Colorimetric Determination of Pb²⁺ in Perfect Aqueous Solution Using Carminic Acid as a Selective Chemosensor

The commercially available natural organic dye, carminic acid (CA), an anthraquinone derivative bearing hydroxyl and carboxyl groups as recognition sites was found to be a colorimetric probe for Pb²⁺ in perfect aqueous solution under neutral conditions with specific selectivity and high sensitivity. Upon addition of Pb²⁺, the absorption maximum of CA showed a large red shift, and the resulted color change from red to purple could be easily identified even by the naked eye. The chemical stoichiometric ratio between CA and Pb²⁺ was determined to be 1:2 through Job plot, Pb²⁺ titration, and kinetic experiments. Moreover, other environmental relevant metal ions induced no or minimal spectral and color changes. The reversibility of Pb²⁺ to CA with EDTA even through several cycles was established for practical applications. The results indicated that CA can be a good candidate for simple, convenient and reversible colorimetric detection of Pb²⁺ in aqueous solution even though it was hard to be applied to determine Pb²⁺ on the water testing by US EPA.