Investigation of the Acid-base Properties of 2-[2-(4-Methoxy-phenylamino)-vinyl]-1,3,3-trimethyl- 3H-indolium Reagent

Vasil Andruch,^a Ioseph S. Balogh,^b Yaroslav R. Bazel,^a Ferenc Billes,^c Mihály Kádár,^d Roland Karosi,^e Gyula Parlagh,^c József Posta,^e András Simon,^d Rastislav Serbin,^a,* Marcel Torok^a
a Department of Analytical Chemistry, P. J. Šafárik University, Moyzesova 11, SK-04154 Ko{ice, Slovak Republic,
  E-mail: rasto00@yahoo.com
^b Department of Chemistry, University of Nyíregyháza, Sóstói út 31/b, Nyíregyháza, Hungary, baloghj@nyf.hu
^c Department of Physical Chemistry, Budapest University of Technology and Economics, Muªªegyetem rkpt. 3, H-1521 Budapest, Hungary, fbilles@mail.bme.hu
^d Department of General and Analytical Chemistry, Budapest University of Technology and Economics, Szt. Gellért tér 4, H-1111 Budapest, Hungary, andras.simon@mail.bme.hu
^e Department of Analytical Chemistry, University of Debrecen, Egyetem tér 1, Debrecen, Hungary,
 postaj@tigris.unideb.hu

Abstract
The acid-base properties of 2-[2-(4-methoxy-phenylamino)-vinyl]-1,3,3-trimethyl-3H-indolium dye were investigated. The reagent exists in a reactive single charged form over a wide range of acidity, from pH 8 to –2.5 H₀. The optimum analytical wavelength of the single charged form is 412 nm, where the molar absorptivity is 4.06 × 10⁴ dm³ mol^–1 cm^–1. Mechanisms of protonation and hydrolysis were discussed. The results suggest the applicability of the dye as an effective analytical reactant. Quantum chemical calculations of the structural and spectral properties of the dye were also carried out.

Keywords: 2-[2-(4-Methoxy-phenylamino)-vinyl]-1,3,3-trimethyl-3H-indolium; spectrophotometry; acid-base properties.