52 PUBLIC HEALTH CHEMISTRY 



to nitrites, and then to ammonia. It thus acts also on 

 any nitrites originally present, so that the process estimates 

 nitrates and nitrites. The reaction is finished when no 

 free nitrites are found in the solution. This is determined 

 by removing 10 c.c. and testing by Griess's test. If 

 nitrites are found to be present, more time must be given. 

 If they are absent, the remainder of the sample water is 

 poured into a 700 c.c. boiling-flask, and the bottle washed 

 out repeatedly with ammonia-free distilled water, the 

 washings being added to the flask, and more water added 

 to bring up the bulk to about 500 c.c. The water is then 

 distilled as in the estimation of free and saline ammonia, 

 and the amount of ammonia determined. This is restated 

 as nitrogen by multiplying by 14/17 (N:NH 3 ). If the 

 sample was found to contain any free ammonia, the amount 

 of this would require to be deducted before assigning the 

 amount found by this process to nitrates and nitrites. 



Indigo Method. Another method, which is a rapid and 

 convenient one, but subject to great irregularity, is the 

 indigo method. 20 c.c. of sample are taken in a beaker, 

 and 20 c.c. of pure strong sulphuric acid are added. From 

 a burette allow standard indigo solution to run into the 

 hot mixture, until the colour of the indigo ceases to be 

 discharged, and a faint greenish tinge becomes permanent. 

 The estimation should be repeated, adding half a c.c. of 

 indigo less to 20 c.c. of the sample, and then the sulphuric. 

 When the colour is discharged, the indigo is run in drop 

 by drop until colour is again permanent. The indigo 

 solution is standardized against standard nitrate solution 

 similarly treated. The strong sulphuric liberates free 

 nitric acid, which in the hot liquid oxidizes the indigo to 

 isatin, which is colourless. Owing to the heat evolved, 

 the titration is best done with the beaker resting on an 

 asbestos mat. 



The method is unreliable in the presence of organic 

 matter, the results being too small. It also requires that 

 all the procedures should be carried out exactly alike for 

 the titration of the sample and the standardizing of the 

 indigo solution. Otherwise it is a very simple, rapid, and 

 delicate method. 



No water used for drinking purposes should contain 



