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Manganese: To the Next Level

[Above: People say, you know, that I’m an idiot or something because all I do is cut lawns for a living, you know? Well, I’m working on it, you know, so I don’t ever have to-- you know, I’m gonna be the head greenskeeper-- hopefully within six years. That’s my--my schedule. But I’m studying a lot of this stuff so I know it, you know. Like, uh, you know... chinch bugs. You know. Manganese. Lot of people don’t even know what that is.]

Johan Gottlieb Gahn discovered manganese in 1774. Personally, I feel that Torbern Bergman and Carl Wilhelm Scheele deserve partial credit, but I'll let you be the judge.

Now I can't find much in the way of sources on this, but Wikipedia says that the name "manganese" owes to the discovery of certain minerals in the land known as Magnesia.

Complicating matters is the fact that there's more than one place by that name, so even the geography is confusing. Let's back up a little further, since we're already on a tangent here.

In the origin myth of the Greek culture, the king of the gods, Zeus, was rather notorious for siring children with human women. Three of these children were Graecus, Makedonos, and Magnes, who became the first leaders of the Greek, Macedonian, and Magnesian peoples. Famous Magnesians include Jason (of Argonaut fame), and Achilles, the hero of the Trojan War.

There's a regional unit of modern-day Greece known as Magnesia, so I would assume this was the site of the Magnesian homeland, the first location to bear the name. Beyond this, the ancient Magnesians established at least two colonies in what is now western Turkey: Magnesia ad Sipylum, now knowns as Manisa, and Magnesia on the Maeander, whose ruins were first excavated in 1891.

A number of minerals were also named after Magnesia, and so it was assumed that these minerals were first discovered in one of the lands that bore the name. Or perhaps they were named for the Magnesian people themselves, or it's all a big coincidence. To be blunt, I find the entire situation ridiculous. With all this stuff bearing the same name, it's impossible to sort out what was intended to honor what. At least, I would assume the purpose was to pay tribute to something. If the real idea was to confuse people, then well done.

Anyway, these were the minerals that were named after Magnesia:

Magnetite: This is an iron oxide mineral which can sometimes become naturally magnetized. Naturally magnetic magnnetite is called lodestone, and this was where humanity first encountered the property of magnetism, hence the name.

Magnesia alba: This is a white oxide of magnesium, atomic number 12. The "alba" part of the name comes from the Latin word for "white", and this was added to distinguish the white magnesium oxide from...

Magnesia negra: A dark colored oxide of manganese, atomic number 25. This mineral was also known as "pyrolusite" and in the 1540 metallurgical text De la pirotechnia, it was called Braunstein for it's dark brown color.

There were two important uses for manganese dioxide. Just as cobalt oxide could be used to tint glass blue, and nickel would give glass a green color, manganese dioxide would turn glass purple. So that's pretty cool.

[Above: Two glass Whitall Tatum insulators, colored purple by manganese. Yours for just $41.]

But there was more. See, medieval glassmakers were working with raw materials that contained trace amount of iron oxides, and this meant that their glass would end up with a greenish tint, even without using any additives to color it. However, if they added manganese dioxide to the glass while it was still molten, the glass would come out completely colorless. And so glassmakers would call the mineral by yet another name: sapo vitri, which is Latin for "glass soap".

So yeah, manganese can't catch a break when it comes to names. "Magnesia negra" doesn't tell you anything, because you can find it in all sorts of countries besides Greece. "Braunstein" just means "brown stone", which isn't very specific at all. Now we have "glass soap"? But it isn't glass, and it's not soap. Sure, it cleans glass in a manner analogous to soap, but I wish they'd have just gone with the purple tinting thing and named it "Purplesaurus Rex." At least it would have been distinct.

According to Mary Lee Weeks in "Discovery of the Elements", manganese may have been isolated in 1770 by the Austrian chemist Ignatius Gottfried Kaim, who published his findings in his dissertation "De metallis dubiis". He heated powdered manganese dioxide with "black flux", which I understand to be a mix of carbon and potassium carbonate. Weeks complained that Kaim's publication was rare and inaccessible, but I found it on Google Books surprisingly quickly. Just goes to show what a difference fifty years makes. Anyway, it's in Latin and I don't think I'm going to find anything important that Weeks hadn't already revealed. Kaim's method sounds very similar to what Gahn did a few years later, so it's at least plausible. But no one paid much attention to Kaim's work, so he didn't get the credit. I will note that Kaim used the term "magnesia vitrariorum", which was yet another name for manganese dioxide.

I'm not sure just when the extra 'n' came into use. Wikipedia explains that it was surely a deliberate corruption of the word "magnesia", because both the black and white minerals were useful to alchemists and glassmakers, and they found the similar names just as intolerable as I do. What I do know is that it was still being called "magnesia negra" all the way up to 1774, so it probably took the discovery of the element to get people to stop confusing it with the other guy.

