RESEARCH ARTICLE

 

Speciation studies of some toxic metal complexes of glycylglycine in propylene glycol-water mixtures

 

 

Mekonen Tirfu Zekariasa^I; Gollapalli Nageswara Rao^{II, *}

^IDepartment of Chemistry, College of Natural and Computational Sciences, Mekelle University, P.O. Box 231, Mekelle, Ethiopia
^IISchool of Chemistry, Andhra University, Visakhapatnam-530003, India

 

 

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ABSTRACT

The formation equilibria of complexes of Pb(II), Cd(II) and Hg(II) with glycylglycine were investigated pH-metrically in propylene glycol-water mixtures (0-60 % v/v) at 303 K and an ionic strength of 0.16 mol L^-1. The dominant species detected were ML⁺ and ML₂H₂²+ for Pb(II); MLH²+ and ML₂H+ for Cd(II) and ML+,ML₂ and ML₂H+ are for Hg(II). The appropriateness of the experimental conditions was verified by introducing errors deliberately in the concentrations of ingredients and volume of the solution. The models containing different numbers of species were refined by using the computer program MINIQUAD75. Selection of the best fit chemical models was based on statistical parameters. The trend in variation of stability constants of the complexes with dielectric constant of the medium was attributed to electrostatic and non-electrostatic forces. Distribution diagrams and plausible equilibria for the formation and possible structures of the complex species are presented.

Keywords: Speciation, complex equilibria, glycylglycine, propylene glycol, stability constants

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Acknowledgements

The first author (MTZ) thanks the Ministry of Education and University Capacity Building Program of the Federal Democratic Republic of Ethiopia for financial assistance.

 

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Received 4 November 2011
Revised 21 April 2012
Accepted 27 July 2012

 

 

* To whom correspondence should be addressed. E-mail: gollapallinr@yahoo.com