38 METHODS OF ANALYSIS [Chap. 



droxitl until the maximum color is developed. Transfer to a colorimetric cylinder, 

 filter if necessary, and compare with the standards in the usual manner. Record as 

 nitrogen in the form of nitrate. 



Reduction Method. — Official. 

 (For water of high chlorin content.) 



16 REAGENTS. 



(a) Sodium or potassium hydroxid solution. — Dissolve 250 grams of the purest 

 hydroxid obtainable in 1250 cc. of water and boil down to 1 liter. 



(b) Aluminium foil. — Use strips about 10 cm. long, weighing about 0.5 gram. 



17 DETERMINATION. 



Place 10 cc. of the sample in a 100 cc. test tube and dilute to exactly 50 cc. ; add 5 cc. 

 of the sodium hydroxid and a strip of the aluminium foil. Close the mouth of the 

 test tube with a rubber stopper carrying a U-shaped glass tube connected with a 

 second test tube containing about 50 cc. of ammonia-free water acidified with hydro- 

 chloric acid, which serves as a trap to catch any ammonia which might escape. Allow 

 to stand at room temperature for 12 hours or more until reduction is complete. Trans- 

 fer the contents of the first tube to a Kjeldahl flask and distil. Cool the distillates 

 and nesslerize as directed under 1 1 ; also nesslerize the solution in the trap. Record 

 as nitrogen in the form of nitrate. 



CHLORIN.— OFFICIAL. 



18 REAGENTS. 



(a) N/20 sulphuric acid. 



(b) N /20 sodium, carbonate. 



(C) Potassium chromate indicator. — Dissolve 5 grams of potassium chromate in 

 water, add a solution of silver nitrate until a slight permanent red precipitate is 

 produced, filter, and dilute to 100 cc. 



(d ) Standard silver nitrate solution. — Dissolve 4.791 grams of silver nitrate in water 

 and dilute to 1 liter; 1 cc. is equivalent to 1 mg. of chlorin. Check by titration against 

 a standardized solution of sodium chlorid. 



19 



DETERMINATION. 



To 100 CC. of the water add a few drops of phenolphthalein. If a red color appears, 

 titrate the carbonates thus indicated to bicarbonates with N/10 or N/20 sulphuric 

 acid. If the water is acid to methyl orange, add N/20 sodium carbonate to neutralize 

 the acidity. Add 1 cc.of the potassium chromate and titrate with the standard silver 

 nitrate. Correct for the amount of silver nitrate necessary to give in 100 cc. of chlorin- 

 free water with 1 cc. of the chromate, the shade obtained at the end of the titration 

 of the sample. lodids and bromids are not usually found in interfering quantities 

 in potable water. However, if they are present make the equivalent correction. 



If chlorids are present in very small quantities concentrate 500 or 1000 cc. in a por- 

 celain dish to 100 cc, rub down the sides of the dish carefully, add 1 cc. of the indicator 

 and titrate as described above. If sufficient chlorids are present in 100 cc. of the water 

 to consume more than 25 cc. of the standard silver nitrate, determine by precipita- 

 tion in nitric acid solution and weigh the silver chlorid. 