Eventually, the Swedish chemist Torbern Bergman took up the study of "black magnesia" and asserted that it had nothing whatsoever to do with the white magnesium oxide. As he said:

"The mineral called black magnesia is nothing other than the calx of a new metal, which must not be confounded with lime nor with magnesia alba."

Aside from Kaim's overlooked dissertation, this seems to be the first hint of the truth behind "black magnesia". By now, chemists had begun to notice that certain minerals resembled the calxes of known metals. Cobalt and nickel were discovered in this fashion, and while some scientists were skeptical of new metals, Bergman had been a supporter of the nickel discovery, and lobbied his peers in Sweden to accept it. So when a new metal oxide came to Bergman's attention, he had the insight to recognize it, and the open-mindedness not to look the other way.

For instance, neither lime (calcium oxide), nor magnesia alba (magnesium oxide) had been successfully reduced into metals, but they were still recognized as calxes. They resembled other calxes, but not completely, nor did they perfectly resemble each other, as Joseph Black concluded in 1755. So even though Bergman couldn't isolate the metal in "black magnesia", he could still say with some confidence that it was something new.

Unable to isolate manganese himself, Bergman handed it off to his friend and colleague Carl Wilhelm Scheele. Scheele studied "black magnesia" for three years, and while he also failed to isolate the metal, he still learned a great deal about its chemistry, and he published his findings in 1774. Bergman submitted a supplement to Scheele's paper, which explained some of the theories surrounding manganese's true nature.

"After he [Scheele] had finished these experiments, I informed him that Mr. Sage supposes manganese to be nothing else than a mineralised mixture of cobalt and zinc. He immediately made several experiments for this purpose, but found not the least mark of either of those metals. Manganese has been classed by all mineralogists among the iron ores. Mr. Pott, however, thought the iron to be mixed only accidentally; and at last Mr. Cronstedt, in his Essay on Mineralogy 1758, placed it among the earths."

So Bergman was convinced that manganese was an undiscovered metal. Scheele's research had shown no signs of cobalt or zinc, and if it had been iron all along, why didn't iron compounds have the same effect on glass? I think Cronstedt's suggestion was that "black magnesia" contained no metal whatsoever. That is, he wondered if the reason Bergman and Scheele couldn't isolate a metal was because there was simply nothing to isolate. But that didn't wash with Bergman, who replied:

"No pure earth colours glass; but all metallic calxes have this property. Manganese, therefore, in this respect shows a great resemblance to the latter, which is further increased by its specific gravity, and its strong attraction for phlogiston."

Bergman was willing to concede, however, that it might not be a brand new metal. Platinum had been introduced to Europe only a few decades earlier, and in that time no one had been able to find or produce a platinum oxide. So perhaps magnesia negra had been this elusive platinum oxide all along, and the two mysteries would solve each other. In any event, Bergman knew there was a discovery to be to be made in "black magnesia", and Scheele's 1774 publication supported this assertion.

Later that same year, Bergman's assistant, Johan Gottlieb Gahn, got involved. He took a crucible, lined it with wet charcoal dust, then put powdered "black magnesia" mixed with oil in the middle. He then added more charcoal dust to cover the "black magnesia". Then he put another crucible on top of the first one, and sealed them together. After heating this container intensely for an hour, he opened the thing up and found a lump of metal.

But Gahn never published his work. We only know about it because Scheele referred to it in his correspondence with Gahn. This can be found in the book "Efterlemnade bref och anteckningar," which is apparently a collected volume of Scheele's letters. From these, we see that Scheele sent Gahn a sample of purified pyrosulite, and looked forward to seeing what Gahn could do with it. That letter was dated May 16, 1774. In the next letter to Gahn, dated June 27, 1774, Scheele thanks Gahn for sending him a manganese regulus, indicating that Gahn's efforts were successful. Worth noting is that Scheele referred to the metal as "regulus magnesiae", so the extra "n" still hadn't taken hold just yet.

In 1775, Torbern Bergman edited a book of chemical lectures by H. T. Scheffer. It was in his annotations that he announced Gahn's discovery of manganese to the world. From p. 483 of the translation by J. A. Schufle:

"Note 3. Without now going through the 14 metals, we have lately gotten knowledge of a fifteenth [metal]. That something metallic is found in brunsten, which makes up its essential part, I have reported in KVAH [Kunglige Vetenskaps Akademien Handlingar: Transactions of the Royal Academy of Science (of Sweden)] proof of its density, its power of coloring glass, and its precipitation with bloodlye. Mr. J. G. GAHN, not knowing of my reports, has at the same time actually brought out of it by reduction a semi-metal, which in tensile strength approaches nearest to platina, and on the other hand does not resemble any of the earlier known [metals], whereupon he soon learned to publish his research. I have also afterwards obtained the regulus of brunsten by reduction, but could not purify it from iron; it is called Magnesium, to avoid excess verbiage, and to distinguish it from the earth magnesia."

