Inorganic Chemistry


The New Metallurgy

If the nineteenth century, particularly the latter half, seems pre-eminently the era of organic chemistry, inorganic chemistry was nevertheless far from remaining at a standstill.

Photography already has been mentioned as an important nineteenth-century application of inorganic chemistry, but in its importance to the economy or the well-being of society it must be regarded as a minor contribution. Another of those small contributions, usually taken for granted but with social significance for all that, was an advance in firemaking techniques. Through all history mankind had been making fires either by applying friction to objects such as wood, which had to be heated to high temperatures before it would catch fire, or by striking sparks that lasted but a moment, as with flint-and-steel. But in time men began to experiment with chemicals which would burst into flame at low temperatures that could be reached with brief friction. In 1827, the English inventor John Walker (c.1781-1859) devised the first practical phosphorus match. It has been much improved in the century and a half since, but the principle is the same.

Photography and the phosphorus match are only two examples of many practical advances in inorganic chemistry that would deserve more than mere mention is a full-dress, detailed history of the science, but in this short work we will concentrate on the larger issues. The most dramatic progress in the applied chemistry of the nineteenth century was made in metals, of which steel was the most important to our economy. Petroleum feeds and fuels our society, but steel forms its support.

Although steelworking was common even three thousand years ago, it was not until the mid-nineteenth century that a technique was devised for producing it cheaply enough and in the huge quantities required for the framework of modern society. The great name here is Henry Bessemer (1813-1898).

Bessemer, an English metallurgist, was attempting to devise an artillery projectile that would spin in flight and move in an accurately predictable path. For this he needed a cannon that was rifled - that is, had spiral grooves cut in the bore wall from breach to muzzle. The barrel had to be made of particularly strong steel to withstand the great pressures required to force the emerging projectiles against the spiral grooves and into a rapid spin. Ordinary unrifled cannon was made of weaker material and steel was very expensive. Unless something was done Bessemer's rifled cannon was impractical.

Iron, as produced, was cast iron, rich in carbon (from the coke or charcoal used to smelt the ore). Cast iron was exceedingly hard, but brittle. The carbon could be painstakingly removed to form wrought iron, which was tough, but relatively soft. A proper amount of carbon was then re-introduced, just enough to form steel, which was both tough and hard.

Bessemer looked for a method that would produce iron plus just enough carbon to form steel without going through the expensive wrought iron stage. To remove the excess carbon in cast iron he sent a blast of air through the molten metal. This air did not cool and solidify the metal. On the contrary, the heat of combination of carbon and oxygen actually raised the temperature of the melt. By stopping the air blast at the right time, Bessemer could obtain steel.

In 1856, he announced his blast furnace. At first, attempts to duplicate his work failed because his method required phosphorus-free ore to begin with. Once this was realized, things went smoothly, steel became cheap, and the Iron Age finally gave way to the Steel Age. (Subsequently, techniques superior to Bessemer's were introduced into steel production.) It is the strength of steel that has made modern skyscrapers and suspension bridges possible; it was steel that armored battleships and provided monster artillery pieces, and steel on which trains ran.

Nor did steelmaking stop at the combination of carbon and iron. The English metallurgist Robert Abbot Hadfield (1858-1940) tested the properties of steel to which quantities of other metals were added. Adding manganese seemed to make steel brittle, but Hadfield added more than previous metallurgists had attempted. By the time the steel was 12 per cent manganese it was no longer brittle. When heated to 1000oC and then quenched in water, it became much harder than ordinary steel. Hadfield patented his manganese steel in 1882, and that event marks the beginning of the triumph of alloy steel.

Other metals were successfully added to steel, chromium, molybdenum, vanadium, tungsten, niobium, forming varieties of alloy steel suitable for particular purposes. By 1919 a non-rusting stainless steel, containing chromium and nickel, had been patented by the American inventor Elwood Hayes (1857-1925). In 1916, Japanese metallurgist Kotaro Honda (1870-1954) found that adding cobalt to tungsten steel produced an alloy cable of forming a more powerful magnet then ordinary steel. This discovery opened the way to still stronger magnetic alloys.

Altogether new metals came into use, too. Aluminium is more common in the earth's soil than iron is; indeed, it is the most common of all metals. However, it remains firmly combined in compounds. Whereas iron has been known and prepared from its ores since prehistoric times, aluminum was not even recognized as a metal until Wohler isolated an impure sample in 1827.

Not until 1855 did a French chemist, Henri Etienne Sainte-Claire Deville (1818-1881), work out an adequate method for preparing reasonably pure aluminum in moderate quantities. Even then, it was far more expensive than steel. It was used for the rattle of Napoleon III's infant son and the cap at the top of the Washington Monument.

