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Molybdenum is an essential trace element for plant growth. Since it occurs in natural waters only in trace amounts, a very sensitive method of determination is needed. With the aid of the following polarographic method it is possible to determine 5x10-10 mol/L resp. 50 ng/L. The principle of the method is based on the reaction between the molybdate ion MoO42- and the complexing agent 8-hydroxy-7-iodo-quinoline-5-sulfonic acid (H2L) to form an MoO2L22- complex, which is adsorbed on the mercury electrode. The adsorbed Mo(VI) is reduced electrochemically to the Mo(V) complex. The hydrogen ions present in the solution oxidize the Mo(V) again, giving rise to the Mo(VI) complex, which is thus newly available for electrochemical reduction. This catalytic reaction is the reason for the high sensitivity of the method.
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