Acid-Base Indicators in Solvents of Low Dielectric Constant

4-11 ACID-BASE INDICATORS IN SOLVENTS OF LOW DIELECTRIC CONSTANT 

In aqueous solution, and in solvents of high dielectric constant, the behavior of indicators generally can be related directly to pH changes (Equation 3-76). With solvents of low dielectric constant the color of an indicator depends on the extent of its ion formation, whether as ion pairs or free ions. The extent of ion formation may bear only an obscure relation to the hydrogen ion activity, which depends on the extent of formation of dissociated protonated solvent. Analogous to Equation (4-13), the reaction between an indicator base In and an acid HA may be represented in its simplest form by 

No simple relation exists between this ratio and the hydrogen ion concentration, as is shown in the following discussion. 

Indicators and glacial acetic acid Kolthoff and Bruckenstein ° found that for perchloric acid in glacial acetic acid the ratio of acid to alkaline forms of p-naphthol-benzein is proportional to the concentration of perchloric acid. On the other hand, the hydrogen ion concentration should vary with the square root of the concentration 

From the viewpoint of titrimetry in glacial acetic acid, the behavior of indicators during the titration of weak bases with perchloric acid is most important. It at first seems that Equation (4-108) could be used in a straightforward way nevertheless, this is not an experimentally useful form because the concentration of molecular perchloric acid, [HCIO4], is unknown. 

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