Modern inorganic chemistry

436 THE ELEMENTS OF GROUPS IB AND MB

USES

Mercury is extensively used in various pieces of scientific apparatus,

such as thermometers, barometers, high vacuum pumps, mercury

lamps, standard cells (for example the Weston cell), and so on. The

metal is used as the cathode in the Kellner-Solvay cell (p. 130).

Mercury compounds (for example mercury(II) chloride) are used

in medicine because of their antiseptic character. The artificial red

mercury(II) sulphide is the artist's 'vermilion^1. Mercury(II) sulphate

is a catalyst in the manufacture of ethanal from ethyne:

C 2 H 2 + H 2 O ^^ CH 3. CHO

COMPOUNDS OF MERCURY

The chemistry of mercury compounds is complicated by the

equilibrium

The relevant redox potentials are :

Hg2+(aq) 4- 2e~ -> Hg(I) : E^ = 0.85 V

+ 2e~ -> 2Hg(I) : E^ = 0.79 V

Hence mercury is a poor reducing agent; it is unlikely to be

attacked by acids unless these have oxidising properties (for example

nitric acid), or unless the acid anion has the power to form complexes

with one or both mercury cations Hg2+ or Hgf +, so altering the

E^ values. Nitric acid attacks mercury, oxidising it to Hg2+(aq)

when the acid is concentrated and in excess, and to Hg2+(aq) when

mercury is in excess and the acid dilute. Hydriodic acid HI(aq)

attacks mercury, because mercury(II) readily forms iodo-complexes

(see below, p. 438).

Oxidation state +1

The mercury(I) ion has the structure

so that each mercury atom is losing one electron and sharing one
electron, i.e. is 'using' two valency electrons. The existence of Hg| +
has been established by experiments in solution and by X-ray
diffraction analysis of crystals of mercury(I) chloride, Hg 2 Cl 2 where