bolbol
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Mercury Oxide?
I recently acquired some Mercury metal and I decided to try and make the yellow oxide of it for my collection.
I made mercury nitrate with it using Nitric Acid and after it was done I added a solution of NaOH to it to get the yellow precipitate.
Weird thing that happened was that upon the addition of the base an immediate dark brown color appeared and it slowly turned yellow when shaking.
I noticed the reaction didnt come to completion so I kept adding more NaOH and suddenly everything turned to the original brown color. I had it sit
for 5 minutes and now it has turned an orange color which I believe is HgO as well with just a difference in particle size.
From Wikipedia:
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The red form of HgO can be made by heating Hg in oxygen at roughly 350 °C, or by pyrolysis of Hg(NO3)2.[5] The yellow form can be obtained by
precipitation of aqueous Hg2+ with alkali.[5] The difference in color is due to particle size, both forms have the same structure consisting of near
linear O-Hg-O units linked in zigzag chains with an Hg-O-Hg angle of 108°.[5]
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Anyone know what the brown colored compound may be?
Should I keep adding more NaOH and see where it goes?
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chem_haruka
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Did you use excess of nitric acid or not? If not ,under alkaline conditions ,Hg2 2+will decompose to Hg and HgO ,and tiny particles of Hg look as
black(mix with yellow ，it will seems as brown color).I think this is the reason.
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bolbol
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The nitric acid was actually in excess.
and wouldn't the concentration of nitric acid be too low to react with the Hg? (The disappearance of the brown color)
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gdflp
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What I'm guessing is that some mercury(I) oxide formed, it is an unstable brown-black solid that spontaneously decomposes to HgO and Hg.
When mercury metal is dissolved in nitric acid, mostly mercury(II) nitrate is formed, but a small amount of mercury(I) nitrate is formed unless the
nitric acid is concentrated, in excess, and the solution is boiled after the dissolution until no NOx is released. This will ensure the
destruction of mercury(I) ions in solution to their thermal instability.
I can't find the Ksp values of either mercury(I) oxide or mercury(II) oxide, but I'm going to guess that mercury(II) oxide has a lower
value, thus it precipitated first, you saw an orange yellow powder, and then upon further addition of sodium hydroxide, mercury(I) oxide came out of
solution and coated the mercury(II) oxide, causing the entire precipitate to appear brown/black. On standing, the mercury(I) oxide decomposed into a
small amount of mercury metal, and more mercury(II) oxide product causing the precipitate to appear orange/yellow again.
[Edited on 3-12-2015 by gdflp]
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bolbol
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That makes more sense.
Althought I didn't think I have to boil the solution to get rid of the Mercury (I) Nitrate. I thought just leaving it on heat would get rid of it. I
will try again with another sample of mercury in a few days to see if there is a change.
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Boffis
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I have precipitated a good deal of mercuric oxide as a result of recycling mercury during some research I have been doing on fulminuric and cyanilic
acids. If the mercury is entirely present as Hg2+ the oxide that precipitates is a pure brilliant yellow but traces of Hg+, as gdflp points out,
causing the colour to turn brown.
To avoid the formation of Hg+ you must boil the solution with a significant excess of strong (35%+) nitric acid. I obtained my details for the
preparation of mercuric nitrate from a book on inorganic preparations which I am pretty sure came from the SM library. It definitely states hot strong
nitric acid.
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