Modern inorganic chemistry

202 GROUP IV

Carbon disulphide is an excellent solvent for fats, oils, rubber,

sulphur, bromine and iodine, and is used industrially as a solvent for

extraction. It is also used in the production of viscose silk; when

added to wood cellulose impregnated with sodium hydroxide

solution, a viscous solution of 'cellulose xanthate' is formed, and

this can be extruded through a fine nozzle into acid, which decom-

poses the xanthate to give a glossy thread of cellulose,

Lead

LEAD(II) CARBONATE

Lead(II) carbonate occurs naturally as cerussite. It is prepared in the

laboratory by passing carbon dioxide through, or adding sodium

hydrogencarbonate to, a cold dilute solution of lead(II) nitrate or

lead(II) ethanoate:

Pb2+ + 2HCO3 -> PbCO 3 i + CO 2 | + H 2 O

If the normal carbonate is used, the basic carbonate or white lead,

Pb(OH) 2. 2PbCO 3. is precipitated. The basic carbonate was used

extensively as a base in paints but is now less common, having been

largely replaced by either titanium dioxide or zinc oxide. Paints

made with white lead are not only poisonous but blacken in urban

atmospheres due to the formation of lead sulphide and it is hardly

surprising that their use is declining.

LEAD(II) CHROMATE(Vl), PbCrO 4

Lead(II) chromate(VI) is precipitated when a soluble chromate(VI)

or dichromate( VI) is added to a solution of a lead salt in neutral or

slightly acid solution:

Pb2+ +CrOr ^PbCrOJ

2Pb2+ + Cr 2 O?" + H 2 O -+ 2PbCrO 4 | + 2H +

The precipitation of lead(II) chromate is used to estimate lead

gravimetrically: the yellow precipitate of lead(II) chromate is filtered

off, dried and weighed. Lead(II) chromate is used as a pigment under

the name "chrome yellow^1.

THE LEAD ACCUMULATOR

The most widely-used storage battery is the lead accumulator. Each
cell consists essentially of two lead plates immersed in an electrolyte