DISSOLVED OXYGEN 345 
increase this by a mixing action. The balance between the available 
oxygen and the required oxygen must probably be established for 
each stream. 
Determination of Dissolved Oxygen. Different methods have been 
in use for determining the oxygen content of a water. The Winkler 
method has been included in standard methods and has certain distinct 
advantages over the Levy method which has been advocated by some. 
It is given in standard methods as follows: 
Reagents. 1. Sulphuric acid, concentrated. (Sp. Gr. 1.83-1.84.) 
2. Potassium permanganate. Dissolve 6.32 gms. of the salt in 
water and dilute the solution to a liter. 
3. Potassium oxalate. A 2 per cent solution. 
4, Manganous sulphate. Dissolve 480 gms. of the salt in water 
and dilute the solution to 1 liter. 
5. Alkaline potassium iodide. Dissolve 700 gms. of potassium 
hydroxide and 150 gms. of potassium iodide in water and dilute to a 
liter. 
6. Hydrochloric acid, concentrated. (Sp. Gr. 1.18-1.19.) 
7. Sodium thiosulphate. A N/40 solution. Dissolve 6.2 gms. of 
chemically pure recrystallized sodium thiosulphate in water and dilute 
the solution to 1 liter with freshly boiled distilled water. Each cubic 
centimeter is equivalent to 0.2 mg. of oxygen or to 0.1895 c.c. of oxygen 
at 0° C. and 760 mm. pressure. Inasmuch as this solution is nat per- 
manent it should be standardized occasionally against a N/40 solution 
of potassium bichromate. The keeping qualities of the thiosulphate 
solution are improved by adding to each liter 5 c.c. of chloroform and 
1.5 gms. of ammonium carbonate before diluting to the prescribed 
volume. 
8. Starch solution. Mix a small amount of clean starch with cold 
water until it becomes a thin paste and stir this mass into 150 to 200 
times its weight of boiling water. Boil for a few minutes, then sterilize. 
It may be preserved by adding a few drops of chloroform. 
Collection of the Sample. Collect the sample in a narrow-necked 
glass-stoppered bottle of 250 to 275 c.c. capacity. The following pro- 
cedure should be followed in order to avoid entrainment or absorption 
of atmospheric oxygen. In collecting from a tap, fill the bottle through 
a glass or rubber tube extending well into the tap and to the bottom of 
the bottle. To avoid air bubbles allow the bottle to overflow for several 
minutes, and then carefully replace the glass stopper so that no air 
bubble is entrained. In collecting from the surface of a pond or tank 
connect the sample bottle to a bottle of 1 liter capacity. Provide each 
