DEFINITION OF ANODE AND CATHODE

2.2 DEFINITION OF ANODE AND CATHODE

A combination of two electrical conductors (electrodes) immersed in an electro- lyte is called a galvanic cell in honor of Luigi Galvani, a physician in Bologna, Italy, who published his studies of electrochemical action in 1791. A galvanic cell converts chemical energy into electrical energy. On short - circuiting such a cell (attaching a low - resistance wire to connect the two electrodes), positive current fl ows through the metallic path from positive electrode to negative electrode. This direction of current fl ow follows an arbitrary convention, established before anything was known about the nature of electricity, and is employed today

12 ELEC TROCHEMIC AL MECHANISMS

despite contemporary knowledge that only negative carriers, or electrons, move in a metal. Electrons, of course, go from negative to positive pole, opposite to the imaginary fl ow of positive carriers. Whenever current is said to fl ow, however, without designating the sign of the carrier, positive current is always implied.

Within the electrolyte, current is carried by both negative and positive carriers, known as ions (electrically charged atoms or groups of atoms). The current carried by each ion depends on its mobility and electric charge. The total of positive and negative current in the electrolyte of a cell is always exactly equivalent to the total current carried in the metallic path by electrons alone. Ohm ’ s law — that is, I = E/R , where I is the current in amperes, E the potential difference in volts, and R the resistance in ohms — applies precisely, under conditions with which we are pres- ently concerned, to current fl ow in electrolytes as well as in metals.

The electrode at which chemical reduction occurs (or + current enters the electrode from the electrolyte) is called the cathode . Examples of cathodic reac- tions are

2 Cu 2 + → Cu −e 2 − Fe 3 + → Fe 2 + −e −

all of which represent reduction in the chemical sense. The electrode at which chemical oxidation occurs (or + electricity leaves the electrode and enters the electrolyte) is called the anode . Examples of anodic reactions are

Zn → Zn 2 + + 2e − Al → Al 3 + + 3e − Fe 2 + → Fe 3 + +e −

These equations represent oxidation in the chemical sense. Corrosion of metals usually occurs at the anode. Nevertheless, alkaline reaction products forming at the cathode can sometimes cause secondary corrosion of amphoteric metals, such as Al, Zn, Pb, and Sn, which corrode rapidly on exposure to either acids or alkalies.

In galvanic cells, the cathode is the positive pole, whereas the anode is the negative pole. However, when current is impressed on a cell from a generator or an external battery — for example, as in electroplating — reduction occurs at the electrode connected to the negative pole of the external current source, and this electrode, consequently, is the cathode. Similarly, the electrode connected to the positive pole of the generator is the anode. It is perhaps best, therefore, not to

T YPES OF CELLS

remember anode and cathode as negative and positive electrodes, or vice versa, but instead to remember the cathode as the electrode at which current enters from the electrolyte and remember the anode as the electrode at which current leaves to return to the electrolyte. This situation is true whether current is impressed on or drawn from the cell.

Cations are ions that migrate toward the cathode when electricity fl ows through the cell (e.g., H + , Fe 2+ ) and are always positively charged whether current is drawn from or supplied to the cell. Similarly, anions are always negatively

charged (e.g., Cl − , OH − , SO 2 4 − ).