Q&A About Mercury

2009-04-04 21:56 by Ian

Hey I just read that mercury will not react with alkalies. I have a water ionizer that can make the water up to 10-11ph using only electricity. Do you think this water would keep mercury from oxidizing or is there still just too much oxygen in water due to the fact that one of its molecules is oxygen?

I don’t know if mercury will react with bases. I would imagine that it won’t react with hydroxide, at least, unless it is already oxidized. Mercury metal is neutral, IE, it has an oxidation state of zero. When a metal reacts with something, there is an electron transfer from the metal to the other reactant. When electrons leave the metal, the metal is no longer neutral, IE, its oxidation state increases. That’s what is being expressed when you see things like…
Na+ (Sodium lost one electron)
Cu+2 (Copper lost two electrons)

Those electrons that were lost by the metal are taken by some other reactant that is more electron-greedy than the metal. Usually this reactant is something like oxygen or chlorine…
Cl- (Chlorine received one electron (from sodium))
O-2 (Oxygen received two electrons (from copper))

Different metals like to donate a fixed number of electrons. For instance, copper will only donate 1 or 2 electrons. Sodium will only donate 1. You can read the metal’s wikipedia page. On the right-hand side will be a list of the metal’s oxidation states. If you look at mercury, you will find that mercury only likes to donate 1 or 2 electrons. So its oxidation states will only ever be…
0 (neutral metal (what I shipped you))
+1 (Hg lost one electron)
+2 (Hg lost two electrons)

Now let’s consider your water ionizer… It works by forcing electrons into the water. By doing this, it reduces (adds electrons to) hydrogen. This makes the hydrogen neutral. When hydrogen is bound to oxygen in water, the hydrogen is acting like a metal and donating its only free electron to the oxygen. When your water ionizer takes electrons out of the wall socket and gives them to hydrogen, the hydrogen bonds to other neutral hydrogen and forms H2 (hydrogen gas) which then bubbles out of the water. The reason it forms hydrogen gas, instead of giving up its electrons to more oxygen in the water is because all of the oxygen in the water already has all the electrons it can take! So consider two water molecules…
2(H2O)

Then your ionizer adds two electrons…
2(H2O) + 2e-

Then we add the release of hydrogen gas, and we balance the equation….
2(H2O) + 2e- → H2 + 2(OH-)

That OH- is what raises the pH of the water. It is called a hydroxide ion. Now we have alkaline water. The oxygen in the hydroxide ion has all the electrons it can take, but it still wants to be electrically neutral. So it will seek out positive ions (like Cu+) and form a new molecule (like CuOH). But in order to do this, the metal has to already be oxidized, because the oxygen can’t take any more electrons from anything.

Now back to mercury. If the mercury is already oxidized (IE, already a salt, or already in a compound), then, yes. The hydroxide ion will probably react with it and form either HgOH or Hg(OH)2, depending on how far the Hg is oxidized.

Now putting the Hg in acid… Acids have lots of free H+ ions. And that means that neutral metals will have another atom to take their electrons…
Hg + H+ → Hg+ + H

This is why metals placed in acid will typically release hydrogen gas and form salts.

Acids are usually oxidizers and bases are usually reducers. The best way to answer your question for sure is to take a small droplet of mercury and drop it into the alkaline water. If you see bubbles come off the Hg, then I am willing to bet that the bubbles are hydrogen and that the product is HgO (mercury (II) oxide). If you want to keep the Hg from oxidizing, you are best off storing it under pure nitrogen or (even better) pure argon. These gases are so nonreactive that the Hg will for all intents and purposes remain neutral indefinitely.

Water is a terribly reactive substance. Storing mercury under water will prevent the mercury from evaporating away, but HgO will eventually form.

Did that answer the question?

—Ian

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