatase and rutile samples. Approximately half of the total amount of hydroxyls present on the Ti02 surface, which underwent neutralization with a diluted (0.01 M) sodium hydroxide solution, has been described as relatively strongly acidic, with a pK value of 2.9, and the remainder as weakly acidic with a pK of 12.7. However, the use of a Langmuir-type adsorption equation as a basis for estimating these acid dissociation constants has raised questions about the real significance of the above values. ... 

Fig. 3 Experimental points of net proton surface excess amounts from the reversible backward titration cycles of sodium montmoril-lonite at different NaCl concentrations. The different lines represent the results of numerical fitting (FITEQL [28]) using the diffuse-double-layer option of the surface complexation model assuming reactions of and Na" ions with permanently charged ion-exchange sites in parallel with protonation/deprotonation reactions on amphoteric edge sites...

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