An electrochemical gradient is produced by the combination of an ions' concentration gradient and the electrical potential gradient across a membrane. This type of gradient affects only charged molecules and atoms as neutral molecules are not influenced by electrical potential differences.
As ions diffuse down their chemical gradient, the change in charge distribution across the membrane creates an electrical potential that affects the flow of other ions and charged molecules. The Nernst Equation can be used to estimate the electrical potential needed to counteract the chemical gradient of an ion and result in an electrochemical gradient of 0. In practice, it is unlikely that only one ion will be moving across the membrane, so the Hodekin-Katz modification of the Goldman Constant Field Equation can be used to more accurately portray the electrical potential gradient needed.