Elucidation of Salt Effects on the Indicator Acidity in Acetonitrile

Abstract

Abstract The salt effects on the proton-transfer from CF3COOH to an acid-base indicator, ‘Methyl Yellow’ [p-(dimethylamino)-azobenzene], were examined in acetonitrile by means of spectrophotometry. The addition of LiClO4 to the indicator (Ind) and CF3COOF mixture (both 2.0 × 10−4 mol dm−3) caused an increase and a successive decrease in the absorbance (λmax = 514 nm) of IndH+. The promoted increase of the IndH+ concentration with increasing concentration of LiClO4 was explained on a quantitative basis concerning the formation of a triple cation, CF3COO−(Li+)2, as well as an ion pair, CF3COO−Li+: Ind–CF3COOH + 2 Li+ IndH+ + CF3COO−(Li+)2. The successive decrease in the absorbance at 514 nm with a large excess of LiClO4 was attributed to an ion-exchange reaction: IndH+ + Li+ IndLi+ + H+. The addition of NaClO4 to Ind–CF3COOH caused only an increase in the IndH+ concentration, and no deprotonation from IndH+. Alkaline-earth metal ions (M2+ = Mg2+, Ca2+, Sr2+, and Ba2+) promoted protonation by the formation of complex species, CF3COO−M2+. Although distinct deprotonation from IndH+ was observed with Mg(ClO4)2, no deprotonation occurred with Ba(ClO4)2. The effects of the chloride ion (Et4NCl) on deprotonation from IndH+ (In + CF3SO3H) were explained by the formation of homoconjugated species (HCl2−), and not by the mere formation of HCl. The apparent changes in pH or the Hammett acidity function with the presence of various salts were elucidated by the ‘complex’ formation in acetonitrile.