T

2,4,5-T 2,4,5-trichlorophenoxyacetic
acid (2,4,5-trichlorophenoxyethanoic
acid): a synthetic auxin formerly
widely used as a herbicide and de-
foliant. It is now banned in many
countries as it tends to become con-
taminated with the toxic chemical
dioxin.

tabunA highly toxic colourless or
brown liquid, C 5 H 11 N 2 O 2 P; r.d. 1.09;
m.p. –50°C; b.p. 247.5°C. It is an
organophosphorus compound, ethyl
N,N-dimethylphosphoramido-
cyanidate. Tabun was discovered in
1936 and belongs to the G-series of
*nerve agents (GA). It was used by
Iraq in the Iran–Iraq war (1980–88).

tactic polymerSee polymer.

tactosol A colloidal sol containing
nonspherical particles capable of ori-
entating themselves. An example of a
tactosol is given by an aged vana-
dium pentoxide sol, in which the
particles are rod-shaped. When a tac-
tosol is placed in a magneticÜeld its
particles arrange themselves along
the lines of force of theÜeld.

Tafel plot The graph of the loga-
rithm of the current density j against
the overpotential ηin electrochem-
istry in the high overpotential limit.
This limit can occur in electrolysis ei-
ther when the overpotential is large
and positive, the electrode being the
anode, or when the overpotential is
large and negative, the electrode
being the cathode. In the case of pos-
itive overpotential, the second expo-
nential in the *Butler–Volmer
equation can be ignored, as it is
much smaller than theÜrst exponen-
tial, and thus

j = j 0 exp[(1 – α)fη]

In the case of a negative overpoten-

tial, theÜrst exponential can be ig-

nored, which gives j = j 0 exp(–αfη), so

that ln(–j) = lnj 0 – αfη. Thus, in the

Tafel plot the value of αcan be ob-

tained from the slope and the value

of j 0 can be obtained from the inter-

cept at η= 0.

Takayama test See haemochro-

mogen test.

talcA white or pale-green mineral

form of magnesium silicate,

Mg 3 Si 4 O 10 (OH) 2 , crystallizing in the

triclinic system. It forms as a sec-

ondary mineral by alteration of mag-

nesium-rich olivines, pyroxenes, and

amphiboles of ultrabasic rocks. It is

soaplike to touch and very soft, hav-

ing a hardness of 1 on the Mohs’

scale. MassiveÜne-grained talc is

known as soapstoneor steatite. Talc

in powdered form is used as a lubri-

cant, as aÜller in paper, paints, and

rubber, and in cosmetics, ceramics,

and French chalk. It occurs chieÛy in

the USA, Russia, France, and Japan.

tannic acidA yellowish complex

organic compound present in certain

plants. It is used in dyeing as a mor-

dant.

tanninOne of a group of complex

organic chemicals (see depsides) com-

monly found in leaves, unripe fruits,

and the bark of trees. Their function

is uncertain though the unpleasant

taste may discourage grazing ani-

mals. Some tannins have commercial

uses, notably in the production of

leather and ink.

tantalumSymbol Ta. A heavy blue-