21.5A: Titanium Metal

The discovery of titanium in 1791 is attributed to William Gregor, a Cornish vicar and amateur chemist. He isolated an impure oxide from ilmenite (FeTiO₃) by treatment with HCl and H₂SO₄. Titanium is the second most abundant transition metal on Earth (6320 ppm) and plays a vital role as a material of construction because of its: Excellent Corrosion Resistance, High Heat Transfer Efficiency, and Superior Strength-To-Weight Ratio. For example, when it's alloyed with 6% aluminum and 4% vanadium, titanium has half the weight of steel and up to four times the strength.

While a biological function in man is not known, it has excellent biocompatibility--that is the ability to be ignored by the human body's immune system--and an extreme resistance to corrosion. Titanium is now the metal of choice for hip and knee replacements.

Properties of titanium

Extraction of Titanium - the Kroll process

Wilhelm J. Kroll (Born November 24, 1889 - Died March 30, 1973) developed the process in Luxemburg around the mid 1930's and then after moving to the USA extended it to enable the extraction of Zirconium as well.

Titanium ores, mainly rutile (TiO₂) and ilmentite (FeTiO₃), are treated with carbon and chlorine gas to produce titanium tetrachloride.

\[\ce{TiO_2 + Cl_2 \rightarrow TiCl_4 + CO_2}\]

Fractionation

Titanium tetrachloride is purified by distillation (BP 136.4) to remove iron chloride.

Reduction

Purified titanium tetrachloride is reacted with molten magnesium under argon to produce a porous “titanium sponge”.

\[\ce{TiCl4 + 2Mg -> Ti + 2MgCl2}\]

Melting

Titanium sponge is melted under argon to produce ingots.

The Kroll process (ISIS Draw .skc file)

Titanium Halides

Titanium(IV) Halides

Formula	Color	MP	BP	Structure
TiF4	white	-	284	fluoride bridged
TiCl4	Colorless	-24	136.4	-
TiBr4	yellow	38	233.5	hcp I- but essentially monomeric cf. SnI4
TiI4	violet-black	155	377	hcp I- but essentially monomeric cf. SnI4

Preparations:

They can all be prepared by direct reaction of Ti with halogen gas (X₂). All are readily hydrolysed.
They are all expected to be diamagnetic.

Titanium(III) halides

Formula	Color	MP	BP	m (BM)	Structure
TiF3	blue	950d	-	1.75	-
TiCl3	violet	450d	-	-	BiI3
TiBr3	violet	-	-	-	BiI3
TiI3	violet-black	-	-	-	-

Preparations:

They can be prepared by reduction of TiX₄ with H₂.

Titanium Oxides and Aqueous Chemistry

Titanium oxides

Formula	Color	MP	m (BM)	Structure
TiO2	white	1892	diam.	r utile - Refractive Index 2.61-2.90 cf. Diamond 2.42

Preparations:

obtained from hydrolysis of TiX₄ or Ti(III) salts.

TiO₂ reacts with acids and bases.
In Acid:
TiOSO₄ formed in H₂SO₄ (Titanyl sulfate)
In Base:
MTiO₃ metatitanates (eg Perovskite, CaTiO₃ and ilmenite, FeTiO₃)
M₂TiO₄ orthotitanates.

Peroxides are highly Colored and can be used for Colorimetric analysis.
pH <1 [TiO₂(OH)(H₂O)x]+
pH 1-2 [(O₂)Ti-O-Ti(O₂)](OH) _x^2-x; x=1-6

[Ti(H₂O)₆]³⁺ -> [Ti(OH)(H₂O)₅]²⁺ + [H+] pK=1.4
TiO²⁺ + 2H⁺ + e- -> Ti³⁺ + H₂O E=0.1V

Representative complexes

TiCl₄ is a good Lewis acid and forms adducts on reaction with Lewis bases such as;

                2PEt3           ->      TiCl4(PEt3)2
                2MeCN           ->      TiCl4(MeCN)2
                bipy            ->      TiCl4(bipy)

Solvolysis can occur if ionisable protons are present in the ligand;

                2NH3            ->      TiCl2(NH2)2     +       2HCl
                4H2O            ->      TiO2.aq         +       4HCl
                2EtOH           ->      TiCl2(OEt)2     +       2HCl

TiCl₃ has less Lewis acid strength but can form adducts also;

                3pyr            ->      TiCl3pyr3

References

1. "Complexes and First-Row Transition Elements", D. Nicholls
2. "Basic Inorganic Chemistry", F.A. Cotton, G. Wilkinson and P.L. Gaus
3. "Advanced Inorganic Chemistry", F.A. Cotton, G. Wilkinson, C. A. Murillo, and M. Bochmann
4. "Chemistry of the Elements", Greenwood and Earnshaw

Contributors and Attributions

• Prof. Robert J. Lancashire (The Department of Chemistry, University of the West Indies)