Internal indicators that will change color with a change of oxidation-reduction potential (ORP) are commonly used. Such indicators are usually soluble organic substances that exist in two states of oxidation, the two forms being in equilibrium and of markedly different color. An example is Ferroin (ferrous 1,10-phenanthroline sulfate), which is used to indicate when sufficient ferrous ammonium sulfate titrant has been added to measure excess dichromate ion in the chemical oxygen demand (COD) test. It should be mentioned that selection of an ORP indicator depends upon the ORP at the stoichiometric end point for the particular reaction involved, just as the selection of an indicator for acidity or alkalinity depends upon the pH of the solution resulting at the equivalence point.