Neutralization titrations aqueous media

The long-chain a-amino acid esters (40), (41), and (42) form bilayers on sonication in water under acidic conditions. Liposomes prepared from (40) and (42) precipitate if the aqueous medium is neutralized by titration with NaOH. Only liposomes made from (41) are stable even in basic solutions, as shown by electron microscopy52). Polypeptide formation in oriented spherical vesicles was confirmed by FT-IR spectroscopy. The liposomal solution of (41) was freeze-dried and the spectrum obtained from the residue was comparable with one of the polycondensed monolayers. The formation of polypeptide vesicles is illustrated in Scheme 4. 

Acid-Base Titrations in Nonaqueous Solvents. It is a fact that the apparent acidity or basicity of a compound is strongly dependent on the acid-base properties of the solvent. For example, very strong acids such as HCl and HNO3 cannot be individually titrated in water because water is sufficiently basic that these acids appear to be totally ionized. Very weak bases, such as amines, cannot be successfully titrated with strong acid in water. Many acids or bases that are too weak for titration in an aqueous medium, however, become amenable to titration in appropriate nonaqueous solvents. As a consequence, there are now many neutralization methods that call for solvents other than water [23-25]. 

It is noteworthy that the analytical procedure for the quantitative determination of TBPO involves reaction with excess stannous chloride in an aqueous medium, followed by back titration of excess stannous ions. The aqueous medium results in the hydrolysis of stannous chloride to produce a solution of stannous hydroxide in aqueous hydrochloric acid. Stannous octoate may not hydrolyze in a neutral aqueous medium. Thus, the absence of complete dissociation and/or hydrolysis prevents the stoichiometric interaction of stannous octoate and TBPO. The possible presence of a TBPO-stannous octoate complex may also play a role in the failure to complete the reaction, as suggested by the observed presence of residual peroxide and residual stannous ions after the decomposition of TBPO has proceeded to the maximum extent. 

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