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Current IssueOctober 30, 2014
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Ask The Expert: Revisiting iron in water
06/08/2001 
 
Question: Being a water softener rep, I'm constantly running into customers that have been told by competitors that they have "ferric iron." Most of the time, there are no particles of iron at all. Could you please direct me to a source of independent information that explains just what this iron is and, more importantly, how to identify it!  

Answer: Your question is one that's confusing to many people, partly because the kinetics of iron oxidation in air are relatively rapid. Keep in mind—when iron is dissolved in water it is usually in the "clear" or ferrous state. When the iron is oxidized it becomes a suspended solid and turns the water red. This is the ferric state. You don't have ferric iron in clear water. The competitor who states there's iron in the water is probably telling the truth, but it's like saying the water is wet. Most groundwater is anoxic—that is, devoid of oxygen—and, as a result, the iron in the groundwater is predominately ferrous, or soluble iron. Ferric iron is formed when the iron is exposed to oxygen (or other oxidants such as chlorine). If the water is withdrawn, it's surely exposed to air; but there's air exposure in most wells through the vent hole. In some cases, although you don't see particles as such, you may notice a slight discoloration to the water. You could probably notice an elevation in the measured turbidity. If you look at a sample of water with some depth, you may notice a tint of yellow or orange; but iron can have other colors, too. The easiest way to determine the difference is to filter the sample through a membrane filter (.22 micron is best, but .45 will probably do) and then measure the iron in the filtered sample. Provided that sample isn't exposed to air, it should give you a relatively accurate reading of the ferrous iron. You could also take the sample and reduce the pH to below about 4 with an acid. This will reduce and redissolve the ferric iron into ferrous form. It is for this reason that most samples of water collected for iron testing have a small amount of an acid in the sample bottle. Iron can be present in other forms as well, for example, as complexes with organic molecules or tied up with microbes. It has been a few years since we did a comprehensive article on iron. It may be time for another look at iron in the not too distant future.

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