288 BIOLOGICAL AND SANITARY MATTERS. 



If the chlorine be high, the method just described may give 

 results below the truth, and the following method may be used : — 



Place about 200 c.c. of water in a wide mouthed stoppered bottle 

 with three pieces of copper-zinc couple. Leave for twenty-four 

 hours in a warm place, or longer if a reaction for nitrites is ob- 

 tained ; then take 100 c.c. and distil off 50 c.c. of this, after 

 making alkaline with sodium carbonate. Estimate the ammonia 

 in the 50 c.c. (or an aliquot part, 5 or 10 c.c. diluted to 50 c.c. 

 are often sufficient) in the manner previously directed. The 

 ammonia found (less the free ammonia present) multiplied by 

 3 '2 will give the nitric acid (as N2O5). 



Preparation of Copper-Zinc Couple. — Cut a number of 

 pieces of sheet zinc 4x1 inch, and immerse them successively 

 in 2 per cent, caustic soda solution, distilled water, 2 per cent, 

 sulphuric acid solution, and distilled water, keeping them for 

 about two minutes in each solution and agitating them. Place 

 them in 3 per cent, solution of crystallised copper sulphate till 

 a firm black deposit is obtained ; rinse them well in distilled 

 water, without undue handling ; and preserve in a stoppered 

 bottle filled with strong alcohol. 



Nitrites. — Dissolve about 0'05 gramme of meta-phenylene- 

 diamine in dilute sulphuric acid (10 c.c.) ; add 10 c.c. of water 

 and allow the mixture to stand. A brownish-pink coloration is 

 produced, if nitrites be present. 



Oxygen absorbed from Permanganate. — Clean out a 

 stoppered bottle with chromic acid and rinse well with distilled 

 water. Take 250 c.c. of water, add 10 c.c. of dilute sulphuric 

 acid and 10 c.c. of standard potassium permanganate solution, 

 mix, and keep at a temperature of about 80° F. (27 "4° C.) for 

 four hours. Add a crystal of potassium iodide and titrate with 

 standard sodium thiosulphate solution till only a faint yellow 

 colour remains ; then add a little starch solution and continue 

 the titration till the blue colour disappears. To 250 c.c. of 

 distilled water add 10 c.c. of sulphuric acid and 10 c.c. of potas- 

 sium permanganate ; and titrate with standard sodium thio- 

 sulphate solution (2 grammes per litre). '■= The difference between 

 the amounts of sodium thiosulphate solution used, divided by 

 the amount of sodium thiosulphate used for the sulphuric acid, 

 potassium permanganate and distilled water, and multiplied 

 by 0-001 will give the weight of oxygen absorbed by 250 c.c. of 

 water. 



Dilute Sulphuric Acid.— Mix 100 c.c. of pure sulphuric 

 acid cautiously with 300 c.c. of water, cool to 80° F., and add 



* The addition of a little, say 0-05 gramme, of sodium salicylate will 

 render this solution permanent. 



