Chromatographic Separation of Heavy Metal Cations on a Silica Gel-G with Amino Acid

Thin layer chromatographic method has been developed for the separation of metal ions such as Cr(VI), Cr(III), As(III), Cd(II), Tl(III) and Hg(II) from their two, three and four component mixtures. The separations were performed on thin layer of silica gel ‘G’ using aqueous l-Alanine as mobile phase. The effect of concentration and pH of mobile phase on the Rf values of individual metal ions were studied and the optimum conditions for separation of metal ions from their mixture were determined.


Introduction
Thin layer chromatography (TLC) is considered to be superior to other chromatographic techniques, because of its simplicity and relatively low cost. Success in TLC depends to great extent upon the proper selection of the mobile phase. The separation possibilities in TLC are greatly enhanced when chromatoplates are developed with mixed solvent systems. TLC has been successfully utilized for various purposes, such as the separation of metal ions from a water sample [1], the characterization of the mobility of metal [2], and the estimation of concentration of toxic metal in industrial waste [3].The use of aqueous solution as a mobile phase in TLC was pioneered by Armstrong and Terrill [4]. Using a surfactant as the mobile phase gained popularity and became more widely applied due to its operational simplicity, cost effectiveness, relative non-toxicity and enhanced separation efficiency [5][6][7][8]. The use of silica gel and an alumina layer with surfactant mediated mobile phase systems [9][10][11][12][13][14][15] has been used to separate various inorganic species. Number of metal ions was systematically chromatographed on thin layer of urea formaldehyde polymer [16].
In this paper chromatographic separation was carried out on silica gel-G using aqueous solution of l-Alanine as mobile phase.

Experimental
Chemicals and Reagents l-Alanine (E. Merck; India); Silica gel-G (E. Merck; India); Hydrochloric acid and Sodium hydroxide. All chemicals were of analytical reagent grade.
Stationary Phase: Silica gel-G. Mobile Phase: The aqueous solution of l-Alanine as the mobile phase.

A. Preparation of Plates:
The TLC plates were prepared by mixing silica gel-G with deminerlised water in 1:2 ratio by weight with constant stirring to obtain homogeneous slurry. It was then immediately applied on the glass plates by dipping method [18]. The plates were allowed to dry over night at room temperature and were used next day for TLC. B. Procedure: Test solutions were spotted onto thin layer plates with the help of a micropipette positioned about 1.0 cm above the lower edge of the TLC plates. The spots were air dried and the plates were then developed with the given mobile phase using by one dimensional ascending technique in glass jars. The development distance was fixed at 10 cm in all cases. Following development, the plates were again air dried and the spots of the Cations were visualized as coloring spots using the appropriate spraying reagent. R f values were then calculated. C. Separation: For the separation, the metal ions to be separated were mixed in equal amounts. A test solution of the resultant mixture was spotted onto the activated TLC plate, and was then air dried. The plates were developed to a distance of 10 cm. The spots were detected and the separated metal cations were identified by their R f values.

Effect of Concentration
This section deals with the separation of Cr(VI); Cr(III); As(III); Cd(II); Tl(III) and Hg(II). Various experiments were carried out at different development time, different pH and at different concentration of l-Alanine for determining optimum condition for separation of the metal ions.
The results dealing with the effect of concentration of mobile phase, i.e. on the R f values of different metal ions such as Cr(VI); Cr(III); As(III); Cd(II); Tl(III) and Hg(II) are represented graphically in fig.1 and tabulated in table 1. The variations in the R f values with concentration in the range of 0.005 to 0.1 M were studied in pH range of 1 to7.     It was observed from Table No 2 and fig 2 that, at low pH, all the six metal ions move with the solvent. It is noted that all metal ions showed very little difference in the R f values at pH 2.0, but as we increase the pH from 2.5 to pH 4.0. Maximum difference in the R f values of different metal ions were seen at pH 3.5. This was required for better separation. However, the behavior of cations changed after the increase in pH 4.0 and above, especially at pH 5.0, Cr(VI) shows double spotting due to formation of hetero poly acid ; Cr(III); Cd(II); & Tl(III) show tailing whereas Hg(II) shows spreading. But As(III) shows compact spot at all pH except pH 1. From these observed values, pH 3.5 has been found out to be ideal for bringing out maximum separation. R f of all mention metal ions was not possible to measure above pH 7. Hence, separation measurements have been carried between pH 3.5 and pH 7.0 in aqueous media of l-Alanine.

Conclusion
Using the above mentioned optimum separating conditions, that is 0.005 M l-Alanine at pH 3.5, qualitative separation of eight binary mixtures; seven ternary mixtures; and four quaternary mixtures of metal ions have been carried out. Various binary, ternary and quaternary separations have been listed in Table 3, 4, and 5 respectively. The R f values of metal cations are given in top to bottom format, as they appear on the chromatographic plate. Photograph of binary, ternary and quaternary separations have been depicted in picture number 1, 2 and 3 respectively.
Experimentally achieved separations on silica gel-G layers developed in aqueous l-Alanine as mobile phase with optimum separating conditions.