2. CHEMICAL AND PHYSICAL INFORMATION
Barium is a dense alkaline earth metal in Group IIA of the periodic table. Naturally
occurring barium is a mix of seven stable isotopes. There are more than 20 known isotopes, but
most of them are highly radioactive and have half-lives ranging from several milliseconds to
several minutes. The free element is a silver-white soft metal that oxidizes readily in moist air
and reacts with water. Barium does not exist in nature in the elemental form but occurs as the
divalent cation in combination with other elements (Agency for Toxic Substances and Disease
Registry [ATSDR], 1992). The physical and chemical properties of barium and selected barium
compounds are presented in Table 1–1. The barium compounds with the following counter ions
are relatively soluble in water: acetate, nitrate, and halides (except fluoride), whereas compounds
with carbonate, chromate, fluoride, oxalate, phosphate, and sulfate counter-ions are quite
insoluble in water (World Health Organization [WHO], 2001).
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Barium makes up 0.05% of the earth’s crust, and the two most prevalent naturally
occurring barium compounds are barite (barium sulfate) and witherite (barium carbonate) ores.
Barium enters the environment through the weathering of rocks and minerals and through
anthropogenic releases. The primary source of barium in the atmosphere is industrial emissions
(ATSDR, 1992). Barium concentrations ranging from 2 × 10
-4
to 2.8 × 10
-2
:
g/m
3
(mean of 1.2 ×
-2
:
g/m
3
) have been detected in urban areas of North America (ATSDR, 1992). Barium is
naturally occurring in most surface waters and in public drinking water supplies. Barium content
in U.S. drinking water supplies ranges from 1 to 20
:
g/L; in some areas barium concentrations as
high as 10,000
:
g/L have been detected (WHO, 1990). Barium is ubiquitous in soils, with
concentrations ranging from 15 to 3000 ppm (ATSDR, 1992).
The primary route of exposure to barium appears to be ingestion from food and drinking
water. Barium is found in many food groups. In most foods, the barium content is relatively low
(<3 mg/100 g) except in Brazil nuts, which have a very high barium content (150-300 mg/100 g)
(WHO, 1990). Bread is considered the largest source of dietary barium, contributing an
estimated 20% of total intake (Ysart et al., 1999). The WHO (1990) reported several published
estimates of dietary intake of barium by humans; daily dietary intake ranged from 300 to 1770
:
g Ba/day, with wide variations; this is equivalent to 4-25
:
g Ba/kg-day, assuming a 70 kg adult
body weight. A daily intake of 0.03-0.60
:
g Ba/kg-day from drinking water can be estimated by
using the drinking water concentration of 1-20
:
g/L, a reference consumption rate of 2 L/day,
and body weight of 70 kg. The range from these two sources combined is 0.004-0.026 mg
3
Ba/kg-day. The chemical and physical properties of barium and selected barium compounds are
shown in Table 1–1.
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Table 1–1. Physical and chemical properties of barium and selected barium compounds
Barium
Barium acetate
Barium
carbonate
Barium
chloride
Barium
hydroxide
Barium oxide
Barium sulfate
CAS Registry
number
a
7440-39-3
543-80-6
513-77-9
10361-37-2
17194-00-2
1304-28-5
7727-43-7
Molecular
formula
Ba
Ba(C
2
H
3
O
2
)
2
BaCO
3
BaCl
2
Ba(OH)
2
@8H
2
O
BaO
BaSO
4
Molecular
weight
137.34
255.43
197.35
208.25
315.48
153.34
233.4
Melting point,
°C
725
41
a
1740 (
" form,
at 90 atm)
a
963 78 1923
1580
(decomposes)
Boiling point, °C
1640
no data
decomposes
1560
550
a
2000
1149 (monoclinal
transition point)
a
Vapor pressure,
mm Hg
10 at 1049 °C
no data
essentially zero
a
essentially zero
a
no data
a
essentially zero
a
no data
a
Water solubility,
forms barium
588 at 0 °C,
0.02 at 20 °C,
375 at 20 °C
a
56 at 15 °C,
38 at 20 °C,
0.00222 at 0 °C,
g/L
hydroxide
750 at 100 °C
0.06 at 100 °C
947 at 78 °C
908 at 100 °C
0.00413 at 100 °C
Specific gravity
3.5 at 20 °C
2.468
4.43
3.856 at 24 °C
2.18 at 16 °C
5.72
4.50 at 15 °C
a
ATSDR, 1992.
Source: Weast and Astle, 1981, unless otherwise noted.
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