CONVECTION

"A mass-transport mechanism that involves bulk movement of a solution (contrast with diffusion that involves individual molecules or ions). We differentiate ""forced"" convection from ""natural"" convection. The simplest example of forced convection is mechanical stirring. If a non-uniform solution is stirred, the solute is ""transported"" from the high concentration parts of the solution to the low concentration parts till the solution becomes completely uniform. Other examples of forced convection are the ""flow"" of a solution through a pipe or a porous separator driven by pressure difference. ""Natural"" convection is very important in electrochemistry. It always occurs at the surface of an electrode carrying current in the absence of ""forced"" convection. As electrode reaction is proceeding, the buildup of reaction products and the consumption of reactants changes the density of the solution layer close to the electrode surface compared to that of the bulk solution. Eventually, this density difference will force the surface solution layer to sink or rise, setting up a ""natural stirring"" action close to the electrode surface which will tend to equalize the surface and bulk concentrations. As a ""rule of thumb,"" natural convection starts after about a minute of current flow. "