Abstract：
:The pKa values of benzoic acid, p-methylbenzoic, and p-aminobenzoic acid (PABA) were determined by potentiometric titration in mixtures of 0-0.5 volume fractions of various cosolvents and water. The differences between the aqueous and semiaqueous pKa values were similar for the three solutes at a particular cosolvent-water mixture for most of the cosolvents studied. The largest differences occurred in the dimethyl sulfoxide (Me2SO)-water system, where the pKa changes were larger for PABA than the other two solutes. The data are analyzed by a consideration of both electrostatic and nonelectrostatic medium effects. The electrostatic medium effect was calculated from the Born Equation while any residual pKa change was attributed to nonelectrostatic medium effects. The residual effects were found to correlate well with indexes of solvent hydrogen bond acceptor ability such as HBA and beta-values. These results provide a rationale for the use of two solvent polarity indexes for more accurate estimates of pKa values of weak electrolytes in semiaqueous solvent systems. Analysis of solubility data of the salt and acid forms of PABA and benzoic acid in ethanol-water and Me2SO-water mixtures suggests that the higher activity of the anionic form of PABA in Me2SO-water mixtures is primarily responsible for the large pKa changes observed for that solute.
journal_name
J Pharm Scijournal_title
Journal of pharmaceutical sciencesauthors
Rubino JT,Berryhill WSdoi
10.1002/jps.2600750217subject
Has Abstractpub_date
1986-02-01 00:00:00pages
182-6issue
2eissn
0022-3549issn
1520-6017pii
S0022-3549(15)47013-3journal_volume
75pub_type
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