40 METHODS OF ANALYSIS [Chap. 



DISSOLVED OXYGEN. 



Method I. (Winkler Method as Modified by Drown and Hazen.^)~Tentativ.. 

 (When less than 0.1 mg. of nitrite nitrogen per liter is present.) 



24 REAGENTS. 



(a) Manganous sulphate solution. — Dissolve 48 grams of nianganous sulphate in 

 100 cc. of water. 



(b) Sodium hydroxid-potassium iodid solution. — Dissolve 360 grams of sodium 

 hydroxid and 100 grams of potassium iodid in 1 liter of water. 



(C) Sulphuric acid. — (Sp. gr. 1.4). Mi.x equal weights of concentrated sulphuric 

 acid and water. 



(d) Standard sodium thiosulphate solution. — Dissolve 6.2 grams of recrystallized 

 sodium thiosulphate in 1 liter of water. This gives a N/40 solution, each cc. of which 

 is equivalent to 0.2 mg. of oxygen or 0.1395 cc. of oxygen at O'^C. and 760 mm. pressure. 

 This solution should be standardized occasionally against N/40 potassium dichromate. 



(e) Starch indicator. — Mix about 2 grams of clean starch with cold water to a thin 

 paste; pour into about 200 cc. of boiling water. Boil for a few minutes. This solu- 

 tion should be freshly prepared. 



25 COLLECTION OF SAMPLE. 



Collect the sample in a carefully calibrated glass stoppered bottle, approximately 

 250 cc. capacity, by means of an apparatus designed to avoid the entrainment or 

 absorption of any oxygen from the atmosphere. Note the temperature. 



26 DETERMINATION. 



Add approximately 2 cc. of the manganous sulphate and 2 cc. of the sodium hydrox- 

 id-potassium iodid, delivering both of these solutions beneath the surface of the 

 liquid by means of a pipette. Insert the stopper and mix the contejits of the bottle 

 by shaking. Allow the precipitate to settle. Remove the stopper; add about 2 cc. 

 of sulphuric acid and mix thoroughly. Rinse the contents of the bottle into a flask; 

 titrate with N/40 sodium thiosulphate, using a few cc. of the starch indicator toward 

 the end of the titration. Do not add the starch until the color has become a faint 

 yellow; titrate until the blue color disappears. Express the results in milligrams 

 per liter and in percentage of saturation.^ This latter determination is the ratio of 

 the amount of gas present to the maximum amount capable of being dissolved by 

 distilled water at the same temperature and pressure. 



Method II. (Winkler Method as Modified by Rideal and Stewart.*) — Tentative. 

 (When more than 0.1 mg. of nitrite nitrogen per liter is present.) 



27 . REAGENTS. 



(a) N/10 potassium permanganate. 



(b) S% potassium oxalate solution. 

 Other reagents are described under 24. 



28 COLLECTION OF SAMPLE. 



Proceed as directed under 25. 



