ESTIMATION OF NITRATES AND NITRITES 223 



Nitrates and Nitrites. 

 216. Estimation of Nitrites. 



Metaphenylenediamine Method {Griess). — This is a 

 colorimetric estimation based on the formation of a 

 colouring matter from metaphenylenediamine in presence 

 of nitrites. The depth of colour is proportional to the 

 amount of nitrites in the solution. 



Reagents. — The following reagents are required : — 



{a) Standard Sodium Nitrite Solution. — i-io grams 

 of pure silver nitrite are dissolved in boiling distilled 

 water and a solution of pure sodium chloride added 

 until no further precipitation of silver chloride occurs. 

 The solution is cooled and made up to i litre with water 

 and allowed to stand several hours. One hundred c.c. 

 of the clear solution are then abstracted and made up 

 to I litre, the dilute solution being filled into small dark 

 glass bottles, which should be kept quite full and stored 

 in the dark. One c.c. of this sodium nitrite solution is 

 equivalent to o-oi mg. of nitrogen. 



(J?) Metaphenylenediamine solution. — Eight grams of 

 metaphenylenediamine hydrochloride are dissolved in 

 water, the solution decolorised if necessary by boiling 

 with animal charcoal, and the filtered liquid rendered 

 slightly acid with hydrochloric acid and diluted to i 

 litre with distilled v/ater. 



{c) Dilute Sulphuric Acid. — Strength i to 3. 



Procedure. — Into a Nessler cylinder (as used for the 

 ammonia estimations) are measured out icx) c.c. of the 

 water to be tested. The outside of the cylinders should 

 be marked at 100 c.c. content, if this has not already 

 been done. One c.c. of standard metaphenylene- 

 diamine solution is then added, together with i c.c. of 

 dilute sulphuric acid, and the liquid stirred well with a 

 glass rod and left to stand for twenty minutes. 



