The Alchemy of Zinc

Back in the day, alchemists apparently spent all their time attempting to turn base metals into gold. I say apparently, since that is the common perception of alchemists. In reality, most alchemists were devoted to what was often referred to as the “Great Work.” According to alchemical theories, much of what is created by nature is imperfect, and by applying the art of alchemy, natural substances can be brought to a higher state of perfection. Since gold was the height of perfection for metals, it was thought that base metals could be transmuted into gold merely by removing these imperfections. The Philosophers Stone wasn’t just about making gold or the “Elixer of Life”, it was a process/device/concept for bringing about perfection.

Color was very important to the alchemists, perhaps since it was one of the few clues with which they had to go on during their experiments. Colors and color changes were rigorously recorded and eventually incorporated into many of the alchemical theories. Certain color sequences were expected during the path towards perfection. In the transmutation of base metals into gold, for example, a color sequence of black to white to yellow to purple was thought to be required. Later theories redefined the sequence as black to white to red. While this fascination with colors may have led them astray on occasion, it has also led to a vast array of wonderful, full color illustrations, intricately drawn and full of alchemical symbolism. Here is one such example.

An experiment often touted as reminiscent of alchemy and demonstrating the relative ease of generating color changes involves the apparent conversion of a copper penny into silver or gold. Simply dissolve some Zn powder in warm NaOH solution, toss in a penny, and watch the penny turn silver as a coating of Zn forms on its surface. Heating the penny turns it gold as the Zn and Cu alloy to form brass. (My kids were duly impressed when I performed this experiment for them, informing me that it was “awesome”, but considering they said the same thing when I turned phenolphthalein red, the “awesomeness” bar may be set pretty low.)

But why does this work? I’ve coated pennies with silver and mercury before, but zinc is more electropositive than copper. Based on electrochemical potentials, zinc shouldn’t plate out on copper -- copper should be plating out on zinc. Apparently I’m not the only person to wonder about this, as web pages devoted to this effect can be found throughout the Internet, for example, here and here.

First of all, Zn dissolves in NaOH to make zincate ion and hydrogen.

Zn + 2OH^- + 2H₂O ----> Zn(OH)₄^2- + H₂

Okay, that’s straightforward chemistry. But how does the copper reduce the Zn(OH)₄^2-? The answer is that it doesn’t. Surprisingly, copper is not the reductant . Copper is not oxidized and it does not go into solution. Copper’s role is to create a galvanic cell with the undissolved portion of the zinc powder. Zinc does not plate out on the penny until the copper is in direct contact with zinc metal. It turns out that zinc metal is the reductant, which sounds bizarre, at least to me. Once the zinc and copper metals are in contact and the galvanic cell is created, the zinc begins to oxidize…

Zn + 4OH^- ----> Zn(OH)₄^2- + 2e^-

And its electrons are now available to reduce the zincate ion near the surface of the penny...

Zn(OH)₄^2- + 2e^- ---> Zn + 4OH^-

So the overall reaction is:

Zn + Zn(OH)₄^2- ----> Zn(OH)₄^2- + Zn

Basically there is no net reaction! Which means the zinc is essentially just migrating from the surface of the zinc powder to the surface of the penny. At first glance, this would appear to violate the laws of thermodynamics, but obviously it doesn’t. I assume there is some sort of entropy effect here, perhaps related to the galvanic cell, but I don’t know the exact cause. In any case, this reaction appears just as mystical as alchemy itself!

If anyone happens to know any more about this process, I’d be happy to hear from you.

Home Grown Titanium

Although my current lack of posting might not indicate it, I've actually come across quite a few interesting articles lately. Unfortunately, they all involve nanochemistry, and since I've already talked too much about nanochemistry recently, I'm imposing a moratorium on that subject for awhile. Fortunately, I do have something else to talk about.

The Goldschmidt reaction.

Not familiar with the Goldschmidt reaction? Perhaps you have heard of it referred to as "the thermite reaction."

