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 defoliant. It is now banned in many
countries as it tends to become contaminated with the toxic chemical
dioxin.
tabun A 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-dimethylphosphoramidocyanidate. 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 polymer See polymer.
tactosol A colloidal sol containing
nonspherical particles capable of orientating themselves. An example of a
tactosol is given by an aged vanadium pentoxide sol, in which the
particles are rod-shaped. When a tactosol is placed in a magnetic Üeld its
particles arrange themselves along
the lines of force of the Üeld.
Tafel plot The graph of the logarithm of the current density j against
the overpotential η in electrochemistry in the high overpotential limit.
This limit can occur in electrolysis either 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 positive overpotential, the second exponential in the *Butler–Volmer
equation can be ignored, as it is
much smaller than the Ürst exponential, and thus
j = j 0 exp[(1 – α)fη]
In the case of a negative overpotential, the Ürst exponential can be ignored, which gives j = j 0 exp(–αfη), so
that ln(–j) = lnj 0 – αfη. Thus, in the
Tafel plot the value of α can be obtained from the slope and the value
of j 0 can be obtained from the intercept at η = 0.
Takayama test See haemochromogen test.
talc A 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 secondary mineral by alteration of magnesium-rich olivines, pyroxenes, and
amphiboles of ultrabasic rocks. It is
soaplike to touch and very soft, having a hardness of 1 on the Mohs’
scale. Massive Üne-grained talc is
known as soapstone or steatite. Talc
in powdered form is used as a lubricant, as a Üller in paper, paints, and
rubber, and in cosmetics, ceramics,
and French chalk. It occurs chieÛy in
the USA, Russia, France, and Japan.
tannic acid A yellowish complex
organic compound present in certain
plants. It is used in dyeing as a mordant.
tannin One of a group of complex
organic chemicals (see depsides) commonly found in leaves, unripe fruits,
and the bark of trees. Their function
is uncertain though the unpleasant
taste may discourage grazing animals. Some tannins have commercial
uses, notably in the production of
leather and ink.
tantalum Symbol Ta. A heavy bluewww.AzShimi.ir www.AzShimi.com
2,4,5-T 2,4,5-trichlorophenoxyacetic
acid (2,4,5-trichlorophenoxyethanoic
acid): a synthetic auxin formerly
widely used as a herbicide and defoliant. It is now banned in many
countries as it tends to become contaminated with the toxic chemical
dioxin.
tabun A 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-dimethylphosphoramidocyanidate. 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 polymer See polymer.
tactosol A colloidal sol containing
nonspherical particles capable of orientating themselves. An example of a
tactosol is given by an aged vanadium pentoxide sol, in which the
particles are rod-shaped. When a tactosol is placed in a magnetic Üeld its
particles arrange themselves along
the lines of force of the Üeld.
Tafel plot The graph of the logarithm of the current density j against
the overpotential η in electrochemistry in the high overpotential limit.
This limit can occur in electrolysis either 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 positive overpotential, the second exponential in the *Butler–Volmer
equation can be ignored, as it is
much smaller than the Ürst exponential, and thus
j = j 0 exp[(1 – α)fη]
In the case of a negative overpotential, the Ürst exponential can be ignored, which gives j = j 0 exp(–αfη), so
that ln(–j) = lnj 0 – αfη. Thus, in the
Tafel plot the value of α can be obtained from the slope and the value
of j 0 can be obtained from the intercept at η = 0.
Takayama test See haemochromogen test.
talc A 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 secondary mineral by alteration of magnesium-rich olivines, pyroxenes, and
amphiboles of ultrabasic rocks. It is
soaplike to touch and very soft, having a hardness of 1 on the Mohs’
scale. Massive Üne-grained talc is
known as soapstone or steatite. Talc
in powdered form is used as a lubricant, as a Üller in paper, paints, and
rubber, and in cosmetics, ceramics,
and French chalk. It occurs chieÛy in
the USA, Russia, France, and Japan.
tannic acid A yellowish complex
organic compound present in certain
plants. It is used in dyeing as a mordant.
tannin One of a group of complex
organic chemicals (see depsides) commonly found in leaves, unripe fruits,
and the bark of trees. Their function
is uncertain though the unpleasant
taste may discourage grazing animals. Some tannins have commercial
uses, notably in the production of
leather and ink.
tantalum Symbol Ta. A heavy bluewww.AzShimi.ir www.AzShimi.com