222 ANALYSIS OF WATER 



215. Absorption of Oxygen. 



{a) Oxygen absorbed in four hours. — One hundred c.c. 

 of the water are placed in a 250 c.c. stoppered bottle 

 and a control of 100 c.c. distilled water in a similar 

 bottle. To each is added 10 c.c. of the permanganate 

 solution and 10 c.c. of dilute sulphuric acid. Both 

 bottles are then stoppered and maintained at 27" C. 

 (80° F.) in a water-bath for four hours. At the end of 

 that time about 10 c.c. of potassium iodide solution are 

 added to each, whereby iodine is liberated, equivalent 

 to the amount of potassium permanganate present, 

 according to the equation 



2KMn04+8H2SO,+ loKI 



= 6K2S04+2MnS04+8H20 + 5l2. 



The iodine formed dissolves in the excess of potassium 

 iodide and is estimated by titration with sodium thio- 

 sulphate, using starch as indicator. The blank experi- 

 ment with permanganate plus distilled water serves 

 to standardise the sodium thiosulphate, which is thus 

 obtained direct in terms of KMnO^, or available oxygen. 

 If many analyses are to be carried out, this standardisa- 

 tion need not be done for each experiment, but must 

 be repeated, nevertheless, at frequent intervals. From 

 the amount of permanganate used up is calculated the 

 amount of oxygen consumed per 100,000 parts of the 

 water. 



{b) Oxygen absorbed in fifteen minutes. — This test is 

 usually only applied to effluents and very questionable 

 waters. It is carried out in exactly the same manner 

 as that described above, except that the samples are 

 kept in the water-bath at 27° C. for fifteen minutes only. 



Unpolluted waters show very little oxygen absorption over the 

 shorter period, and even after four hours should not give a higher 

 value than o-io per 100,000 parts of water. 



