Medical brain notes 14

  INTRODUCTION An  amperometric method  or  amperometry  is concerned with the measurement of current under a constant applied voltage ; and under such experimental parameters the concentration of the  ‘analyte’  exclusively determines the quantum and magnitude of the current. Hence, these measurements may be employed effectively to record the alteration in concentration of an ion in question in the course of a titration, and ultimately the end-point is established. This specific process is commonly referred to as  amperometric method  or  amperometry.   In this particular case, the total current flowing shall remain almost equal to the current carried by the ions that undergoes equal electrolytic migration together with the current caused on account of the diffusion of the ions. Thus, we have :   I = Id + Im   where   I = Total current,   Id = Diffusion current, and   Im = Migration current.   An ...

  THEORY Assuming that the migration current (Im) is virtually eliminated by the addition of a reasonably enough supporting electrolyte then the only cardinal factor which would affect the limiting current would be the rate of diffusion of the electro-active substance from the main body of the solution to the surface of the electrode.   Thus, we may have :   Diffusion current = Limiting current – Residual current   It follows from above that the diffusion current is directly proportional to the concentration of the electro-active substance present in the solution. Now, if a situation is created whereby a portion of the electro-active substance is eliminated by interaction with a specific reagent, the diffusion current shall decrease significantly. It represents the fundamental underlying principle of amperometric method or amperometry. Hence, at an appropriate applied voltage the apparent diffusion current is measured as a function of the volume of the titrating solu...

  INSTRUMENTATION The amperometric titrations can be accomplished by any one of the  three  methods, namely :   ( i ) Amperometric titrations with the dropping mercury electrode,   ( ii ) Amperometric titrations with a rotating platinum microelectrode, and   ( iii ) Amperometric titrations with twin-polarized microelectrodes (or  Biamperometric Titrations  or  Dead-stop-end-point method ).   These  three  techniques will be discussed in the sections that follow.   1. AMPEROMETRIC TITRATIONS WITH THE DROPPING MERCURY ELECTRODE   Figure 17.2 ( a ) and ( b ) illustrates the schematic diagram of amperometric titrations with the dropping mercury electrode having a titration-cell and an electric circuit respectively.   The titration-cell Figure 17.2 ( a ) essentially comprises of a pyrex 100-ml, four-necked, flat-bottomed flask. A semimicro burette (B) (graduated in 0.01 ml), a 2-way gas-inlet tube (A) to enable N 2 ...