talline solid, H 2 S 2 O 7 ; r.d. 1.9; m.p.
35 °C. It is commonly encountered
mixed with sulphuric acid as it is
formed by dissolving sulphur triox-
ide in concentrated sulphuric acid.
The resulting fuming liquid, called
oleumor Nordhausen sulphuric acid,
is produced during the contact
process and is also widely used in the
sulphonation of organic com-
pounds. See also sulphuric acid.
dithionate A salt of dithionic acid,
containing the ion S 2 O 6 2–, usually
formed by the oxidation of a sulphite
using manganese(IV) oxide. The ion
has neither pronounced oxidizing
nor reducing properties.
dithionic acidAn acid, H 2 S 2 O 6 ,
known in the form of its salts
(dithionates).
dithioniteSee sulphinate.
dithionous acid See sulphinic
acid.
divalent(bivalent)Having a va-
lency of two.
DLASee diffusion limited aggre-
gation.
D-lines Two close lines in the yel-
low region of the visible spectrum of
sodium, having wavelengths 589.0
and 589.6 nm. As they are prominent
and easily recognized they are used
as a standard in spectroscopy.
dl-isomerSee optical activity;
racemic mixture.
DLVO theory A theory of colloid
stability proposed in the 1940s by the
Soviet scientists Boris Derjaguin and
Lev Landau and independently by the
Dutch scientists Evert Verwey and
Theo Overbeek. The DLVO theory
takes account of two types of force in
a stable colloid: the van der Waals’
force, which is attractive and binds
particles together, and electrostatic
repulsion. The total interaction po-
tential can be calculated as a func-
tion of distance, with colloid stability
being attained when the two forces
balance each other. The DLVO theory
is the basis for understanding colloid
stability and has a considerable
amount of experimental support.
However, it is inadequate for the
properties of colloids in the aggre-
gated state, which depend on short-
range interactions taking into
account the speciÜc properties of
ions, rather than regarding them as
point particles.p-DMAB See p-dimethylaminoben-
zadehyde.
DMFSee dimethylformamide.DMGSee dimethylglyoxime.DMSOSee dimethyl sulphoxide.
DNA(deoxyribonucleic acid)The ge-
netic material of most living organ-
isms, which is a major constituent of
the chromosomes within the cell nu-
cleus and plays a central role in the
determination of hereditary charac-
teristics by controlling protein syn-
thesis in cells. DNA is a nucleic acid
composed of two chains of *nu-
cleotides in which the sugar is de-
oxyriboseand the bases are
*adenine, *cytosine, *guanine, and
*thymine (compare rna). The two
chains are wound round each other
and linked together by hydrogen
bonds between speciÜc complemen-
tary bases to form a spiral ladder-
shaped molecule (double helix: see
illustration).
When the cell divides, its DNA also
replicates in such a way that each of
the two daughter molecules is identi-
cal to the parent molecule. The
hydrogen bonds between the com-
plementary bases on the two strands
of the parent molecule break and the
strands unwind. Using as building
bricks nucleotides present in the
nucleus, each strand directs the183 DNA
d