The Direct Effects of Alkali Metal and Alkaline Earth Metal Perchlorates on the Equilibria of Acid-Base Indicators (Sulfonephthaleins) in Acetonitrile Solution

Abstract

Abstract With the addition of alkali metal (M+) and alkaline earth metal (M2+) perchlorates, the cause of the color changes of several sulfonephthaleins of various pKa values, i.e., Thymol Blue, Phenol Red, Bromothymol Blue, and Bromocresol Green, in acetonitrile solutions were examined by means of spectrophotometry and 1H NMR. Upon the addition of ≥ 0.1 mol dm-3 LiClO4 to 1.0 × 10-4 mol dm-3 Phenol Red, the γ-sultone ring of the sulfonephthalein was cleaved to give absorption bands at around 410 and 500 nm. The band at 500 nm was assigned to the acid form, and that around 410 nm to a quinonoid structure (the intermediate form) which would be formed by deprotonation from the acid form. The effects of NaClO4 were much smaller than those of LiClO4. Magnesium and barium perchlorates caused larger effects than LiClO4; however, a nonmetallic salt, Et4NClO4, caused almost no effects. In addition, a purple-red colored species (λmax = 563 nm), a basic form, was also produced from Phenol Red by the addition of ≥ 0.2 mol dm-3 Mg(ClO4)2. Interestingly, the metal ions assisted not only the acid effects of CH3SO3H, but also the base effects of Et3N or 1,1,3,3-tetramethylguanidine on Phenol Red or Bromothymol Blue in acetonitrile. 1H NMR spectra of the sulfonephthaleins confirmed the opening of the γ-sultone rings in the presence of the metal salts. The ring cleavage upon the addition of the metal ions was considered from the standpoint of not ``medium effects'', but a direct chemical interaction between the metal ions and the sulfonate ion (function) in the sulfonephthaleins.