Acids and Bases in Alcohol-Water Mixtures

Although acid-base titrations in alcohol-water mixtures have been studied extensively, we do not consider them in detail since titration curves and indicator equilibria in ethanol-water and methanol-water mixtures can be calculated in the same way as in water. Values of the autoprotolysis constants of the mixtures, are close to for mixtures containing only a moderate amount of alcohol. On the other hand, even a trace of water in ethanol causes a large increase in J SH According to Gutbezahl and Grunwald, pXsH is 14.33, 14.88, 15.91, and 19.5 for ethanol-water mixtures containing 20, 50, 80, and 100 wt % ethanol. 

Acid-base indicators can be characterized according to the charges present on their acid and alkaline forms. 

J Sulfonephthaleins (for example, bromophenol blue, bromocresol green, 

2 Cationic acid indicators (for example, methyl yellow and neutral red) undergo a small change in p, because of the small change in charge separation in the reaction InH In -t- 

Methyl red represents a special case in which the intermediate form (red 2) acts as a neutral molecule rather than a dipolar ion. Consequently, its first transition (at low pH) is characteristic of a cationic acid, whereas its second transition is characteristic of a molecular acid. The salt effects with methyl red in ethanol and methanol also point to a neutral molecular species as the intermediate form. Bear in mind that the degree of dipolar ion character is expected to diminish with decreasing dielectric constant of the solvent. 

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