Ok there’s this question on my chemistry review that I can’t figure out. It states:

A sample of well water is known to contain a high concentration of iron. what solution could you use to test the water to get a positive precipitate test for the dissolved iron? I don’t understand it because how can the iron dissolve in water?

I tried writing that NaOH would be able to create a precipate with iron because FeOH would be insoluble together but Na is more reactive than Fe so that wouldn’t displace.

Can someone explain the answer to me (by the way the teacher wrote that there are many solutions and NaOH is one of them)


closed as off-topic by Mithoron, Todd Minehardt, A.K., Jon Custer, Jan Dec 6 '18 at 16:50

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  • $\begingroup$ Have a look here for a specific test for dissolved iron wiredchemist.com/chemistry/instructional/laboratory-tutorials/… $\endgroup$ – Waylander Dec 5 '18 at 18:04
  • $\begingroup$ I'm voting to close this question as off-topic. You have multiple issues understanding what is being asked of you that would require a rather elaborate answer touching on too many topics. Furthermore, it is best if your teacher clears your misconceptions. $\endgroup$ – Jan Dec 6 '18 at 16:50

Iron dissolved in water is $\ce{Fe^{2+}}$. It can be present as $\ce{FeS,FeCO3,FeSO4.7H2O}$, but (for natural water sources) usually as $\ce{Fe(HCO3)2}$. In order to precipitate iron, you may use any method used to remove $\ce{Fe^{2+}}$ from water (so you should somehow obtain $\ce{Fe^{3+}}$, which will precipitate in $\ce{Fe(OH)3}$ form). Industrial water treatment usually offers two options:

  1. Oxidation using $\ce{O2}$: $\ce{4Fe(HCO3)2 + O2 + 2H2O = 4Fe(OH)3 + 8CO2}$

Sometimes (especially when water is really "high" in iron), you can simply observe water in the bucket "rusting" with time ($\ce{Fe(OH)3}$ has brown-yellowish colour). Industrial methods use aeration columns - this is much faster process than "wait for water in the busket".

  1. Oxidation using iron removers (they may contain $\ce{MnO2, KMnO4, H2O2, O3,}$ etc.)

One of examples is $\ce{4Fe(HCO3)2 + 3MnO2 + 2H2O = 4Fe(OH)3 + MnO + Mn2O3 + 8CO2}$.

So, basically, there are two options:

  1. Open reservoir (or put $\ce{O2}$ there). Wait.
  2. Add something.
  • $\begingroup$ There's elementary misconception in OP question - thinking iron was supposed to be metallic. Another thing is that counterion is pretty much irrelevant, you'd'n't make mistake with FeS without this discussion. $\endgroup$ – Mithoron Dec 5 '18 at 23:30

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