Derivation of the Equation for the Instantaneous Diffusion Current at the Cylindrical Drop Mercury Electrode

The photographic method of analysis, introduced in 1922 by Professor J. Heyrovsky, is dependent upon electrolysis at the dropping electrode of the solution being analyzed. The method is based on interpreting current voltage curves that are obtained when solutions of electro reducible or electro oxidisable substances are electrolyzed in a cell in which one electrode consists of mercury falling drop-wise from a very fine capillary glass tube and the4 other electrodes is a pool of mercury. The circuit shown in Fig 1, is one used in the polarographic method of analysis. It consists of an accurate slide wire in which a steady current flows. The voltage across the slide wire can adjusted to any desired value by varying the number of batteries and the resistance. This voltage is measured by the voltmeter connected across the batteries and the resistance R. The slide wire contact is moved to vary the voltage applied to the two electrodes. As mentioned above the current voltage curves are obtained by gradually increasing the applied emf, and recording the current indicated by the galvanometer O. The current through the bulk of the solution is carried by movement of all the ions present, but only ions reducible or oxidisable at the microelectrodes can undergo electron exchange and carry the current to the microelectrodes. As in other physical methods used for analytical purposes, polarographic quantitative determination are calibrated by running standards of iron composition and plotting the results