TITANIUM: HISTORY, STRUCTURE, PROPERTIES, REACTIONS, USES - CHEMISTRY - 2024

The titanium is a transition metal which is represented by the chemical symbol Ti. It is the second metal to appear from block d of the periodic table, just after scandium. Its atomic number is 22, and it occurs in nature as many isotopes and radioisotopes, of which ⁴⁸ Ti is the most abundant of all.

Its color is silver gray, and its parts are covered by a protective layer of oxide that makes titanium a metal very resistant to corrosion. If this layer is yellowish, it is titanium nitride (TiN), which is a compound that is formed when this metal burns in the presence of nitrogen, a unique and distinguished property.

Titanium rings. Source: Pxhere.

In addition to what has already been mentioned, it is highly resistant to mechanical impacts despite being lighter than steel. That is why it is known as the strongest metal of all, and its very name is synonymous with strength. It also has strength and lightness, two characteristics that make it a desirable material for aircraft manufacturing.

Likewise, and no less important, titanium is a biocompatible metal that is pleasant to the touch, which is why it is used in jewelery to make rings; and in biomedicine, such as orthopedic and dental implants, capable of integrating into bone tissues.

However, its best known uses reside in TiO ₂, as a pigment, additive, coating and photocatalyst.

It is the ninth most abundant element on Earth, and the seventh within metals. Despite this, its cost is high due to the difficulties that must be overcome to extract it from its minerals, among which are rutile, anatase, ilmenite and perovskite. Of all the production methods, the Kroll Process is the most widely used worldwide.

History

Discovery

Titanium was identified for the first time in the ilmenite mineral in the Manaccan Valley (United Kingdom), by the pastor and amateur mineralogist William Gregor, back in 1791. He was able to identify that it contained an oxide of iron, since its sands moved through the influence of a magnet; but he also reported that there was another oxide of an unknown metal, which he called "manacanite."

Unfortunately, although he turned to the Royal Geological Society of Cornwall and other outlets, his contributions did not cause a stir for not being a recognized man of science.

Four years later, in 1795, the German chemist Martin Heinrich Klaproth independently recognized the same metal; but in rutile ore at Boinik, now Slovakia.

Some claim he named this new metal 'titanium' inspired by its toughness in resemblance to the Titans. Others claim that it was due more to the neutrality of the mythological characters themselves. Thus, titanium was born as a chemical element and Klaproth was later able to conclude that it was the same manacanite as the mineral ilmenite.

Isolation

Since then, attempts began to isolate it from such minerals; but most of them were unsuccessful, since the titanium was contaminated with oxygen or nitrogen, or formed a carbide that was impossible to reduce. It took almost a century (1887) for Lars Nilson and Otto Pettersson to prepare a sample that was 95% pure.

Then, in 1896, Henry Moissan managed to obtain a sample with up to 98% purity, thanks to the reducing action of metallic sodium. However, these impure titaniums were brittle by the action of oxygen and nitrogen atoms, so it was necessary to design a process to keep them out of the reaction mixture.

And with this approach originated the Hunter Process in 1910, devised by Matthew A. Hunter in collaboration with the General Electric at Rensselaer Polytechnic Institute.

Twenty years later, in Luxembourg, William J. Kroll devised another method using calcium and magnesium. Today, the Kroll Process remains one of the leading methods for producing metallic titanium on commercial and industrial scales.

From this point on, the history of titanium follows the course of its alloys in applications for the aerospace and military industries.

Structure and electronic configuration

Pure titanium can crystallize with two structures: a compact hexagonal (hcp), called the α phase, and a body-centered cubic (bcc), called the β phase. Thus, it is a dimorphic metal, capable of undergoing allotropic (or phase) transitions between the hcp and bcc structures.

The α phase is the most stable at ambient temperature and pressure, with the Ti atoms surrounded by twelve neighbors. When the temperature is increased to 882 ° C, the hexagonal crystal becomes a cubic, less dense, which is consistent with the higher atomic vibrations caused by heat.

As the temperature increases, the α phase opposes greater thermal resistance; that is, its specific heat also increases, so that more and more heat is needed to reach 882 ° C.

What if instead of increasing the temperature, the pressure does? Then you get distorted bcc crystals.