Surely everyone who has ever taken a freshman chemistry course has read or heard about the thermite reaction. Most of you have probably seen it in action. The pyrotechnics are impressive and most freshman chemistry lecturers simply cannot resist demonstrating it in front of a class. In its most common incarnation, aluminum and Fe2O3 (or Fe3O4) powders are mixed and ignited. The aluminum is converted to Al2O3 while the iron oxide is reduced to the metallic state. Significant quantities of heat are released, and if the experiment is set up correctly, molten iron will drip out of the bottom of the reaction vessel. Although iron oxide is the material most associated with the thermite reaction, copper and manganese oxides can also be used.

In a continuation of "the type of experiments I'd like to try at home when my wife is away" category, I recently came across a method for generating titanium metal in your garage using TiO2 and the thermite reaction. A full description of the technique as well as a video of the pyrotechnics are included. Metallic titanium was actually recovered, which is amazing since titanium tends to oxidize in air at temperatures near its melting point. In order to generate the temperatures necessary to melt the titanium, CaSO4 was added to generate additional heat. CaSO4 reacts with aluminum in its own version of the thermite reaction to form CaS. A more detailed description of the process involved can be found here.

This would have been an awesome experiment for alchemists to have performed back in the day. Simple, yet impressive. Perhaps the substitution of iron oxide with some form of gold oxide (or other suitable gold compound) might have resulted in the appearance of molten gold, always a good way to impress the wealthy patrons upon whom the alchemists depended. Unfortunately, although aluminum salts were known to the alchemists as far back as ancient Greece, aluminum metal was not produced until the 1800's. And it's the chemical energy stored in the metal which drives the whole reaction.

Chemical Symbolism

So, they are now making nano particles from volcanic lava.

Yes.

From lava.

Really.

When is this nano madness going to end?

Okay, so perhaps I'm exaggerating here a little bit. It would be more correct to say that Dang Sheng Su and his team are making nano particles using volcanic lava. Lava obtained from Mt. Etna in Sicily contains nanosized Fe2O3 particles, which can be reduced to iron nanoparticles at 700C in the presence of hydrogen. These iron particles are apparently good templates for the production of carbon nanotubes and nanofibers when exposed to ethylene and hydrogen at high temperatures.

I know I tend to rag on nano-technology a bit too much on this blog. My first post on nanoparticles wasn't too complimentary. But I've been coming around lately as I read more and more articles on the subject. Much of the research may eventually just turn out to be hype (which can probably be said about any emerging field), but it certainly seems as if there is a lot of potential in this area. Who knows? Maybe I'll be offered a job in nanotechnology sometime in the future. I wouldn't want my views on this blog to limit my opportunities. :)
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Back in 1803, John Dalton developed a series of atomic symbols for some of the known elements at that time. Here is his representation of the table of elements. It cannot be called a periodic table yet since it doesn't demonstrate the periodicity of the elements.

Picture obtained from Wired

Now it's not like there wasn't already a series of symbols for most of these elements already. Alchemists had been using a set of elemental symbols for centuries before Dalton came along, but a lack of standardization was a significant problem, especially since alchemists often used many different symbols for the same element. Certainly, part of the problem was due to the lack of a central authority (IUPAC hadn’t been invented yet). There are at least 20 different versions of the symbol for gold that I know of, and probably a lot more. But the biggest roadblock to standardization among the alchemists was their need for secrecy. Treatises by alchemists were often ambiguous and confusing, apparently to confuse the competition. For example, alchemists often used the term "mercury" for the element mercury, the element gold, the liquid fraction of certain reactions, and various metaphysical concepts such as spiritual goodness. You really need a course to understand what these alchemists were trying to say.

I'm guessing that Dalton was doing his part to disassociate chemistry from the art of alchemy by refusing to use any of the common alchemical symbols of the time. Ironically, his symbol for hydrogen is the same as (one of) the alchemical symbols for gold. Remove the circle from the symbol for phosphorus and you have the alchemical symbol for phosphorus. His symbol for sulfur is the same as the alchemical planetary symbol of earth. Nevertheless, it was a start. It wasn't until Berzelius started using letters based on the Latin version of the element names that everything became manageable. Can you imagine writing out chemical formulas for large organic molecules using Dalton's symbols? If that wouldn't drive you toward Inorganic chemistry, then nothing would. ;P