I'm trying to produce Iron (III) Nitrate by way of high-purity iron reagents, and nitric acid. Concentrated acid passivates the metal, so I'm trying to use a dilute solution; something in the 8.75% or (very roughly) 2M area.
My problem is that I am simply not dissolving as much metal as I think I should be. Not by an order of magnitude or so. Maybe my calculations are wrong, or maybe I'm failing to understand something.
I have: Nitric Acid, 68%-70% - roughly 15.8M. I have diluted it in an equal volume of water, which should get me to 35%, or 7.9M. I diluted THAT solution, 50mL in water, to a total volume of 200mL. (I failed to allow for volume gain or loss while mixing, but for these purposes, close enough is good enough) I should now have 8.75% nitric acid, or 1.975M (call it 2M).
200 mL of a 2M solution should contain 0.4 moles of nitric acid
+ NO + 2H2
4 moles of nitric acid should dissolve 1 mole of iron. 0.4 moles nitric acid should dissolve 0.1 mole of iron.
0.1 mole of iron is 5.5g.
My 200mL of dilute acid has failed to dissolve 0.5g of iron. I haven't weighed out the remaining material yet, because I'm hoping it will suddenly realize what I'm trying to do, and dissolve in a fit of helpfulness, but that's kind of a long shot.
I know I can't expect a complete reaction, but is there any way to get closer then this? Alternately, am I making some damn stupid mistake in my math, or my thinking?
Thanks for any help.