Apparently Bergman had already settled on a name, and I have to wonder if Gahn or Scheele were consulted before he went public with it. For whatever reason, it didn't stick, and the word "magnesium" would end up being used for Element 12 once it was discovered. I don't know when or why the name was changed, but I do know that Antoine Lavoisier's 1789 list of "Simple Substances" called it "manganese". Lavoisier's nomenclature system became very influential, so if he didn't coin "manganese" he probably helped cement it as the accepted name.

Like I said at the beginning, I'm dubious that Gahn alone deserves the credit for discovering manganese. He certainly isolated it all by himself, but he only worked on the problem because Scheele supplied the purified ore, and because Scheele and Bergman had so thoroughly convinced themselves that there was a metal in there for Gahn to isolate. To me, that sounds like a collaboration.

As for the isolation itself, Weeks praised Gahn's technique in "Discovery of the Elements," but I'm not sure I'm quite as impressed.

I've covered this many times before, but metals like copper can be isolated from their oxides by heating them in the presence of a reducing agent, such as carbon monoxide.

CuO + CO → Cu + CO2

The basic idea is to heat up the metal oxide while it's in the presence of a substance that craves oxygen even more than the metal does. The carbon monoxide gas surrounds the copper oxide, and yanks apart every copper-oxygen bond it can get at. This forms carbon dioxide, a much more stable molecule. Since carbon dioxide is so stable, it won't react to the metallic copper left behind. The reaction chamber can then be cooled to room temperature and the copper metal collected for use.

Copper's just about the easiest metal to do this with, if we don't count metals like mercury which bond to oxygen so weakly that the process isn't even needed. Humans probably learned to smelt metal by playing with copper ore in a campfire, because a campfire is hot enough to melt the copper metal as it's produced, thus exposing fresh oxide surfaces to the reaction.

Things became more challenging when humans tried to smelt iron ore. This is because iron has a much higher melting point, and it was impractical to smelt it using the same method that had worked so well for copper, lead, and tin. Humans got around this problem by inventing the bloomery, which smelted powdered iron oxide in solid state. The process reduced the iron oxide to a porous mass called a "bloom", which could then be hammered into a more solid shape later.

Now I don't think carbon monoxide was on anyone's radar in the late 18th Century. Humans had been using it to smelt metals for millennia, but they probably didn't understand how it worked. You could simply add charcoal to the reaction vessel and it would produce carbon monoxide by reacting to the limited supply of oxygen in the sealed vessel. So I imagine a lot of alchemists and chemists simply credited the absence of fresh air and the presence of elemental carbon.

So if you adopted this method with manganese dioxide, and tried to smelt it with just enough charcoal to produce an blanket of CO, you'd get this:

MnO2 + CO → MnO + CO2

The calx is reduced all right, but only part-way. Instead of giving up both of its oxygen atoms, the manganese atom only gives up one. The carbon monoxide is converted to carbon dioxide, and so it's reducing power is used up.

Now, you might wonder why we don't just add more CO to force the reaction to keep going. But that's like riding two motorcycles at once and expecting to go twice as fast. It doesn't work that way. One CO molecule reacts with one unit of manganese dioxide to produce MnO and carbon dioxide. A second CO molecule would have no impact on this. It would simply find another manganese dioxide unit to react with. Any extra CO molecules left over at the end would simply do nothing.

To put it another way, carbon monoxide isn't a strong enough reducing agent to reduce manganese dioxide to the free metal. It could do the job with copper and iron, but this manganese a bridge too far. But why should this be?

The problem with manganese is that it's got more than one stable oxide, because it has multiple oxidation states. Like any element, atoms of manganese are surrounded by electrons, and the outermost shell of electrons, the valence shell, are the ones that define how manganese interacts with with the world. Manganese's valence shell is of principal quantum number 4, and it contains two electrons in an s-subshell. These electrons can be shared with other atoms to form chemical bonds.

Normally, having only two valence electrons would mean that an atom can only have a valence of +2. But manganese is a transition metal, which means that some of the inner electrons can get involved as well. For manganese, the next shell down is principle quantum number 3. This shell contains an s, p, and d subshell, and the d-subshell is the one that matters here. The energy level of this 3d subshell happens to be very near that of the 4s subshell. So near, in fact, that any electrons in the 3d subshell can participate in chemical bonds.

The d-subshell can hold up to ten electrons. In the case of manganese, only five electrons occupy the 3d subshell. This is an interesting situation, because a half-full subshell is rather stable. This puts manganese in a very versatile situation for chemical bonding. It can share one or two of its 4s electrons, or it can share some or all of its five 3d electrons, or it can share all seven of them at once. So manganese can exist in oxidation states anywhere from -3 to +7, although some of these are easier to achieve than others.

