Editing Chemical oxygen demand (section)

===Measurement of excess===
An excess amount of potassium dichromate (or any oxidizing agent) must be present for all organic matter to be completely oxidized. Once oxidation is complete, the amount of excess potassium dichromate must be measured to ensure that the amount of Cr<sup>3+</sup> can be accurately determined. To do so, the excess potassium dichromate is [[titration|titrated]] with [[ferrous ammonium sulfate]] (FAS) until all of the excess oxidizing agent has been reduced to Cr<sup>3+</sup>. Typically, the oxidation-reduction indicator [[ferroin]] is added during this titration step. Once all the excess dichromate has been reduced, the ferroin indicator changes from blue-green to a reddish brown. The amount of [[ferrous ammonium sulfate]] added is equivalent to excess potassium dichromate added to the original sample. Note: Ferroin indicator is bright red from commercially prepared sources, but it exhibits a green hue when added to a digested sample containing potassium dichromate. During the titration, the color of the indicator changes from a green hue to a bright blue hue and then to a reddish brown upon reaching the endpoint. Ferroin indicator changes from red to pale blue when oxidized.<ref>{{cite web|url=http://antoine.frostburg.edu/chem/senese/101/glossary/f.shtml|title=General Chemistry Online: Glossary|website=antoine.frostburg.edu}}</ref>