Lead Arsenate 43 



NOTES 



With large beakers such as are most al- 

 ways used in this washing process it is an 

 easy matter to put a stirring rod thru the 

 bottom or sides of the beaker. The student 

 should learn to stir without touching the 

 stirring rod to the beaker. In such a case as. 

 the one in hand the stirring is best done by 

 the force of the entering wash water. 



The quality of the sodium arsenate on the 

 market varies greatly. The best grades are 

 crystallin and hydrated. The inferior grades 

 are frequently anhydrous, massiv, of spongy 

 appearance and carry considerable sodium 

 carbonate and sulfate. 



If the two salts are not used in equivalent 

 proportions the litmus paper will show which 

 was taken in excess. Lead nitrate turns the 

 paper red while sodium arsenate turns it 

 blue. 



The washing takes out the excess of either 

 salt that may have been used, and the sodium 

 nitrate formd by the reaction. In the pres- 

 ence of any considerable quantity of these 

 salts the small particles of lead arsenate are 

 flocculated, that is, many thousands are 

 brot together in one floe. As the concen- 

 tration of the soluble salts is lowerd by wash- 

 ing the particles separate, are deflocculated. 



