Adsorption of carminic acid, a dye onto glass powder

The adsorption of carminic acid (CA) on glass powder (GP) has been studied polarographically. The adsorption is a two-stage process comprising a fast initial phase and a slower second phase. The rate constant for the first phase increased slightly with increasing dye concentration while the rate constant for the second phase remained almost constant. This indicated the presence of two diffusional processes. In the first phase, CA molecules are adsorbed at the outer sites of the glass particles in a fast process that dominates the initial kinetics of adsorption. In the second phase, dye molecules slowly diffuse inward and adsorb to the inner sites of the glass particles. The rate of adsorption increases with increasing temperature. The thermodynamic parameters for activated state have been evaluated using the Arrhenius and Eyring equations. The equilibrium isotherms have been analyzed using the Freundlich and Langmuir equations. The parameters of the isotherms show that the adsorption process is favorable. (C) 2003 Elsevier Science B.V. All rights reserved.