Methyl Red: A Fluorescent Sensor for Hg2+ over Na+, K+, Ca2+, Mg2+, Zn2+, and Cd2+

, Diganta Kumar Das; , Priyanka Goswami; , Champa Barman; , Biva Das

doi:2012.17.S1.75

Environmental Engineering Research 2012;17(S1): S75-S78. DOI: https://doi.org/10.4491/eer.2012.17.S1.S75

Methyl Red: A Fluorescent Sensor for Hg²⁺ over Na⁺, K⁺, Ca²⁺, Mg²⁺, Zn²⁺, and Cd²⁺

Diganta Kumar Das ^†, Priyanka Goswami , Champa Barman , and Biva Das

Department of Chemistry, Gauhati University, Guwahati 781014, Assam, India

Corresponding Author: Diganta Kumar Das ,Tel: +91-361-2570535, Fax: +91-361-2570535, Email: digantakdas@gmail.com

Received: September 17, 2012; Accepted: October 10, 2012.

ABSTRACT

Methyl red (MR), a very well known acid base indicator (pK_a = 5.2), shows fluorescence emission in the range 320 nm to 480 nm, with an emission maximum at 375 nm on excitation by photons at 310 nm. A fluorescent intensity titration of MR by Hg²⁺ ion, a well known toxic heavy metal ion, shows quenching of the fluorescent intensity of MR. The quenching continues till the concentration ratio between MR and Hg²⁺ ion becomes 1:1. Similar titration with a number of other metal ions, namely Na⁺, K⁺, Ca²⁺, Mg²⁺, Zn²⁺, and Cd²⁺, has very small effect on the fluorescent intensity of MR. Thus MR acts as a selective fluorescent sensor for Hg²⁺ ion by selective fluorescence quenching. The efficiency of fluorescence quenching value is found to be 8.0. The binding ratio and binding constant (β) between MR and Hg²⁺ ion are calculated from the fluorescence and UV/visible spectral data, and are found to be 1:1 and log β =4.68. The Hg:MR complex was found to be stable in the pH range 5.5 to 8.2.

Keywords: Fluorescence | Hg²⁺ ion | Methyl Red | Quenching | Sensor