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In these metallic crystals, the valence electrons of the 3d and 4s orbitals intervene in the bond that joins the Ti atoms, according to the electronic configuration:

3d ² 4s ²

It has just four electrons to share with its neighbors, resulting in nearly empty 3d bands, and therefore titanium is not as good a conductor of electricity or heat as other metals.

Alloys

Even more important than what has been said regarding the crystalline structure of titanium, is that both phases, α and β, can form their own alloys. These can consist of pure α or β alloys, or mixtures of both in different proportions (α + β).

Likewise, the size of their respective crystalline grains influences the final properties of said titanium alloys, as well as the mass composition and the relationships of the added additives (a few other metals or N, O, C or H atoms).

Additives have a significant influence on titanium alloys because they can stabilize some of the two specific phases. For example: Al, O, Ga, Zr, Sn and N are additives that stabilize the α phase (denser hcp crystals); and Mo, V, W, Cu, Mn, H, Fe, and others are additives that stabilize the β phase (less dense bcc crystals).

The study of all these titanium alloys, their structures, composition, properties and applications, are the object of metallurgical works that rely on crystallography.

Oxidation numbers

According to the electron configuration, titanium would need eight electrons to completely fill the 3d orbitals. This cannot be achieved in any of its compounds, and at most it gains up to two electrons; that is, it can acquire negative oxidation numbers: -2 (3d ⁴) and -1 (3d ³).

The reason is due to the electronegativity of titanium and that, in addition, it is a metal, so it has a greater tendency to have positive oxidation numbers; such as +1 (3d ² 4s ¹), +2 (3d ² 4s ⁰), +3 (3d ¹ 4s ⁰) and +4 (3d ⁰ 4s ⁰).

Note how the electrons of the 3d and 4s orbitals are leaving as the existence of the cations Ti ⁺, Ti ²⁺ and so on is assumed.

The oxidation number +4 (Ti ⁴⁺) is the most representative of all because it corresponds to that of titanium in its oxide: TiO ₂ (Ti ⁴⁺ O ₂ ^2-).

Properties

Physical appearance

Grayish silver metal.

Molar mass

47.867 g / mol.

Melting point

1668 ° C. This relatively high melting point makes it a refractory metal.

Boiling point

3287 ° C.

Autoignition temperature

1200 ° C for pure metal, and 250 ° C for finely divided powder.

Ductility

Titanium is a ductile metal if it lacks oxygen.

Density

4.506 g / mL. And at its melting point, 4.11 g / mL.

Heat of fusion

14.15 kJ / mol.

Heat of vaporization

425 kJ / mol.

Molar heat capacity

25060 J / mol · K.

Electronegativity

1.54 on the Pauling scale.

Ionization energies

First: 658.8 kJ / mol.

Second: 1309.8 kJ / mol.

Third: 2652.5 kJ / mol.

Mohs hardness

6.0.

Nomenclature

Of the oxidation numbers, +2, +3 and +4 are the most common, as they are referred to in the traditional nomenclature when naming titanium compounds. Otherwise, the rules of the stock and systematic nomenclatures remain the same.

For example, consider TiO ₂ and TiCl ₄, two of the best-known compounds of titanium.

It has already been said that in TiO ₂ the oxidation number of titanium is +4 and, therefore, being the largest (or positive), the name must end with the suffix -ico. Thus, its name is titanic oxide, according to the traditional nomenclature; titanium (IV) oxide, according to the stock nomenclature; and titanium dioxide, according to the systematic nomenclature.

And for TiCl ₄ we will proceed more directly:

Nomenclature: name

-Traditional: titanic chloride

-Stock: titanium (IV) chloride

-Systematic: titanium tetrachloride

In English this compound is often referred to as 'Tickle'.

Each titanium compound can even have its own names outside the naming rules, and will depend on the technical jargon of the field in question.

Where to find and production

Titaniferous minerals

Rutile quartz, one of the minerals with the highest content of titanium. Source: Didier Descouens

Titanium, although it is the seventh most abundant metal on Earth, and the ninth most abundant in the earth's crust, is not found in nature as a pure metal but in combination with other elements in mineral oxides; better known as titaniferous minerals.

Thus, to obtain it it is necessary to use these minerals as raw material. Some of them are:

-Titanite or sphene (CaTiSiO ₅), with iron and aluminum impurities that turn their crystals green.

