Sulfur is one of the few chemical elements found in its pure state in nature. Consequently, it was known and used by the ancients. Many of those uses are still employed to this day, so it is a good thing that sulfur is rather common, at least locally. Historically, sulfur was mined near volcanic activity and thermal springs where it often occurs. In a few third world countries that is still a source of income for a significant number of people.
As the use of sulfur (mostly as sulfuric acid) increased in the 19th century, mining sulfur near volcanic regions could not keep up with demand, so new sources had to be developed. It was known that vast amounts of sulfur occur in association with salt domes in and near the Gulf of Mexico, but there was no way to mine it due to water and shifting sand. Thus, in 1894 a brilliant process was devised by German-American engineer Herman Frasch to solve the problem.
In the Frasch Process, three concentric pipes are sunk to the sulfur deposit. Superheated water is then pumped betwixt the outermost pipe and the next one. Sulfur melts at 115 C, so water under pressure can be heated to high enough a temperature to melt it. However, sulfur is more dense than water so water pressure alone will not move it up the pipe. To solve that, compressed air is pumped down the innermost pipe which whips the sulfur to a froth. This is less dense than water, so the sulfur in pushed to the surface. When the air/sulfur/water mixture reaches atmospheric pressure, the water flashes to vapor, forcefully ejecting the sulfur from the pipe. The pipe is designed to send the sulfur to a collection area. Many tons per day can be extracted by this method.
These days, very sulfur is produced this way. The last Frasch plant in the United States closed in 2000. Sulfur now is almost exclusively produced as a byproduct from the desulfurization of oil and gas before using it as fuel. Because of the large amount of energy to heat the water that is required for the Frasch process, byproduct sulfur is cheaper, and besides the sulfur must be removed from the fuels anyway to avoid acid rain, formerly a serious problem in North America.
Sulfur, symbol S, atomic number (Z) = 16, is extremely common. It is produced in the cores of massive stars by the alpha process, fusing silicon and helium to sulfur-32, the most common isotope. Stable isotopes of masses 33, 34, and 36 also exist, as well as many artificial, radioactive ones. Sulfur is just below oxygen in the periodic table, making it a chalcogen. Since it is just below oxygen, one would expect it to have properties similar to oxygen. To some extent that is the case, but there are significant differences as well.
Obviously, sulfur is a solid rather than a gas. That can be explained to some extent by its higher Z. Also, sulfur exists mainly as eight membered rings, so sulfur molecules are much heavier than the diatomic oxygen molecule. That is most of the reason that oxygen and sulfur have melting points that are over 300 C different. Sulfur DOES form compounds that are analogous to oxides where a sulfur atom extracts two electrons from another element, often a metal, to form sulfides. Some of the most important ores of many metals are sulfides, such as sphalerite, zinc sulfide, and galena, lead sulfide.
However, sulfur has a much more complex chemistry than does oxygen, because it is a third row element. This gets into quantum mechanics, but essentially sulfur can do something that oxygen can not, and that is form more than two chemical bonds. This is because sulfur, like all third row elements, has d orbitals where second row elements do not. This means that sulfur can form up to six bonds per atom, opening the way to lots of interesting things. In addition, sulfur is much less electronegative than is oxygen, so it can both receive electrons (an oxidizing agent) and donate electrons (a reducing agent). Except when combining with fluorine, the most electronegative element, oxygen typically is only an oxidizing agent.
Sulfur is everywhere. It is critical in biochemistry, being a component of at least two common amino acids, the building blocks of proteins. This is very important, because the tertiary structure (the three dimensional shape after folding) of proteins is largely determined by disulfide bridges where two sulfur atoms on different amino acids (in many cases amino acids that are far apart in the chain) combine, thus “tacking” the skeleton into shape. Keratin, the protein of nails and hair, is particularly rich in sulfur containing amino acids, making keratin very tough. This is also why burning hair smells quite different than burning meat, since meat is relatively poorer in sulfur containing amino acids.
