The polarographic estimation of calcium and the reduction of the cobaltinitrite ion (1946)
AuthorsMacartney, T. G.show all
The nomenclature of polarography is made clear by the diagram which shows a typical current-voltage curve or polarogram, as the automatically recorded photographic record is called. The growth and falling of the mercury drop gives rise to a condenser current (1) which causes the polarograms to [Diagram] exhibit a jagged appearance, often very marked. In all polarographic work it is the mean current which is measured. When a potential difference is applied across a solution, only a very slight residual current flows till the P.D. reaches a sufficiently high value to decompose one component. As the P.D. increases from the decomposition voltage, the current increases rapidly, till it reaches a constant value; the polarographic diffusion current. The polarograph records variations in current as the P.D. changes, for just one electrode. This is made possible by using a large unpolarisable electrode (usually anode) and a dropping mercury electrode (usually cathode). The latter is subject to concentration polarisation only, for the current flowing depends upon the rate at which the electro-oxidisable or electroreducible substance can diffuse to the mercury drop. To prevent this substance from undergoing a nett attraction or repulsion by electrostatic forces, a large amount of an indifferent electrolyte (supporting electrolyte) is added to make a homogeneous electrostatic field round the drop. This eliminates the migration current, leaving the current to be carried by the diffusion of ions alone. Generally the dropping mercury electrode is the cathode and the reducible substance is an ion, either cation or anion. The diffusion current is reached when the rate of discharge of ions equals the rate at which the ions diffuse to the surface of the mercury drop. This is the concentration polarisation of the cathode for the amount of current flowing is controlled by the rate at which ions reach the drop. [Diagram] A simple circuit of the polarograph is shown. The resistances R₂ and R₃ are of equal value, so that when the pointer P. is on the centre of R₄ no current flows through the cell. Moving P to the right puts an increasing negative potential on the dropping electrode. In the present work the instrument used was a Cambridge Instrument Coy model, the property of the Soil Survey Division of the D.S.I.R. The only advantage over manually operated polarographs lies in the automatic photographic recording device giving continuous polarograms. Only one capillary was used throughout the course of the work and frequent readings showed that it maintained a constant drop-time. The characteristics of this capillary are given in Part II. To remove oxygen from solution, pure hydrogen from a cylinder was passed for a time depending on the volume of solution. For 50 mls. of solution the hydrogen was bubbled through tor twenty minutes.