-Brookite (Orthorhombic TiO ₂).

-Rutile, the most stable polymorph of TiO ₂, followed by the minerals anatase and brookite.

-Ilmenite (FeTiO ₃).

-Perovskite (CaTiO ₃)

-Leucoxene (heterogeneous mixture of anatase, rutile and perovskite).

Note that there are several titaniferous minerals mentioned, although there are others. However, not all of them are equally abundant and, likewise, they can contain impurities that are difficult to remove and that put the properties of the final metallic titanium at risk.

This is why sphene and perovskite are often used for the production of titanium, as their calcium and silicon content are difficult to remove from the reaction mixture.

Of all these minerals, rutile and ilmenite are the most used commercially and industrially due to their high content of TiO ₂; that is, they are rich in titanium.

Kroll process

Selecting any of the minerals as raw material, the TiO ₂ in them must be reduced. To do this, the minerals, together with coal, are heated red hot in a fluidized bed reactor at 1000 ° C. There, the TiO ₂ reacts with chlorine gas according to the following chemical equation:

TiO ₂ (s) + C (s) + 2Cl ₂ (g) => TiCl ₄ (l) + CO ₂ (g)

TiCl ₄ is an impure colorless liquid, since at that temperature it is dissolved together with other metallic chlorides (iron, vanadium, magnesium, zirconium and silicon) originated from the impurities present in the minerals. Therefore, TiCl _{4 is} then purified by fractional distillation and precipitation.

Once purified, the TiCl ₄, a species easier to reduce, is poured into a stainless steel container to which a vacuum is applied, to eliminate oxygen and nitrogen, and filled with argon to ensure an inert atmosphere that does not affect the titanium. produced. Magnesium is added in the process, which reacts at 800 ° C according to the following chemical equation:

TiCl ₄ (l) + 2Mg (l) => Ti (s) + 2MgCl ₂ (l)

Titanium precipitates as a spongy solid, which is subjected to treatments to purify it and give it better solid forms, or is used directly for the manufacture of titanium minerals.

Reactions

With the air

Titanium has a high resistance to corrosion due to a layer of TiO ₂ that protects the interior of the metal from oxidation. However, when the temperature rises above 400 ° C, a thin piece of metal begins to burn completely to form a mixture of TiO ₂ and TiN:

Ti (s) + O ₂ (g) => TiO ₂ (s)

2Ti (s) + N ₂ (g) => TiN (s)

Both gases, O ₂ and N ₂, are logically in the air. These two reactions occur rapidly once the titanium is heated red hot. And if it is found as a finely divided powder, the reaction is even more vigorous, making titanium in this solid state highly flammable.

With acids and bases

This TiO ₂ -TiN layer not only protects titanium from corroding, but also from attack by acids and bases, so it is not an easy metal to dissolve.

To achieve this, highly concentrated acids need to be used and boiled to a boil, obtaining a purple solution resulting from the aqueous complexes of titanium; for example, ⁺³.

However, there is an acid that can dissolve it without many complications: hydrofluoric acid:

2Ti (s) + 12HF (aq) 2 ^3- (aq) + 3H ₂ (g) + 6H ⁺ (aq)

With halogens

Titanium can react directly with halogens to form the respective halides. For example, your reaction to iodine is as follows:

Ti (s) + 2I ₂ (s) => TiI ₄ (s)

Similarly with fluorine, chlorine and bromine, where an intense flame is formed.

With strong oxidants

When titanium is finely divided, it is not only prone to ignition, but also to react vigorously with strong oxidizing agents at the slightest source of heat.

Part of these reactions is used for pyrotechnics, since bright white sparks are generated. For example, it reacts with ammonium perchlorate according to the chemical equation:

2Ti (s) + 2NH ₄ ClO ₄ (s) => 2TiO ₂ (s) + N ₂ (g) + Cl ₂ (g) + 4H ₂ O (g)

Risks

Metallic titanium

Titanium powder is a highly flammable solid. Source: W. Oelen

The metallic titanium by itself does not represent any risk to the health of those who work with it. It is a harmless solid; Unless, it is ground as a fine particle powder. This white powder can be dangerous due to its high flammability, mentioned in the reactions section.

