In nature, iron is found as oxides and in minerals where it is combined with silicon or sulfur. The soluble iron content of surface waters rarely exceeds 1 mg/L, while ground waters often contain higher concentrations due to leaching from the surrounding strata.

The National Secondary Drinking Water Standard for iron is 0.3 mg/L, as iron concentrations in excess of that level impart a foul taste and cause staining of laundry and porcelain surfaces.  High concentrations in surface waters can indicate the presence of industrial effluents or runoff from mining operations.  In systems which use iron piping, an iron concentration in the process stream may indicate corrosion.

Iron, Total and Soluble  and Iron, Total and Ferrous, using the Phenanthroline Method
Ferrous iron reacts with 1, 10-phenanthroline to form an orange-colored chelate.  When determining total iron, thioglycolic acid solution is added to reduce ferric iron to the ferrous state prior to reaction with the phenanthroline reagent.  The reagent formulation minimizes interferences from various metals.  Results are expressed as ppm (mg/L) Fe.

Iron, Total, using the PDTS Method
The colorimetric method for determining total iron uses thioglygolic acid to dissolve particulate iron and to reduce ferric iron to the ferrous state.  Ferrous iron then reacts with PDTS (3-(2-pyridyl)-5,6-bis(4-phenylsulfonic acid)-1,2,4-triazine disodium salt) in acid solution to form a purple-colored chelate.  Results are expressed as ppm (mg/L) Fe.