224 ANALYSIS OF WATER 



At the same time fractions of a cubic centimetre of 

 standard sodium nitrite solution (o-2, 0-4, 0-6, o-8, and 

 I c.c.) are measured into separate Nessler cylinders and 

 made up to 100 c.c. with distilled water. To each is 

 then added i c.c. of metaphenylenediamine solution 

 and I c.c. of dilute sulphuric acid. These solutions are 

 allowed to stand under the same conditions as the 

 sample test for twenty minutes, a comparison and 

 matching of the tints being then made, as in the case of 

 ammonia. 



217. Griess-Ilosvay Method. 



This method is extremely delicate, and is carried out 

 in the same manner as the above. The solutions con- 

 sist of standard sodium nitrite as before, and a mixture 

 of (i)0'5 gram of sulphanilic acid in 150 c.c. of dilute 

 acetic acid, and (2) the colourless solution poured off 

 from boiling o-i gram |8-naphthylamine with 20 c.c. of 

 water diluted to 150 c.c. with dilute acetic acid. An 

 intense pink colour is rapidly produced which is pro- 

 portional to the amount of nitrite present. 



218. Total Nitrates and Nitrites. 



The nitrates and nitrites together may be estimated 

 in a number of ways, the nitrate figure being obtained 

 by subtracting the value for nitrites found by one of the 

 above methods. 



{a) Nitrometer Method. — This is a very accurate 

 method, but takes rather a considerable time to carry out. 

 From 100 to 250 c.c. of water are evaporated to dryness 

 on the water-bath (the residue from the " total solids " 

 may be used), and extracted with distilled water to remove 

 calcium and magnesium carbonates. The solution is 

 evaporated to small bulk (about 5 c.c.) and the dissolved 