Thus, sulfur is an essential nutrient for both plants and animals. Do not worry about getting enough in your diet, because if you get enough protein to survive, you most likely gets all of the sulfur that you need. However, some agricultural land is becoming deficient in sulfur, particularly in places like Europe where agriculture has been practiced for millenia. When this occurs, it is easily replaced with fertilizers that contain sulfur, and they are relatively cheap. Calcium sulfate is generally used since it is more readily absorbed by plants than is elemental sulfur. It can also be mined, so does not require synthesis. We shall get back to this material later. Studies show that sulfur is needed by plants in amounts similar to phosphorous, so it is indeed important.
Several vitamins also contain sulfur, so it is a critical competent of materials in addition to proteins. Many drugs have sulfur in them, including penicillin. A sulfur containing compound, allicin, gives garlic its characteristic smell. Many sulfur compounds have characteristic odors such as hydrogen sulfide (the smell of rotten eggs), sulfur dioxide (the choking gas given off when sulfur burns), butyl thiol (skunk oil, also added in trace amounts to natural gas as a warning agent, since natural gas itself in odorless), and others too numerous to mention.
In addition to nutritional uses, there are thousands of industrial and consumer uses for sulfur, usually as sulfur compounds but sometimes the element itself. You have probably seen sulfur at the drug store (usually called “flowers of sulfur”), and it is used as a dusting powder and is fairly effective for athlete’s foot. The sulfur combines with perspiration to form low levels of sulfurous acid which is a mild antiseptic. Since antiquity sulfur has been burnt to produce sulfur dioxide to bleach and preserved dried fruit. Also for hundreds of years compounds of sulfurous acid, called sulfites, have been used to keep wine from going off due to bacterial action. Some people have a rather severe allergic reaction to sulfites, so by law all wine that has been treated with them has to carry a “contains sulfites” label.
An interesting use for elemental sulfur is in forensic work. Shoeprints, when in soil, are often preserved using plaster of Paris, actually calcium sulfate! However, shoeprints in snow are instantly destroyed using plaster. Sulfur is melted and used instead. Even though the temperature is higher than plaster, the heat capacity of sulfur is very much lower than the heat capacity of the water in the plaster, so a light treatment with molten sulfur does not destroy the print.
By far the largest use of sulfur is to produce sulfuric acid, probably the most important industrial chemical after water. There is hardly an industrial process that does not involve this material in a direct or an indirect way. The battery in your car (unless you have a Leaf or a hybrid) contains sulfuric acid both as the electrolyte and as a reactant in the electricity producing chemical system. The importance of sulfuric acid to modern life can not be overemphasized. There are a couple of reasons that this material is so important.
First, it is the cheapest strong acid, being made from sulfur, water, and air. The modern production method is to burn sulfur to sulfur dioxide, then using a catalyst, further oxidizing it to sulfur trioxide (some of that interesting chemistry because of those d orbitals), then combining the SO3 with water, H2O to form sulfuric acid, H2SO4. In this way sulfuric acid can be obtained in a high state of purity and in high concentration.
Second, sulfuric acid has a very high boiling point, higher than any of the other common acids. This allows it to be used to produce those other acids, since it does not boil over into the receiver. To make hydrochloric acid, for example, salt (sodium chloride) is treated with sulfuric acid and heat, and the anhydride (meaning literally “without water”) hydrogen chloride, HCl, if formed. This is dissolved in water to form hydrochloric acid. If you have a pool, you have probably used muriatic acid, which is just an obsolete name for hydrochloric acid.
Sulfuric acid used to be called “The Old Grey Mare of Industry” because of its multitude of uses, in comparison with the farm animal that was at the time the chief source of power on the family farm. Although farming is not done with horses so much these days, sulfuric acid is more important than ever. Unless some special properties are required, sulfuric acid is the acid of choice for most chemical processes because of its low cost and high purity.
There are lots of other sulfur compounds that are important, and you use or touch them every day. Drywall (the so called Sheet Rock, which is a brand name) is merely plaster of Paris that is mixed with water and placed into molds lined with heavy paper, then allowed to harden. Enormous quantities of these sheets are used in commercial and residential construction. It is inexpensive compared to many other wall coverings, easy to work with (I am quite good at handing drywall), durable (unless you have a habit of hitting walls with your fist), and, extremely importantly, fireproof (there is not enough paper to sustain combustion). Plaster is also often used to line fire resistant filing cabinets, not only because it is an excellent insulator, but also gives off water vapor when heated, thus soaking up energy and reducing the intensity of a fire.
