Editing Chemical oxygen demand (section)

==Preparation of ferroin indicator reagent==
A solution of 1.485 g 1,10-[[phenanthroline]] monohydrate is added to a solution of 695&nbsp;mg FeSO<sub>4</sub>·7H<sub>2</sub>O in distilled water, and the resulting red solution is diluted to 100 mL.

===Calculations===
The following formula is used to calculate COD:

:<math>\mathrm{COD} = \frac{8000 (b - s)n}\text{sample volume}</math>

where ''b'' is the volume of FAS used in the blank sample, ''s'' is the volume of FAS in the original sample, and ''n'' is the normality of FAS. If milliliters are used consistently for volume measurements, the result of the COD calculation is given in mg/L.

The COD can also be estimated from the concentration of oxidizable compound in the sample, based on its stoichiometric reaction with oxygen to yield CO<sub>2</sub> (assume all C goes to CO<sub>2</sub>), H<sub>2</sub>O (assume all H goes to H<sub>2</sub>O), and NH<sub>3</sub> (assume all N goes to NH<sub>3</sub>), using the following formula:

:COD = (''C''/FW)·(RMO)·32

Where 
:''C'' = Concentration of oxidizable compound in the sample,
:FW = Formula weight of the oxidizable compound in the sample,
:RMO = Ratio of the # of moles of oxygen to # of moles of oxidizable compound in their reaction to CO<sub>2</sub>, water, and ammonia

For example, if a sample has 500 Wppm (Weight Parts per Million) of phenol:

:C<sub>6</sub>H<sub>5</sub>OH + 7O<sub>2</sub> → 6CO<sub>2</sub> + 3H<sub>2</sub>O

:COD = (500/94)·7·16*2 = 1192 Wppm