50 PUBLIC HEALTH CHEMISTRY 



2. Diphenylamine Test (C 6 H 6 ) 2 NH. Take 5 c.c. of 

 sample, add as much diphenylamine solution, mix, and 

 run down pure strong sulphuric, when a blue colour forms 

 at junction of liquids in presence of nitrates. 



QUANTITATIVE TESTS. 



i. Phenol-sulphonic Acid. This reagent is made by 

 adding 6 grm. of pure carbolic acid to 3 c.c. of aq. dest. 

 and then adding mixture to 37 c.c. of pure sulphuric acid. 



A standard solution of potassium nitrate is required, 

 0-072 grm. to i litre, and then I c.c. = o-oi mgr. N as 

 nitrates. This contains i part N in 100,000 parts of 

 standard. 



PROCESS. To two porcelain dishes are added respectively 

 10 c.c. of the sample and 10 c.c. of the standard. These 

 are placed on the water-bath until their contents are just 

 evaporated to dryness. To each of the residues add i c.c. 

 of phenol-sulphonic acid, and mix well with a glass rod (if 

 a large amount of nitrates is present the liquid will turn 

 red). Set aside for fifteen minutes, and then wash out 

 each dish successively into two clean 100 c.c. Nessler 

 glasses with 25 per cent ammonia solution in distilled 

 water. Add more ammonia until effervescence ceases, 

 and make up to mark with aq. dest. The nitrates present 

 convert the phenol-sulphonic acid into picric acid, with 

 which the ammonia forms a picrate having a yellow colour, 

 and the amount of this is proportional to the amount of 

 nitrates present. The two glasses are now compared as 

 to tints, and the darker one is diluted as before described 

 under Ilosvay's test for nitrites. If the water is very 

 pure, a larger amount of sample should be evaporated 

 down, say 20 c.c., 50 c.c., or 100 c.c., and a smaller quantity 

 of standard, say 5 c.c. If rich in nitrates, then less should 

 be taken of the sample, say 5 c.c. or i c.c. 



Aluminium Process. If aluminium foil be added to a 

 strongly alkaline water, decomposition of the water ensues 

 with the evolution of hydrogen, which in the presence of 

 nitrites or nitrates reduces these, converting their contained 

 nitrogen into ammonia. Thus : 



4A1 + 4NaOH + 4H 2 O = 2Al 2 Na 2 O 4 + 6H 2 : and 

 3KNO 3 + I2H 2 = 3KOH + 6H 2 O 



