Metrohm 757 VA Computrace - Manual (Page 57)

4.2 Safety considerations concerning mercury
757 VA Computrace – Hardware
53
4.2.2 Toxicity of mercury and its compounds
Mercury and its compounds are toxic since they react with enzymes containing sul-
fur and decompose them with the formation of HgS. The toxicity depends on the
chemical and physical state of the mercury [4, 8 – 10]:
• Metallic liquid mercury is readily resorbed by the skin and finds its way
through glandular passages into lower skin regions where it is oxidized and
carried on as a salt.
• The low-solubility mercury (I) compounds and metallic mercury in the form
of a coherent liquid have low toxicity when taken up orally (but not through the
skin!).
• Mercury (II) compounds are more readily soluble and therefore much more
toxic: LD100 (the 100% lethal dose) for oral take-up is approx. 0.2…1 g.
• Mercury vapor is highly toxic: vapor with an Hg concentration exceeding the
TLV of 0.1 mg/m 3 air causes chronic poisoning after prolonged breathing for 5
to 8 hours per day.
Despite the large number of laboratories involved in polarographic/voltammetric
work, sensible and proper handling (see section 4.2.3) has ensured that not one
single case of mercury poisoning has been reported to date. The real Hg concen-
trations measured in the laboratory atmosphere are consistently far below the TLV
(threshold limit value).
4.2.3 Handling of mercury
Several safety rules, described in detail in what follows, must be observed in the
handling of mercury owing to its toxicity (see section 4.2.2):
• Working in a fume cupboard
The handling of mercury should, if possible, always be carried out in a fume
cupboard (hood). It must be ensured that no metal drops or spilling drop on
the floor or the lab bench and that no evaporation of the metal occurs.
• Working over plastic trays
Movements with vessels containing mercury must be carried out in, or at least
above, rigid seamless trays made from plastic or enamelled metal. The sup-
plied 6.2711.030 Drip pan made of polystyrene is eminently suitable for this.
• Collecting mercury from the measuring vessel
If work is performed with the MME, at the end of the determination the analy-
sis solution contains mercury which must be collected for later disposal. This
can be done by collecting the analysis solutions in a large vessel and then
decanting, by filtering the analysis solutions or by siphoning off the mercury
using vacuum.