218 BACTERIOLOGICAL AND ENZYME CHEMISTRY 



shaking machine. At intervals of about three days about 

 20 c.c. of the solution may be examined :-- 



(a) for ammonia by means of Nessler solution ; 



(b) for nitrites by means of acetic acid, potassium iodide 

 and starch, and 



(c) for nitrates by means of the Stoddart test. 



The following further details in regard to these tests may 

 be useful : — 



Ammonia gives a reddish-brown precipitate, or in dilute 

 solutions, a yellowish-brown coloration, with an alkaline 

 solution of potassium mercury iodide, known as Nessler's 

 reagent. The depth of coloration is proportional to the 

 amount of ammonia present. 



Nitrites. — Acetic acid liberates nitrous acid from a solution 

 containing nitrites ; the nitrous acid, in its turn, liberates 

 iodine from potassium iodide, and the free iodine gives a 

 blue coloration with starch. 



Nitrates. — The Stoddart test affords a ready means of 

 determining the presence of nitrate : 10 c.c. of the sample, 

 filtered from suspended solids, are poured into a test-tube of 

 rather thick glass. About as much pyrogallol as will cover 

 a sixpence is then dissolved in the solution and 2 c.c. of 

 strong nitrate-free sulphuric acid carefully added from a 

 pipette. So as to form a layer in the lower portion of 

 the solution. Dry powdered sodium chloride (salt) about 

 equal in quantity to the pyrogallol is now added, and if 

 nitrate is present a purple band is formed immediately 

 above the sulphuric acid layer. The intensity of the 

 coloration is roughly proportional to the amount of nitrate 

 present. 



It will be found that a progressive change takes place; 

 first of all, formation of ammonia will be noticed, with no 

 nitrite or nitrate ; this attains a maximum, and then decreases 

 with simultaneous appearance of nitrite, but little or no 

 nitrate ; finally the nitrites disappear and there is left a 