To be sure, iron is a transition metal as well, but its electron configuration has six d-electrons, which is a little more than half-full, and so this places limits on the oxidation states it can realistically achieve. Generally speaking, iron prefers to share only two or three electrons at a time. This leaves five electrons in its d-subshell unshared, which keeps the d-subshell half full, which is a nice place to be. So iron (II) and iron (III) are really the only kinds of iron salts you tend to see. Usually, iron salts can be either one, such as iron (II) sulfate or iron (III) sulfate. But oxygen is very aggressive about electron sharing, and so when iron bonds with oxygen the higher oxidation state is preferred. Iron (II) oxide is stable, but when you heat it enough, some of it decomposes into iron (III) oxide. So where smelting is concerned, the +3 oxidation state of iron is the only one you really need to worry about. If you can reduce iron (III) oxide at all, it'll go to an oxidation state of 0, which is the free metal.

But because manganese has a wider variety of oxidation states, a one-step reduction isn't necessarily a given. Manganese dioxide represents manganese at +4. It's usually not the most stable oxidation state, but it is when oxygen is involved. Manganese (II) oxide is also stable. Not quite as stable as manganese (IV) oxide, but in a vessel full of hot carbon monoxide it'll do.

I assume this was the problem Bergman and Scheele kept running into. They knew how to reduce metals in general, but manganese posed a whole new obstacle that 18th century metallurgy had never encountered before. So the tried and true methods simply weren't enough. Instead of reducing the oxide into a free metal, it reduced the oxide into another oxide.

This is why humanity had only discovered 14 metals by 1774. Most of the undiscovered metals were simply too reactive to be pulled apart from their compounds. Manganese, at least, was willing to meet chemists halfway. A lot of metal oxides would have refused to react with hot carbon monoxide altogether, but manganese would give up one of its two oxygens, just not both.

And maybe this was where Gahn stood out from among his colleagues. Under normal smelting conditions, the manganese dioxide would react, just not to completion. Gahn wouldn't have known about oxygen or oxidation states or d-electrons, but he did have one thing going for him, and it was was the phlogiston theory. And while I've been giving phlogiston a lot of flak in the last few posts I've made, it did provide a practical model for smelting metals.

See, we know today that when you smelt a metal like copper, you're taking the metal calx and stripping it of its oxygen to leave the bare metal. According to the phlogiston theory, it's the other way around, and you're adding phlogiston to a calx to synthesize the metal. So you could think of a metal as a "phlogisticated calx".

There are problems with this theory, mainly because phlogiston doesn't exist and oxygen does. But the theory still makes it very clear what you need when the process doesn't work the way it's supposed to. If Gahn had been working with a more difficult metal, he would have gotten no reaction at all, and he probably would have given up on it. But since it reacted without yielding a metal, he may have interpreted this as a partial phlogistication of the calx. Normal smelting conditions could force some phlogiston into manganese calx, just not enough to produce a metal.

So he had to use a stronger means of phlogistication, which modern chemistry would call a stronger reducing agent. Po-TAY-to, po-TAH-to.

Gahn didn't know that elemental carbon was a stronger reducing agent than carbon monoxide, but he did know that more charcoal meant more phlogiston, and a hotter temperature would facilitate the reaction. When Henry Cavendish tried to saturate air with phlogiston, he used red hot charcoal to do it, and it worked. Gahn was applying the same idea to metal ore. Instead of just having a lump of charcoal in his reaction vessel, he lined his crucible with carbon and mixed his ore with carbon and oil, and put some more carbon on top. I don't know how large his crucible was, but I imagine it was fairly small. Whatever air might have been sealed inside would have been consumed very early on.

The point of all of this was that Gahn wanted to use more carbon, with less air. I'm not sure what purpose the oil served, but Gahn may have seen this as a rich source of phlogiston in liquid form. And smothering the ore in cokey-oil would ensure that it would be completely surrounded by phlogiston when he turned up the heat.

In modern terms, you have this:

MnO2 + 2C → Mn + 2CO

I don't know how much heat Gahn used, but it was probably pretty hot. He didn't skimp on the carbon, so why hold back on the temperature? In any case, the reaction converted manganese from an oxidation state of +4 all the way down to 0. Conversely, carbon's oxidation state was increased from 0 to +4.

Again, I'm not sure that I'm terribly impressed with his solution to the problem, since it was just "More Charcoal, More Heat." Don't get me wrong, it worked. He discovered manganese. That's more than I did. I just don't see this as a terribly inspired approach. Gahn basically took what Bergman and Scheele probably tried and just turned it up to eleven. Then again, I'm looking back with the benefit of 241 years of hindsight. It always seems obvious after the fact, but maybe Gahn's brilliance wasn't in turning up the dial, it was in realizing that there was a dial to be turned.

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