and Laboratory Methods. 1895 



MICRO-CHEMICAL ANALYSIS. 

 XX. 



SILVER GROUP CONTINUED— LEAD. 



//. The addition of dilute Sulphuric Acid precipitates Lead Sulphate. 

 Pb(N03)2+H2S04=PbS04+2HN03 



Method. — To the rather dilute test drop add a drop of dilute sulphuric acid 



by the flowing in method. A dense granular precipitate is at once produced. 



This precipitate is generally at first amorphous, but soon becomes crystalline, or 



^^_^ to be more accurate, part of the tiny crystalline grains 



/ ^ "^"^^m^^ soon grow to a size and development permitting their 



^ni^ ^-\ \'^ 4r\'^ ^^^^ recognition. Crystallites, lenticular grains, plates, 



jyVry/^ % and thin rhombs are seen (Fig. 81). 



J ^ ^j\ /\ ^ ^s^> Remarks. — In solutions of substances containing 



'\/^:y~--^ ^/i ^^ other elements in larger amount than the lead, the 



o^L^ (^"v^ ^^ precipitate of lead sulphate is usually amorphous, but 



'^ I ■ ■ I when the lead is in excess, or in large amount, the 



p, ' ' ' precipitate is generally crystalline. 



When the reagent drop is added at once to the 

 test drop as in testing for calcium, strontium and barium, the precipitate of lead 

 sulphate is apt to be amorphous. 



Recrystallized from hot concentrated sulphuric acid in the manner employed 

 in the cases of strontium and barium (q. v.) the crystals first obtained closely 

 resemble those given by strontium, while a little later crystallites with feathery 

 arms appear, which cannot be distinguished from those of the barium salt. This 

 is to be expected since the sulphates of lead and barium are isomorphous. If 

 after heating with concentrated sulphuric acid, the preparation is placed in a 

 desiccator, thus excluding moisture from the air, the crystals separating have, 

 according to Schultz, * the formula PbSO^. HgSO^- H2O. 



Lead sulphate can also be recrystallized by heating with strong hydrochloric 

 acid or with strong nitric acid. In either case there is generally some decompo- 

 sition and there appear on cooling, in addition to the rhombs of lead sulphate, 

 crystals of lead chloride or lead nitrate. 



Although it is seldom that the analyst will be deceived as to the character 

 of an amorphous white precipitate obtained with sulphuric acid, or that he will 

 meet with a precipitate consisting of the mixed sulphates of lead, barium, and 

 strontium, he must be on his guard against such cases. When in doubt, ad- 

 vantage may be taken of the fact that lead sulphate is soluble in moderately 

 concentrated solutions of sodium or potassium hydroxides, or concentrated solu- 

 tions of ammonium acetate. In this manner a satisfactory separation may 

 usually be effected. The decanted or filtered solution containing the lead can 



*Pogg. An. 133, 137. 



