Chemistry Reference
In-Depth Information
sodium sulfate eliminates interference from molybde-
num and other metals (NIOSH, 1977). A spectropho-
tometric method for the quantitative determination
of tungsten and molybdenum in the presence of each
other in the trace amounts expected in biological sam-
ples has been described by Cardenas and Mortenson
(1974). The method involves the selective formation
and extraction of the complexes of the metals with a
dithiol reagent (toluene-3,4-dithiol) under specifi c con-
ditions of acidity and temperature. Interference from
other components found in biological samples has
been found to be negligible. Estimation of trace con-
centrations of tungsten occurring in air and in water
can be performed by instrumental neutron activation
analysis (NAA) by use of automatic
and tensile strength of steel; it plays a vital role in the
production of a number of other alloys noted for their
hardness, such as chromium, cobalt, and tungsten alloy
used for tipping and facing lathe tools. Many drills and
cutting edges of tools are tipped with tungsten carbide,
which gives them a hardness comparable to that of
diamond. The metal is used for making fi laments for
incandescent lamps, and tungstates are used in X-ray
tubes, fl uorescent lamps, and lasers, and as pigments
in dyes and inks. Tungsten has acquired importance in
nuclear and space technology in the nozzles of rocket
motors and protecting shields for spacecraft (Rieck,
1967), as well as in heat-resistant coatings (Matejicek
et al
., 2005). The use of heavy metal tungsten alloys in
weapons has been introduced in small-caliber ammuni-
tion (“green bullets”) as a replacement for lead (ORNL,
1998) and in kinetic-energy penetrators as a replace-
ment for depleted uranium (ORNL, 1996). An increas-
ing interest has recently been shown in WO
3
thin fi lms,
with grain sizes of approximately 60 nm, for a variety
of applications such as optoelectronics, microelectron-
ics, selective catalysis, and environmental engineering
(Ramana
et al
., 2006).
-ray spectroscopy
(Cawse, 1974; Salmon, 1974). The detection limit of
these authors' method, using a thermal neutron fl ux
up to 10
14
n/cm
2
/sec for activation, was 3 ng/m
3
for
industrial air and 10
γ
g/kg for rainwater. The preci-
sion of the method was estimated at 5-10%. NAA can
also be used for the determination of trace amounts of
tungsten in biological material (Wester, 1974). Induc-
tively coupled plasma mass spectrometry (ICP-MS),
atomic emission spectrometry (ICP-AES), and high-
resolution ICP-MS (HR-ICP-MS) have been developed
to meet new demands on monitoring tungsten levels
in air (Rose, 1994) and solutions (Wang, 1999).
µ
4 ENVIRONMENTAL LEVELS
AND EXPOSURES
4.1 General Environment
There are few documentations of anthropogenic
release of tungsten into the general environment. His-
torical mining activities in Cornwall have resulted in
metal-polluted estuarine sediments that contain high
levels of tungsten (up to
3 PRODUCTION AND USES
3.1 Production
Wolframite and scheelite are the common, naturally
occurring sources of tungsten, the richest deposits
being found in China, Alaska, and Mexico. The world
production is in the order of 80,000 metric tons per
year. The ore is crushed and ground, concentrated by
various physical processes, converted to the oxide, and
reduced to the metal. Tungsten carbide is produced by
heating the fi nely powdered metal intimately mixed
with carbon in an atmosphere of hydrogen in an elec-
tric furnace. In the production of tungsten carbide
tools, the carbide is sintered with cobalt, which acts
as a binder. The sintered material is then ground to its
fi nal shape. Other metals may be added, such as chro-
mium, nickel, titanium, and tantalum, depending on
the properties required for the fi nal product.
650 mg/kg) (Yim, 1976).
A clear link from such sites to human exposures
remains to be established.
Inhalation of air and consumption of food are the
dominating pathways of tungsten exposures to the
general population. Air pollution resulting from
industrial activities or coal power plants may span
15-19 ng/m
3
in air (Germani
et al
., 1981; Ondov
et al
.,
1989). Background levels were reported as <10 ng/m
3
in air and (Jagielak and Mamont-Ciesla, 1979) and
in certain vegetables, 6.3-39
≈
g/kg fresh weight (in
onions) (Bibak
et al
, 1998). However, because of incom-
plete data, the total tungsten intake has been diffi cult
to estimate accurately (ATSDR, 2005).
The dietary intake of tungsten, estimated by NAA in
four subjects, ranged from 8.0-13.0
µ
g/day over a total
of eight total diet estimations (Wester, 1974). Drinking
water sampled in the three largest Swedish cities varied
in tungsten concentration from 0.03-0.1
µ
3.2 Uses
Tungsten is a valuable metal because it has the high-
est melting point of all metals, it has great strength at
high temperatures, and it has good conductivity for
electricity and heat. It is used to increase the hardness
g/L, depend-
ing on the sampling site. NAA was used (Boström and
Wester, 1967). In a survey of atmospheric trace elements
µ
