Dictionary of Chemistry [6th Ed.]

used to produce other titanium com-

pounds and pure titanium metal.

titanium dioxideSee titanium(iv)

oxide.

titanium(IV) oxide (titania; tita-

nium dioxide)A white oxide, TiO 2 ,

occurring naturally in various forms,

particularly the mineral rutile. It is

used as a white pigment and as a

Üller for plastics, rubber, etc.

titrationA method of volumetric

analysis in which a volume of one

reagent (the titrant) is added to a

known volume of another reagent

slowly from a burette until an end

point is reached (see indicator). The

volume added before the end point is

reached is noted. If one of the solu-

tions has a known concentration,

that of the other can be calculated.

TNTSee trinitrotoluene.

tocopherol See vitamin e.

Tollens reagent A reagent used in

testing for aldehydes. It is made by

adding sodium hydroxide to silver ni-

trate to give silver(I) oxide, which is

dissolved in aqueous ammonia (giv-

ing the complex ion [Ag(NH 3 ) 2 ]+). The

sample is warmed with the reagent

in a test tube. Aldehydes reduce the

complex Ag+ion to metallic silver,

forming a bright silver mirror on the

inside of the tube (hence the name

silver-mirror test). Ketones give a neg-

ative result. It is named after Bern-

hard Tollens (1841–1918).

tolueneSee methylbenzene.

topazA variably coloured alu-

minium silicate mineral,

Al 2 (SiO 4 )(OH,F) 2 , that forms or-

thorhombic crystals. It occurs chieÛy

in acid igneous rocks, such as gran-

ites and pegmatites. Topaz is valued

as a gemstone because of its trans-

parency, variety of colours (the wine-

yellow variety being most highly

prized), and great hardness (8 on the

Mohs’ scale). When heated, yellow or

brownish topaz often becomes a

rose-pink colour. The main sources of

topaz are Brazil, Russia, and the USA.

torrA unit of pressure, used in

high-vacuum technology, deÜned as

1 mmHg. 1 torr is equal to 133.322

pascals. The unit is named after

Evangelista Torricelli (1609–47).

torsion angleIn a nonlinear chain

of atoms A–B–C–D, the angle be-

tween the plane containing atoms

ABC and the plane containing BCD.

The torsion angle can have any value

from 0°to 180°. If the chain is

viewed along the line BC, the torsion

angle is positive if the bond AB

would have to be rotated in a clock-

wise sense (less than 180°) to eclipse

(i.e. align with) the bond CD. If the

rotation of AB has to be in an anti-

clockwise sense, the torsion angle

would be negative.

There is a terminology relating to

the steric arrangements of atoms

based on the size of the torsion angle

(see also conformation). The syn

arrangement is one in which the size

of the torsion angle is ± 90 °. The anti

arrangement is one with a torsion

titanium dioxide 530

t

synperiplanar

antiperiplanar

+synclinal

+anticlinal

–synclinal

–anticlinal

–30°

0°

+30°

sp sp

+sc

+90°

+ac

+150°

ap

–150°

• ac

–90°

• sc

Torsion angle