and Laboratory Methods. 



2011 



acid. 



MICRO-CHEMICAL ANALYSIS. 

 XX. 



SILVER GROUP CONTINUED— LEAD. 



IV. Ammonium Dichromaie added to solutions of salts oj Lead precipitates 

 Lead Chromate. 



2Pb(N03)2+(NH4)2Cr207+H20=2PbCr04+2NH4N03-i-2HN03. 



Method. T'o the drop of the solution to be tested add a drop of dilute nitric 

 Next to the test drop thus prepared place a drop of a solution of the 

 reagent. Heat the two drops simultaneously and 

 while they are hot cause the reagent to flow into the 

 test drop. Thin, slender, yellow prisms (mono- 

 clinic ?) are immediately formed (Fig. 83). These 

 prisms are sometimes grouped in radiating masses. 

 More often they occur singly. 



Remarks. The addition of the reagent to a cold 

 neutral solution seldom yields more than a heavy 

 amorphous precipitate bright yellow in color. 



From barium chromate the lead salt is readily 

 distinguished by adding a minute amount of sodium 

 or potassium hydroxide ; under this treatment lead 

 chromate is converted into basic chromate, orange red or even bright red in 

 color of variable composition. Too much alkali will completely dissolve the lead 

 chromate ; but if to this alkaline solution ammonium carbonate is added, red 

 basic chromates are precipitated. 



Mercurous salts when treated as described above yield bright red crystals of 

 peculiar form and variable composition (see Mercury). Although it is not likely 

 that these salts can be confused with the lead compound, if any doubt should 

 arise, they can be differentiated by treating with ammonium hydroxide, in which 

 the mercurous salts are not soluble while the lead compound dissolves. 



The behavior of mixtures of silver and lead has already been described under 

 the former element. Lead does not form a dichromate. In the presence of 

 silver interesting mixed chromates of silver and lead separate. 



Potassium chromate yields results similar to those obtained with the dichro- 

 mate. 



The chromates of strontium and barium have already been discussed under 

 those elements. 



Thallous salts yield with ammonium dichromate long acicular crystals of 

 thallous dichromate TLCr,0,. 



Fig. 83. 



Exercises for Practice. 



See suggestions given under Mercury. 

 Cornell University. 



E. M. Chamot. 



