This chapter studies electrochemistry. The cell potential is very simply related to the Gibbs energy change of the reaction taking place in the cell. Measuring the cell potential is a straightforward process: attaching a digital voltmeter to the cell enables one to make a precision measurement in seconds. As a result, such measurements give access to thermodynamic properties in a particularly simple way. Since cell reactions generally involve ions, these measurements of cell potentials are especially useful for finding the standard thermodynamic functions of ions in solution. Data derived from cells, in the form of standard electrode potentials, are very useful for understanding the relative oxidizing or reducing power of different species. The cell potential is related in a simple way, given by the Nernst equation, to the concentration of the species in the cell. This leads to important practical applications in which cell potentials are used to measure concentrations.