When titanium is ground, its reaction with oxygen and nitrogen is faster and more vigorous, and it can even burn explosively. That is why it represents a terrible fire risk if where it is stored it is hit by flames.

When burning, the fire can only be put out with graphite or sodium chloride; never with water, at least for these cases.

Likewise, their contact with halogens should be avoided at all costs; that is, with some gaseous leakage of fluorine or chlorine, or interacting with the reddish liquid of bromine or the volatile crystals of iodine. If this happens, the titanium catches fire. Nor should it come into contact with strong oxidizing agents: permanganates, chlorates, perchlorates, nitrates, etc.

Otherwise, its ingots or alloys cannot represent more risks than physical blows, since they are not very good conductors of heat or electricity and are pleasant to the touch.

Nanoparticles

If the finely divided solid is flammable, it must be even more so that made up of titanium nanoparticles. However, the central point of this subsection is due to the TiO ₂ nanoparticles, which have been used in countless applications where they merit their white color; like sweets and candies.

Although its absorption, distribution, excretion or toxicity in the body is not known, they have been shown to be toxic in studies on mice. For example, they showed that it generates emphysema and redness in their lungs, as well as other respiratory disorders in their development.

By extrapolation from mice to us, it is concluded that breathing TiO ₂ nanoparticles affects our lungs. They can also alter the hippocampus region of the brain. Additionally, the International Agency for Research on Cancer does not rule them out as possible carcinogens.

Applications

Pigment and additive

Talking about the uses of titanium is necessarily referring to that of its compound titanium dioxide. TiO ₂ in fact covers about 95% of all applications concerning this metal. The reasons: its white color, it is insoluble, and it is also non-toxic (not to mention the pure nanoparticles).

That is why it is usually used as a pigment or additive in all those products that require white colorations; such as toothpaste, medicines, candy, papers, gems, paints, plastics, etc.

Coatings

TiO ₂ can also be used to create films to coat any surface, such as glass or surgical tools.

By having these coatings, the water cannot wet them and runs on them, like rain would on car windshields. Tools with these coatings could kill bacteria by absorbing UV radiation.

Dog urine or chewing gum could not be fixed on asphalt or cement by the action of TiO ₂, which would facilitate its subsequent removal.

Sunscreen

TiO2 is one of the active components of sunscreen. Source: Pixabay.

And finally, with regard to TiO ₂, it is a photocatalyst, capable of originating organic radicals that, however, are neutralized by silica or alumina films in sunscreen. Its white color already clearly indicates that it must have this titanium oxide.

Aerospace industry

Titanium alloys are used to make large airplanes or fast ships. Source: Pxhere.

Titanium is a metal with considerable strength and hardness in relation to its low density. This makes it a substitute for steel for all those applications where high speeds are required, or large wingspan aircraft are designed, such as the A380 aircraft in the image above.

That is why this metal has many uses in the aerospace industry, as it resists oxidation, it is light, strong and its alloys can be improved with the exact additives.

Sport

Not only in the aerospace industry does titanium and its alloys take center stage, but also in the sports industry. This is because many of its utensils need to be lightweight so that their wearers, players or athletes, can handle them without feeling too heavy.

Some of these items are: bicycles, golf or hockey sticks, football helmets, tennis or badminton rackets, fencing swords, ice skates, skis, among others.

Also, although to a much lesser degree due to its high cost, titanium and its alloys have been used in luxury and sports cars.

Pyrotechnics

The ground titanium can be mixed with, for example, KClO ₄, and serve as a firework; that in fact, those who make them in pyrotechnic shows do.

Medicine

Titanium and its alloys are the metallic materials par excellence in biomedical applications. They are biocompatible, inert, strong, difficult to oxidize, non-toxic, and integrate seamlessly with bone.

This makes them very useful for orthopedic and dental implants, for artificial hip and knee joints, as screws to fix fractures, for pacemakers or artificial hearts.

Biological

The biological role of titanium is uncertain, and although it is known that it can accumulate in some plants and benefit the growth of certain agricultural crops (such as tomatoes), the mechanisms where it intervenes are unknown.

It is said to promote the formation of carbohydrates, enzymes, and chlorophylls. They conjecture that it is due to a response of plant organisms to defend themselves against the low bioavailable concentrations of titanium, since they are harmful to them. However, the matter is still in the dark.

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