Analysis of Heavy and Transition Metals with Spectrophotometric Detection

3 Analysis of Heavy and Transition Metals with Spectrophotometric Detection 

A significantly better separation between iron(III), copper, and nickel is accomplished on a polymethacrylate-based cation exchanger using pure tartaric acid as the eluent. As the respective chromatogram in Fig. 3-153 reveals, iron(III) still elutes near the void volume, which renders the quantitation of this signal more difficult. The addition of ZnEDTA to the PAR reagent is very advantageous, because it enables the simultaneous detection of alkaline-earth metals. 

A reversal of elution order for the ions Pb2+, Co2+, Zn2+, and Ni2+ is obtained with the use of oxalic acid as the sole complexing agent. Such a separation is displayed in Fig. 3-155. Under these conditions, the metal separation is controlled by anion and cation exchange processes. The degree to which both mechanisms contribute to the separation process is different for each metal ion. The anion exchange mechanism predominates where stable anionic oxalate complexes are formed. In metal ions which do not form stable oxalate complexes, such as Cu2+, the cation exchange mechanism is 

If the pH value in the PAR reagent is lowered to 8.8 by adding a phosphate buffer, gallium(III), vanadium(IV)/(V), and mercury (II) can be detected and may be separated from the other heavy metals with PDCA as the eluent [151]. Fig. 3-157 shows the separation of vanadium(V) that was applied as ammonium(meta)vanadate, NH4V03. Under these conditions, vanadium(IV) elutes after about 17 minutes the two most important oxidation states of vanadium being easily distinguished. Gallium(III), of particular importance for the semiconductor industry, elutes near the void volume. 

Since the chromium(III)-PDCA complex can be measured directly at 520 nm, this method allows the determination of chromium(III) in an excess of chromium(VI) utilizing direct UV/Vis detection. Conversely, it is possible to determine traces of chro-mium(VI) in pure chromium(III) by applying post-column derivatization with 1,5-DPC and abstaining from pre-column derivatization with PDCA. 

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