58 LABORATORY MANUAL OF MICROBIOLOGY 



(e) Zinc dust is freed from traces of nitrates and nitrites 

 by treating some of the pure commercial zinc dust with a 

 dilute solution of acetic acid and heating on a water bath for 

 1 hour. 



The zinc is filtered off by means of suction, washed with dis- 

 tilled water and dried. 



Test for Nitrites. — Place 10 to 15 cubic centimeters of the 

 solution to be tested into a test tube and add 1 to 2 cubic centi- 

 meter portions of solution (a) first, then an equal amount of 

 solution (b). The formation of a red color in solution indicates 

 the presence of nitrites. This test is sensitive to 1 X 10~^ milli- 

 gram of N02~ ions in 1 liter of solution. If the solution, which 

 is to be tested for the presence of nitrite, contains free inorganic 

 acids, it must be first neutraHzed with a solution of a base. The 

 same is true of the following three methods. 



Test for Nitrates, in the Absence of Nitrites. — This test is 

 based upon the reduction of nitrates to nitrites by zinc dust. 



Place 10 to 15 cubic centimeters of the solution to be tested in a 

 test tube and add 1 to 2 cubic centimeter portions of reagents 

 (a) and (h) as in the test for nitrites. Add also a very small 

 pinch (about 10 to 20 milligrams) of zinc dust (e). The produc- 

 tion of a red color indicates the presence of nitrates. Sensitivity 

 of test, 1 milligram NOs" ions in 1 liter of solution. 



Test for Hydroxylamine in the Absence of Nitrites. — This 

 test is based upon the oxidation of hydroxylamine (NH2OH) to 

 nitrites by iodine. 



Place 10 to 15 cubic centimeters of the solution into a test 

 tube. Add 3 to 5 cubic centimeters of solution (a) and about 5 

 drops of solution (c). Shake and allow to stand 2 to 3 minutes 

 in the cold. Add solution (d), drop by drop, until the iodine 

 is just decolorized. Now add 2 to 3 cubic centimeters of solu- 

 tion (h). The formation of a red color indicates the presence of 

 hydroxylamine in the test solution. 



Sensitivity of test, 3 X 10-^ milligram of NH2OH in 1 liter. 



Test for Nitrates and Hydroxylamine in the Presence of 

 Nitrites.^ — The destruction of nitrites is first brought about by a 

 diazo-reaction, which consists in adding the sulfanihc acid reagent 

 (a) to the test solution and boiling. 



1 Blom, J., Ber. deut. chem. Gesell, 59 : 121, 1926. 



