zinc chloride rather than ammonium
chloride. The electrical characteristics are similar to those of the
Leclanché cell but the cell works better at low temperatures and has
more efÜcient depolarization characteristics. A number of alkaline secondary cells can be designed for use
as dry cells. In these, the electrolyte
is a liquid (sodium or potassium hydroxide) held in a porous material or
in a gel. Alkaline dry cells typically
have zinc–manganese dioxide, silver
oxide–zinc, nickel–cadmium, or
nickel–iron electrode systems (see
nickel–iron accumulator). For specialized purposes, dry cells and batteries have been produced with solid
electrolytes. These may contain a
solid crystalline salt, such as silver iodide, an ion-exchange membrane, or
an organic wax with a small amount
of dissolved ionic material. Such cells
deliver low currents. They are used
in miniature cells for use in electronic equipment.
dry ice Solid carbon dioxide used as
a refrigerant. It is convenient because
it sublimes at –78°C (195 K) at standard pressure rather than melting.
drying oil A natural oil, such as linseed oil, that hardens on exposure to
the air. Drying oils contain unsaturated fatty acids, such as linoleic and
linolenic acids, which polymerize on
oxidation. They are used in paints,
varnishes, etc.
DSC Differential scanning calorimetry. See thermal analysis.
D-series See absolute configuration.
DTA Differential thermal analysis.
See thermal analysis.
dubnium Symbol Db. A radioactive
*transactinide element; a.n. 105. It
was Ürst reported in 1967 by a group
at Dubna near Moscow and was
conÜrmed in 1970 at Dubna and at
Berkeley, California. It can be made
by bombarding californium–249 nuclei with nitrogen–15 nuclei. Only a
few atoms have ever been made.
A
• Information from the WebElements site
Dulong and Petit’s law For a
solid element the product of the relative atomic mass and the speciÜc
heat capacity is a constant equal to
about 25 J mol
–1 K
–1
. Formulated in
these terms in 1819 by the French
scientists Pierre Dulong (1785–1838)
and Alexis Petit (1791–1820), the law
in modern terms states: the molar
heat capacity of a solid element is approximately equal to 3R, where R is
the *gas constant. The law is only approximate but applies with fair accuracy at normal temperatures to
elements with a simple crystal structure.
A
• Original paper
Dumas, Jean Baptiste André
(1800–84) French chemist, who became an apothecary in Geneva,
where in 1818 he investigated the
use of iodine to treat goitre. He then
took up chemistry and moved to
Paris. In 1826 he devised a method of
measuring *vapour density. He went
on to discover various organic compounds, including anthracene (1832),
urethane (1833), and methanol
(1834), which led him in 1840 to propose the theory of types (functional
groups).
Dumas’ method 1. A method of
Ünding the amount of nitrogen in an
organic compound. The sample is
weighed, mixed with copper(II)
oxide, and heated in a tube. Any nitrogen present in the compound is
converted into oxides of nitrogen,
which are led over hot copper to reduce them to nitrogen gas. This is
187
Dumas’ method
d
www.AzShimi.ir www.AzShimi.com
chloride. The electrical characteristics are similar to those of the
Leclanché cell but the cell works better at low temperatures and has
more efÜcient depolarization characteristics. A number of alkaline secondary cells can be designed for use
as dry cells. In these, the electrolyte
is a liquid (sodium or potassium hydroxide) held in a porous material or
in a gel. Alkaline dry cells typically
have zinc–manganese dioxide, silver
oxide–zinc, nickel–cadmium, or
nickel–iron electrode systems (see
nickel–iron accumulator). For specialized purposes, dry cells and batteries have been produced with solid
electrolytes. These may contain a
solid crystalline salt, such as silver iodide, an ion-exchange membrane, or
an organic wax with a small amount
of dissolved ionic material. Such cells
deliver low currents. They are used
in miniature cells for use in electronic equipment.
dry ice Solid carbon dioxide used as
a refrigerant. It is convenient because
it sublimes at –78°C (195 K) at standard pressure rather than melting.
drying oil A natural oil, such as linseed oil, that hardens on exposure to
the air. Drying oils contain unsaturated fatty acids, such as linoleic and
linolenic acids, which polymerize on
oxidation. They are used in paints,
varnishes, etc.
DSC Differential scanning calorimetry. See thermal analysis.
D-series See absolute configuration.
DTA Differential thermal analysis.
See thermal analysis.
dubnium Symbol Db. A radioactive
*transactinide element; a.n. 105. It
was Ürst reported in 1967 by a group
at Dubna near Moscow and was
conÜrmed in 1970 at Dubna and at
Berkeley, California. It can be made
by bombarding californium–249 nuclei with nitrogen–15 nuclei. Only a
few atoms have ever been made.
A
• Information from the WebElements site
Dulong and Petit’s law For a
solid element the product of the relative atomic mass and the speciÜc
heat capacity is a constant equal to
about 25 J mol
–1 K
–1
. Formulated in
these terms in 1819 by the French
scientists Pierre Dulong (1785–1838)
and Alexis Petit (1791–1820), the law
in modern terms states: the molar
heat capacity of a solid element is approximately equal to 3R, where R is
the *gas constant. The law is only approximate but applies with fair accuracy at normal temperatures to
elements with a simple crystal structure.
A
• Original paper
Dumas, Jean Baptiste André
(1800–84) French chemist, who became an apothecary in Geneva,
where in 1818 he investigated the
use of iodine to treat goitre. He then
took up chemistry and moved to
Paris. In 1826 he devised a method of
measuring *vapour density. He went
on to discover various organic compounds, including anthracene (1832),
urethane (1833), and methanol
(1834), which led him in 1840 to propose the theory of types (functional
groups).
Dumas’ method 1. A method of
Ünding the amount of nitrogen in an
organic compound. The sample is
weighed, mixed with copper(II)
oxide, and heated in a tube. Any nitrogen present in the compound is
converted into oxides of nitrogen,
which are led over hot copper to reduce them to nitrogen gas. This is
187
Dumas’ method
d
www.AzShimi.ir www.AzShimi.com