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Electrode potential is the combined potential of two electrodes that undergo reactions in a cell. Typically measured in volts, it can differ based on the substances the electrodes are made of. Hydrogen is often considered the standard, with a potential of zero volts, from which the reactions of other elements are compared to. Electrochemical processes can be predicted by understanding the electrode potential of a material, as can the chances of it corroding under particular circumstances. Also called Electromotive Force (EMF), this potential is generally measured using a galvanic cell, which consists of two chambers, each with an electrode and connected by a piece of paper called a salt bridge.
The electric current of a cell is typically measured by a voltmeter. There may be no current; the difference in potential between the electrodes, in this case, is usually the same as the EMF of the entire cell. The EMF is generally measured at a temperature of 77°F (about 25°C), and a typical sea level atmospheric pressure. Such conditions are used to gauge the standard electrode potential in a two half-cell configuration. A hydrogen electrode is typically on one side of this layout, while the potential of the one on the other side is usually determined by experiment.
Once this standard value is derived, then it can be predicted how a substance will react. Usually, the higher a negative value of the potential, the more likely the element will dissolve in a solution. No reaction is often seen in non-reactive metals. These are typically placed at the bottom of a standard electrode potential table, which identifies the EMF of different types of elements. The elements are generally arranged in order of their tendency to react, so positively charged ones are higher up because they react more often than negatively charged substances.
Standard conditions aren’t always available, so a calculation called the Nernst equation can be used to determine the electrode potential. It accounts for the temperature, a gas constant, how many particles called electrons are transferred, and ion concentration. While hydrogen is the standard reference, elements often used for electrodes include zinc and copper, iron, aluminum, or calcium.
The electrode potential of each of these elements is used to calculate a galvanic cell’s voltage. A separate calculation of atomic half-reactions is sometimes used to derive the EMF as well. The process of determining the potential of the electrodes can therefore require an understanding of scientific principles as well as mathematics.