Another extremely large use of sulfur is in the production of synthetic detergents. Soap does not contain a significant amount of sulfur (except as impurities in the starting materials) and is made from vegetable or animal fat and either sodium or potassium hydroxide. Now, only alkali metal soaps are water soluble. When soap is used in areas where the water is hard (containing iron, calcium, magnesium or a combination of them as dissolved salts), those metal ions replace the sodium or potassium, causing the soap to precipitate. This is the famous bathtub ring and soap scum.
There are a couple of ways to combat this phenomenon. One is to add materials that preferentially react with those ions and prevent them from reacting with the soap. Probably the most effective of these are the phosphates, but they have some significant environmental concerns because they are not removed at the sewerage treatment stage, so can enter waterways where they act as nutrients, getting algal growth out of whack, causing a “bloom” that then dies and, while rotting, deplete oxygen from water, killing fish. Borax and carbonates are also used (without the negative environmental effects), but are not nearly as effective as phosphates.
Another approach is to soften the water, exchanging the offensive ions with sodium. This is done with ion exchange resins which contain sodium and preferentially attract the other ions and release sodium ones. However, this technology is not that cheap and requires periodic recharging with salt to remove the other ions. Here in the Bluegrass, where the water is extremely hard due to calcium (making it ideal for making Bourbon whiskey, by the way) many people have water softeners. As an aside, do not believe all of the hype that you see on the TeeVee (particularly on the Fox “News” Channel) about that electronic gizmo that is supposed to “soften” your water whilst leaving in the “good” minerals. I shall do a piece on that soon.
A completely different approach is to use something other than soap. Here is where sulfur comes into play. There is a class of sulfur compounds called alkylbenzenesulfonates that are affected by hard water much less than is soap. This is not to say that they are completely immune, but just much less affected. Looking at my bottle of Clairol Herbal Essences shampoo (dishwashing detergent is very similar), the two main ingredients after water are sodium laureth sulfate and sodium lauryl sulfate, both of this class of compounds. Laundry detergent usually also contains some water softening agents as well, because fabric is more difficult to clean than what little hair that I have or dishes.
Back when these agents were first introduced, there was a huge problem with mountains of foam after sewerage treatment. It turns out that these first generation detergents were not biodegradable, but the modern ones are and the sewerage treatment plants now break them down to simpler, nontoxic molecules that do not form suds.
Another large use for sulfur to to make rubber usable. This process is called vulcanization since heat is involved and the Roman god Vulcan was associated with heat and with sulfur. Natural rubber is not an extremely useful substance in its raw form. It oxidizes readily, gets gummy in hot weather and stiff in cold weather. As a matter of fact, about the only thing that it was good for was removing pencil marks, hence the name “rubber”. However, when treated with elemental sulfur (other agents are also used in modern practice), and heated, something remarkable happens. Remember the disulfide bridge in proteins? The same thing happens with rubber, the sulfur forming those same kinds of bonds betwixt the long chain rubber molecules, making the entire structure more stable. Depending on how much sulfur is added, very soft rubbers, like crepe shoe soles up to very hard rubbers (like Ebonite, as hard and stiff as oak) can be made. Carbon black also improves the chemical and mechanical properties of rubber, making it tougher. The tire industry uses large amounts of both carbon black and sulfur to make modern, long wearing, low rolling resistance automobile tires. Now you know why tires are typically black.
That is about enough for tonight. I hope that you have a greater appreciation of sulfur than before. It really is quite a remarkable element, due largely to those d orbitals!
Well, you have done it again! You have wasted many more perfectly good einsteins of photons reading this smelly piece. And even though Eric Cantor stops acting like a second grader, taking his ball and leaving, when he reads me say it, I always learn much more than I could ever possibly hope to teach by writing this series. Thus, please keep those comments, questions, corrections, and other feedback coming. Tips and recs are also much appreciated. I shall stay around tonight as long as comments warrant, and shall return tomorrow after Keith’s show (I really like being able to say that again!) for Review Time. Remember, no science or technology subject is off topic here.
Warmest regards,
Doc
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a smelly subject?
Warmest regards,
Doc