656 LABORATORY HINTS 



Place 100 c.c. of the water in a large porcelain dish. 

 Add a few drops of 5 per cent, potassium chromate. 

 Run in from a burette standard AgXOg solution (i c.c. =^o"ooi grm. 

 of CI) until a reddish tint remains. 

 Silver chromate is formed which is red. 

 If sa}' r; c.c. are required then these are the parts per 100,000. 

 Since 17 x q-coi x -100.000 = j--, 



lOD 



To obtain the results in terms of sodium chloride multiply the 

 result by i'647 since CI x r647 = NaCl. 



This factor is obtained from the ratio li^^^i _ 5^ = i'647- 



t-i 3S-5 

 (5) To Detect and Estimate Nitrites. 



These generally indicate recent organic pollution. 

 Nitrites are eventually oxidized to nitrates. 



Sometimes nitrates are reduced to nitrites by ferrous salts or in 

 deep wells due to the metal of the bore pipes, but these are the 

 exceptions. 



Nitrites are usually found in sewage effluent. 



Test (i). 



Place 100 c.c. of the water in a Nessler cylinder. 



Add 6 drops of dilute HCl. 



Add 6 drops of naphthylamine hydrochloride. 



Add 6 drops of sulphanilic acid. 



Stir with a glass rod. 



A pink colour indicates nitrites. 

 If no coloration in fifteen minutes nitrites are absent. 

 Test (2). 



Place 100 c.c. of water in a Nessler cylinder. 

 Add a few drops of H2SO4. 

 Add I crystal of KI and mix well. 



Add 5 c.c. of fresh starch solution. A blue colour indicates nitrites. 

 Test (3). 



To 100 c.c. of water add a few drops of diluted HCl. 

 Add 5 c.c. of metaphenylene-diamine solution. 

 Mix and allow to stand. 



A yellowish-orange colour indicates nitrites. 

 If the reagent is not colourless, shake it with a little animal 

 charcoal and filter. 

 To estimate the amount match this colour by using a standard 

 sodium nitrite solution (i c.c. = o'ooi grm.). 



(6) To Detect and Estimate Nitrates. 



These are usually present. 



Strata containing red sandstone, &c., impart them to the water. 



