658 LABORATORY HINTS 



Suppose the Water Sample is the lighter of the two. 



Pour out some of the darker, say first 15 c.c, then 10 c.c. 

 as above, then the amount of the orif^inal solution remain- 

 ing- in the standard cyHnder is - ^ ^ '^° 

 ° -^ 100 X 1 00 



Therefore in the sample we have : — 

 85 X 90 X 00005 X 100,000 



ICO X 100 X 50 



Method (2). 



= 0*766 parts N as nitrate per 100,000 parts. 



Dissolve 2 grm. of sodium hydrate in 100 c.c. of distilled 

 water. 



This is a pure solution of caustic soda free from nitrates. 



Add 100 c.c. of the sample water and a piece of aluminium foil. 



Permit to stand for twenty-four hours in a dark place. 



Then all the oxidized nitrogen is converted into ammonia. 



Distil this solution and Nesslerize as above. 



Before distilling test for nitrites to see that all have been reduced 

 to nitrates. 



Method (3). 



Coat some zinc foil with copper by exposing it to copper sul- 

 phate until it becomes black. 



Put zinc foil copper-coated into a bottle with 250 c.c. of the 

 sample water, add 0*5 grm. oxalic acid and leave for twenty- 

 four hours. 



All oxidized nitrogen is converted into ammonia. 



Distil and Nesslerize. 



(7) To Estimate the Oxidizable Organic Matter in Water (Oxygen 

 absorbed) . 



This can be done in three hours at 37° C, or in four hours at 80° F. 



The putrescent organic matter is chiefly oxidized in the first thirty 

 minutes. 



Oxygen may be absorbed by nitrites, HoS, ferrous salts, &c. 



In a stoppered bottle of 400 c.c. capacity place 200 c.c. of sample 

 and add 10 c.c. of KMnO^ solution (i c.c. =o"oooi grm. of available 



Add 10 c.c. of H2SO4 (i in 4). 



Place in incubation at 37° C. for three hours. 



If the pink colour quickly fades add more KMnO^ always counting 

 the c.c. used. 



At the end of three hours at 37° C. a pink colour should be present. 



Cool under the tap. 



Add 5 c.c. of KI 10 per cent, solution. 



Run in from burette sodium thiosulphate solution (i grm. in 

 1,000 c.c.) until the yellow colour has nearly disappeared. 



