D. D. VAN SLYKE 



oxidizable substances in a cell in which one electrode consists of mercury 

 falling from a fine capillary. 



When a current at gradually increasing voltage is passed to a 

 small mercury electrode through a solution containing a solute capable 

 of giving or receiving electrons ("redox solute") at a given potential, 

 relatively little current is obtained until the decomposition potential 

 of the redox solute is reached. Then the current rapidly rises, with 

 further increase in voltage until a plateau is reached at which the 

 redox solute is providing its maximal flow of electrons. This flow, and 

 the resultant current, are proportional to the concentration of the redox 

 solute, which determines the rate at which its molecules diff'use to the 

 electrode and discharge or receive electrons. Since diff'usion is a proc- 

 ess independent of the voltage, increase of voltage above that at which 

 electrolytic decomposition of the redox solute equals its rate of diff'usion 

 to the mercury electrode does not further increase the flow of electricity; 

 the concentration of the active solute thus forms a bottleneck which 

 limits the current. The curve of current vs. voltage then reaches a 

 plateau, the height of which, in milliamperes, is proportional to the 

 concentration of active redox solute (32-34). 



Maintenance of the proportionality requires a continually 

 renewed surface of the mercury electrode; both renewal of the surface 

 and setting its area at a small size are obtained by using mercury 

 dropping from a fine capillary, of about 0.03 mm. diameter, as the 

 electrode; a drop is delivered about once in two to four seconds. 



The current fluctuates somewhat as each drop of mercury 

 expands and falls, but the average current, i, in microamperes is given 

 by the "Ilkovic equation": 



i = 605 n D"'' C m'' t"'' (3) 



C is the millimoles of redox solute per liter, n is the number of electrons 

 exchanged per molecule of redox solute in the electrolytic decomposi- 

 tion, m is the weight of mercury flowing from the capillary per second, 

 and t is the time required for formation of one drop of mercury. The 

 procedure is adapted to analysis of highly dilute solutions, 0.001 molar 

 and lower concentrations. 



Measurement of the current in such a system provides a measure 

 of the concentration of the redox solute. Furthermore, if two different 

 redox solutes are present with diff"erent decomposition potentials, they 



ii6 