In 1886 a young American student of chemistry, Charles Martin Hall (1863-1914), hearing his teacher say that anyone discovering a cheap way of making aluminum would grow rich and famous, determined to tackle the problem. Working in his home laboratory, he discovered that aluminum oxide could be made to dissolve in a molten mineral called cryolite. Once the oxide was in solution, electrolysis would produce aluminum itself. In the same year, the French metallurgist Paul Louis Toussaint Heroult (1863-1914) devised essentially the same method for producing the metal. The Hall-Heroult method made aluminum cheap and fit for even the most common use - down to the kitchen pots and pans.

Aluminum's greatest value lies in its lightness (one-third the weight of steel). This quality makes it of particular use to the aircraft industry, which also devours quantities of magnesium, an even lighter metal. Practical methods were devised in the 1930's for extracting magnesium metals from the salts dissolved in the ocean, giving us now a virtually inexhaustible source of the metal. (Bromine and iodine - to say nothing of salt itself - are now profitably obtained from the ocean. A problem of growing importance for the future is that of extracting fresh water itself from the ocean.)

Metals such as titanium show great promise also. Titanium is a common metal, highly resistant to acids, intermediate in lightness between aluminum and steel, and properly prepared, the strongest of the metals, weight for weight. Zirconium is similar, but is less common and is heavier.

Titanium's outlook for the future is particularly bright in connection with the supersonic planes being designed and built. Planes moving through even the thin upper atmosphere at speeds that are multiples of the speed of sound undergo considerable frictional resistance from the air. Their outer skin must withstand high temperatures, and here titanium is particularly suitable, for it retains its strength at high temperature better than do other metals.

Nitrogen and Fluorine

Nitrogen surrounds us in the atmosphere, but there it is present in elementary form. To most organisms it is useful only in compound form. As it happens, nitrogen is almost inert and reacts to form compounds only with difficulty. Despite the great abundance of air soil is often short of nitrates (the most common type of nitrogen compound), and they must be supplied in the form of animal wastes or chemical fertilizers. Nitrates are also ingredients of gunpowder, and are used, indirectly, in the formation of the newer explosives, such as nitrocellulose and nitroglycerine.

The earth's supply of nitrates is produced through the action of thunderstorms. The nitrogen and oxygen of the air combine in the vicinity of lightening bolts to form compounds. These compounds dissolve in the raindrops and are brought to earth. In addition, certain types of bacteria utilize elementary nitrogen from the air to produce nitrogen compounds. But as man's requirement of nitrates, both for fertilizers and explosives, grew, it became difficult to rely on natural sources alone. The German chemist Fritz Haber (1868-1934) investigated methods for combining atmospheric nitrogen with hydrogen to form ammonia. The ammonia could then be converted easily to nitrates. By 1908 Haber had managed to perform the task by placing nitrogen and hydrogen under high pressure and using iron as a catalyst.

With the coming of World War I, and the blockade of Germany by the British fleet, Germany could no longer obtain natural nitrate from the Chilean desert (the best natural source). The German chemist Karl Bosch (1874-1940) had changed the Haber process from a laboratory demonstration into an industrial operation. By the middle of the war he was producing all the nitrogen compounds Germany needed.

Just the reverse was the case of fluorine, so active that it existed only in compounds and defied the efforts of chemists to set it free. Since the time of Lavoisier chemists were certain the element existed; so certain were they, in fact, that Newlands and Mendeleev included it in their periodic tables, though no man had ever seen it. To be sure, electrolysis would break fluorine away from its various compound molecules. However, as soon as the gas was in elemental form it would react with whatever was closest and become part of a compound again. (Fluorine is the most active of all chemical elements.)

Many chemists tacked the problem in the nineteenth century, from Davy onward. It was left to the French chemist Ferdinand Frederic Henri Moissan (1852-1907) to succeed. Moissan decided that since platinum was one of the few substances that could resist fluorine, there was nothing to do but prepare all his equipment of platinum, regardless of expense. What's more, he lowered the temperature of everything to -50oC to reduce fluorine's fierce activity. In 1886, he passed an electric current through a solution of potassium fluoride in hydrofluoric acid in his all platinum equipment and achieved his goal. The pale yellow gas, fluorine, was finally isolated.

Though this was a great feat, Moissan became even more famous for another achievement that was not really an achievement at all. Charcoal and diamond are both forms of carbon and differ only in that the carbon atoms in diamond are packed with great compactness. It follows that if great pressure is placed on charcoal, the atoms might rearrange more compactly to form diamond. Moissan tried to accomplish this by dissolving charcoal in molten iron and letting the carbon crystallize out as the iron cooled.

