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Greener Alternative to Qualitative Analysis for Cations without H2S and Other Sulfur-Containing Compounds

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Indu Tucker Sidhwani and Sushmita Chowdhury

Department of Chemistry, Gargi College, University of Delhi, Delhi-110049, India

J. Chem. Educ., 2008, 85 (8), p 1099

DOI: 10.1021/ed085p1099

Publication Date (Web): August 1, 2008

Abstract

Qualitative analysis of inorganic salts and mixtures is an essential experiment carried out in high school, undergraduate, and postgraduate levels all over the world especially in India and South Asia. The classical technique uses H₂S, which is highly toxic and has adverse effects on humans and the environment. Increasing awareness concerning a healthy environment has prompted us to develop a green scheme for detection of cations. NH₄⁺ and K⁺ are detected directly in group zero. Pb²⁺ and Ag⁺ are detected in group I as chlorides using dilute HCl. Ca²⁺, Sr²⁺, Ba²⁺, and Pb²⁺ are precipitated in group II as sulfates using Na₂SO₄(aq) and ethanol. Cu²⁺, Cd²⁺, Fe³⁺, Mn²⁺, Co²⁺, Ni²⁺, and Mg²⁺ are precipitated as hydroxides using NaOH and H₂O₂ in group III. The precipitate is treated with NH₃(aq) leaving a residue (group IIIA: Fe³⁺, Mn²⁺, Mg²⁺ as hydroxides) and the centrifugate (group IIIB: Cu²⁺, Cd²⁺, Ni²⁺, Co²⁺ as soluble ammines). Al³⁺, Zn²⁺, Sn²⁺/Sn⁴⁺ (as soluble hydroxo complexes), and Cr³⁺ (as CrO₄^2-) are detected in group IV, in the presence of each other by using selective reagents. The separation is sharp; individual cations are detected by spot tests and conventional methods. This scheme is simple and fast and makes green chemistry accessible to students.