Why won't iron dissolve as easily in HNO₃ as it does in HCl?

I just completed a method development lab in which we looked further into the determination of iron using the dichromate method. Instead of using HCl (which we used in a previous iron determination), I used HNO3. The problem I encountered was that the iron did not dissolve well in the nitric acid. We also observed that the solution turned slightly black when adding 0.5 M stannous chloride. Could the nitrate ion have an effect on the oxidation/reduction of the iron and could it also oxidize the tin (II) to tin (IV)?
Jared Anderson

When you dissolve the iron in HCl, you'll get Fe²⁺ (at least, until the O₂ in air starts to work on it). It should dissolve in dilute HNO₃, but some of the iron will go over to iron(III). If you put iron in concentrated HNO₃, it becomes resistant to corrosion or dissolution. This process is called passivation. is some disagreement about how passivation works, but it probably involves laying down an oxide film of some kind.

As for the tin, probably. If the concentration of the nitric acid was high enough it shouldn't have any problem oxidizing tin(II) to tin(IV).

Author: Fred Senese senese@antoine.frostburg.edu

Copyright © 1997-2010 by Fred Senese
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Last Revised 02/23/18.URL: http://antoine.frostburg.edu/chem/senese/101/redox/faq/passivation.shtml