By 1893, it seemed to him he had succeeded. He produced several tiny, impure diamonds together with a sliver of good diamond, over half a millimeter in length. It is possible, however, that Moissan was the victim of a hoax and that some assistant had seeded the iron. We now know, from theoretical considerations, that under the conditions Moissan used, diamond cold not have formed.

An American inventor, Edward Goodrich Acheson (1856-1931), also attempted the formation of diamond from more ordinary forms of carbon. He failed, but in the process, while heating carbon intensely in the presence of clay, he obtained an extremely hard substance which he named carborundum. It proved to be silicon carbide (a compound of silicon and carbon), and formed an excellent abrasive.

To form diamonds, pressure higher than any available in the nineteenth century to be used, together with high temperatures which would make it possible for atoms to alter their positions with reasonable ease. The American physicist Percy William Bridgman (1882-1961) spent half a century, beginning in 1905, devising equipment that would yield higher and higher pressures. Various elements and compounds took up new forms, ones in which atoms and molecules packed unusually compact arrangements. Varieties, of ice, for instance considerably denser than water and with a melting point higher than the boiling point of water at ordinary pressures, were produced. (Such high-pressure forms revert to ordinary forms as soon as the pressure is relieved, usually. Diamond is an exception.) In 1955, using Bridgman's techniques, truly synthetic diamonds were finally produced.

Inorganic-Organic Boundary

With the beginning of the twentieth century a vast area of study overlapping organic and inorganic chemistry became apparent.

The English chemist Frederick Stanley Kipping (1863-1949) began, in 1899, research on organic compounds containing the element silicon which, next to oxygen, is the most common element in the earth's rocky crust. Over a period of forty years he managed to synthesize a large number of organic compounds containing one or more of these atoms so characteristic of the inorganic world. Indeed, it was possible to obtain indefinitely long chains made up of silicon and oxygen atoms in alternation.

This work might be viewed as purely inorganic, but each silicon atom has a valence of four, of which only two are used in combination with oxygen. The other two can be bound to any of a variety of organic groupings. In World War II and afterward, such inorganic/organic silicones gained importance as greases, hydraulic fluids, synthetic rubbers, water repellents, and so on.

Ordinary organic compounds are composed of carbon atoms to which other atoms are attached. In general, the majority of the "other atoms" are hydrogens, so that organic compounds may be spoken of as the hydrocarbons and their derivatives. The fluorine atom is almost as small as the hydrogen atom and will fit anywhere the hydrogen atom will. One would expect that there should exist a whole family of fluorocarbons and their derivatives.

An early experimenter with fluoro-organic compounds was The American chemist Thomas Midgley Jr. (1889-1944). In 1930 he prepared freon, with a molecule consisting of a carbon atom to which two chlorine atoms and two fluorine atoms are attached. It is easily liquefied so that it can be used as a refrigerant, in place of those other easily liquefied gases ammonia and sulfur dioxide. Unlike them, freon is odorless and non-toxic and completely non-flammable as well. It is now used almost universally in home refrigerators and air-conditioners.

During World War II, fluorine and fluorine compounds were used in connection with the work on uranium and the atomic bomb. Greases were needed that would not be attacked by fluorine and, for the purpose, fluorocarbons were used, since these already have undergone maximum attack by fluorine.

Fluorine forms a very tight bond with carbon, and fluorocarbon chains are more stable and more inert then hydrocarbon chains. Fluorocarbon polymers are waxy, water-repellent, solvent-repellent, electrically insulating substances. A fluorocarbon plastic (Teflon) was come into use, in the 1960's, as a film to cover frying pans, which then no longer require fat for frying.

Inorganic complexity does not require the carbon atom at all in some cases. The German chemist Alfred Stock (1876-1946) began the study of boron hydrides (compounds of boron and hydrogen) in 1909. He found that fairly complicated compounds could be built up, compounds analogous, in some ways, to the hydrocarbons.

Since World War II, boron hydrides have gained an unexpected use as rocket fuel additives designed to increase the push that forces rockets into the upper atmosphere and outer space. Further, the boron hydrides proved of theoretical interest, because the ordinary formulas of the type first devised by Kekule were inadequate to explain their structure.

But all these accomplishments, however ingeniously and painstakingly arrived at, however essential to the modern way of life, were extraneous to the most serious business of twentieth-century chemistry. The pure scientist was probing beneath the surface of the atom.