Chronopotentiometry Sand equation

Transition time (for chronopotentiometry) — Electrolysis time required before the surface concentration of a redox species drops to zero in - chronopotentiometry. In constant-current chronopotentiometry (see also -> constant-current techniques), the transition time r is given by the - Sand equation ... 

Nowadays, most chemists know the name "Sand" only because it appears attached to a partic ilar equation in texts that describe chronopotentiometry. This technique can be useful in the diagnosis of electrode reactions (l). A current step i is Impressed across an electrochemical cell containing iinstirred solution, the potential of the working electrode is measured with respect to time and the transition time x is noted. According to the Sand equation, the product ix /2 should be constant for an uncomplicated linear diffusion-controlled electrode reaction at a planar electrode. 

In chronopotentiometry, a current is applied to the electrode and the potential changes to a value at which the flux of the electroaclive species is sufficient to supply the applied current. After a certain time, the flux of redox species to the surface cannot sustain this current and the potential changes rapidly to a new value at which another species (often solvent or electrolyte) is reduced (or oxidized). This time, termed the transition time t, follows the Sand equation ... 

In chronopotentiometry, conditions are chosen so that transport in the solution is rate controlling. A solution contains 0.001 M Cd(N03) (with 1M KN03) and the constant current applied is 0.10 A cm-2. The quarter-time potential is (L57 V vs. standard calomel. Use Sand s equation to calculate the transition time, x, and plot the electrode potential as a function of time. (Bockris)... 

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