4 Safety
757 VA Computrace – Hardware
52
4.2
Safety considerations concerning mercury
4.2.1
Properties of mercury
The most important properties of mercury (Hg) are listed in the Table below. This
compilation allows the following summary:
•
Mercury is a
heavy metal
with a very high density and is
liquid
at room tem-
perature.
•
Mercury is mobile at room temperature and tends to
form drops
because of
its high surface tension. The surface tension is around 6 times greater than
that of water, Hg is thus not wetted by water.
•
Mercury has a relatively
high electrical conductivity
(at room temperature it
is only some 60 times lower than that of silver).
•
Mercury has a relatively
high vapor pressure
compared with other metals.
Mercury vapor is some seven times heavier than air (so that it sinks rapidly
and specifically to the floor).
•
The
odor threshold is very high
relative to the threshold limit value (TLV).
•
Air saturated with Hg vapor (which naturally does not occur in practice) con-
tains approximately 250 times the amount of Hg specified by the TLV at room
temperature.
Properties of mercury
Property
Value
Ref.
Density
ρ
(liquid mercury)
13.5451
g/cm
3
(at
Θ
= 0 ° C)
[1]
Density
ρ
(mercury vapor)
8.959
g/dm
3
(at
Θ
= 0 ° C)
[2]
Melting point
Θ
F
–38.86
° C
(at
p
air
= 1.01325 bar)
[3]
Melting enthalpy
∆
H
F
2.295
kJ/mol
(at
p
air
= 1.01325 bar)
[3]
Boiling point
Θ
V
356.73
° C
(at
p
air
= 1.01325 bar)
[3]
Boiling enthalpy
∆
H
F
59.1
kJ/mol
(at
p
air
= 1.01325 bar)
[3]
Vapor pressure
p
0.0253
Pa
(at
Θ
= 0 ° C)
0.17
Pa
(at
Θ
= 20 ° C)
0.391
Pa
(at
Θ
= 30 ° C)
0.81
Pa
(at
Θ
= 40 ° C)
1.69
Pa
(at
Θ
= 50 ° C)
[2, 4]
Mass concentration
ρ
in air
(after reaching equilibrium)
2.0
mg/m
3
(at
Θ
= 0 ° C)
13.6
mg/m
3
(at
Θ
= 20 ° C)
29.6
mg/m
3
(at
Θ
= 30 ° C)
62.7
mg/m
3
(at
Θ
= 40 ° C)
126
mg/m
3
(at
Θ
= 50 ° C)
[2, 4]
Evaporation rate
85
µ
g/h
·
cm
2
(at
Θ
= 25 ° C)
[2]
Surface tension
σ
4.67
·
10
–3
N/cm
(at
Θ
= 20 ° C)
[5]
Electrical conductivity
κ
1.044
·
10
4
S/cm
(at
Θ
= 20 ° C)
[6]
Odor threshold
13
mg/m
3
[2]
Threshold limit value (TLV) for air
for mercury
for organic mercury com-
pounds (calculated as Hg)
0.1
mg/m
3
0.01
mg/m
3
[4, 7]
[2, 4, 7]
