322 .METALS AND THEIR COMBINATIONS. 



Poisonous properties and antidotes. Compounds of lead are directly 

 poisonous, and it happens, not infrequently, that water passing through leaden 

 pipes or collected in leaden tanks becomes contaminated with lead. Water 

 free from air and salts scarcely acts on lead ; but if it contain air, oxide of lead 

 is formed, which is either dissolved by the water or is decomposed by the 

 nitrates or chlorides present in the water, the soluble nitrate or chloride of lead 

 being formed. 



If the water contains carbonates and sulphates, however, these will form 

 insoluble compounds, producing a film or coating over the lead, preventing 

 further contact with the water. Rain water, in consequence of its containing 

 atmospheric constituents, and no sulphates, acts as a solvent on lead pipe ; 

 spring and river waters generally do not. 



Water containing lead will show a dark color on passing hydrogen sulphide 

 through it ; if the quantity present be very small, the water should be evapo- 

 rated to ^ or even -^ of its original volume before applying the test. 



The constant handling of lead compounds is one of the causes of lead 

 poisoning (painters' colic). As an antidote, mangesium sulphate should be 

 used, which forms with lead an insoluble sulphate ; the purgative action of 

 magnesia is also useful. (In lead works workmen often drink water containing 

 a little sulphuric acid.) 



Tests for lead. 

 (Use a 5 per cent, solution of lead acetate or lead nitrate.) 



1. Hydrogen sulphide or ammonium sulphide added to the solution 

 produces a black precipitate of lead sulphide (Plate III.,, 1), insol- 

 uble in dilute acids or alkalies. A very delicate reaction : 



Pb(N0 3 ) 2 + H 2 S PbS + 2HN0 3 . 



2. Dilute sulphuric acid or a solution of a sulphate gives a white 

 precipitate of lead sulphate, PbSO 4 . This is one of the four insoluble 

 sulphates. (See test 2 for sulphates.) 



3. Other reagents which give precipitates with solutions of lead 

 salts are : 



Hydrochloric acid or solution of a chloride, producing white lead 

 chloride, PbCl 2 . (See test 3 for hydrochloric acid.) 



Potassium iodide, producing yellow lead iodide, PbI 2 (Plate III., 

 6). (See test 2 for iodides.) 



Potassium chromate, producing yellow lead chromate (chrome-yel- 

 low), PbCr0 4 (Plate II., 6). (See test 2 for chromates.) 



Alkali carbonates, producing white basic lead carbonate. 



Alkali phosphates, producing white lead phosphate, PbHPO 4 . 



Solution of sodium hydroxide, producing white lead hydroxide, 

 Pb(OH) 2 , which dissolves in excess of the alkali, forming sodium 

 plumbite, Pb(ONa) 2 . (See comments on tests for zinc, page 317.) 



