The Implications of Using Diffusive or Convective Control

If the overall mass control comprises three components - migration, convection and diffusion - which form of mass transport is the most effective  

Migration in the absence of a swamping electrolyte is somewhat more effective than is diffusion, but migration can be ignored if a swamping electrolyte is added to the solution. Diffusion still occurs even if the solution is stirred, but convection is so much more efficient that we can ignore the diffusion completely. 

Potential differences can only be measured accurately at zero current. Accordingly, when measuring a potential at an electrode, it is best to use a voltmeter with as high an internal resistance as possible. It is also advisable for the resistance of the leads and other experimental paraphernalia to be as small as possible, thereby minimizing the IR drop. 

The passage of current through an electrochemical cell implies that the internal composition of the cell changes. The magnitude of the current tells us how much change has occurred, while the sign of the current tells us whether the reaction at the electrode of interest was oxidation or reduction. 

In summary, then, the biggest difference between measurements of potential and current (potentiometry and amperometry, respectively) is that potentials are measured with a zero current wherever possible, implying that no compositional changes occur inside the cell during measurement, whereas compositional changes do occur during the measurement of current. 

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