Metrohm 757 VA Computrace - Manual (Page 56)

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 µg/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]