426 ELEMENTARY CHEMICAL MICROSCOPY 



sheaves of acicular prisms appear and later there are formed 

 long thin prisms with square ends, giving polarization colors 

 and parallel extinction. Nitron nitrate has a very low solubility 

 even in warm water, hence the reaction is a delicate one. The 

 sheaves of white crystals, appearing brownish by reflected light, 

 are characteristic. 



In dilute solutions none of the salts of the common acids inter- 

 fere save iodides and bichromates. With these salts there may 

 be obtained crystals which closely resemble the nitrate but these 

 crystals disappear upon even gentle warming; nitron nitrate will 

 not. 



From concentrated solutions there may be obtained under 

 favorable conditions, precipitates with chlorates, perchlorates, 

 phosphates, chromates, bichromates, iodides, ferro- and ferri- 

 cyanides, oxalates and tartrates, but in no case in dilute solu- 

 tions with gentle warming should there be any difficulty in 

 differentiating between such precipitates and the crystals ob- 

 tained with nitrates. 



NITRITES. 



a. With Silver Nitrate there is obtained a felted mass of fine 

 needles with long acicular prisms at the outer edge of the mass, 

 changing into short stout prisms with imperfectly developed 

 ends. These crystals are colorless under the microscope and do 

 not show their greenish tint until viewed in masses by reflected 

 light. 



b. With Potassium Iodide and Starch. Add to the drop to be 

 tested a crystal of potassium iodide, then a little potato starch 

 and finally a trace of dilute sulphuric acid. The hydroiodic 

 acid set free by the acid is oxidized by the nitrous acid; iodine 

 is liberated and stains the starch blue or violet or black. 



Always test the potassium iodide, with starch and dilute sul- 

 phuric acid, to ascertain its purity and to be certain that no 

 appreciable blueing of the starch takes place with the reagents 

 alone. 



Only traces of iodine are liberated from iodide when treated 



