60 
bulletin of the bureau of fisheries. 
When hydrochloric acid is added, the higher oxides or hydroxides of man- 
ganese are dissolved, forming manganese chloride, MnCl 2 , and the excess of oxygen 
liberates its equivalent of chlorine, which reacts with potassium iodide and liberates 
iodine. Each atom of oxygen dissolved in the water thus liberates two atoms of 
iodine, and the amount of iodine is determined by titrating with sodium thiosulphate 
solution that has been standardized. 
CALCULATION OF OXYGEN CONTENT. 
The dissolved oxygen may be calculated by several formulse, but in this work 
all results have been stated in cubic centimeters per liter, and the following formula 
has been used : 
0.055983 X 1,000 x b X n 
N'V 
In this formula b equals the number of cc. of potassium bichromate used in stand- 
ardizing thiosulphate, that is, 25 cc; N' equals number of cc. of thiosulphate 
required to titrate the 25 cc. of potassium bichromate; n equals number of cc. of 
thiosulphate required to titrate sample of water; V equals capacity of bottle less 
2 cc. that is deducted for the water displaced by the 2 cc. of solutions added. 
Since 25 cc. of bichromate was always used the formula has been simplified 
thus: 
0.055983 X 1,000 X25n 1,399.57 Xn 
N'xV . ~ N'XV 
which equals 
1,399.57 X no. cc. thiosulphate to titrate sample 
No. cc. thiosulphate to titrate 25 cc. N /100, K 2 Cr 2 0 7 X capacity of bottle 
which equals cubic centimeters of oxygen per liter. 
This method has proved very satisfactory, especially when one considers the 
rough handling to which the apparatus and chemicals have been subjected and the 
great variety of conditions under which the analyses have been carried out. In a 
very few cases an oxygen sample has been found not to agree with the check sample 
taken at the same depth, but in most instances this has been traced to incomplete 
mixing after adding the potassium hydroxide and potassium iodide. 
Winkler (1914 and 1914a) has modified the above method for the determination 
of oxygen in waters containing organic matter and nitrites. Standard Methods of 
Water Analysis (American Public Health Association, 1920) also gives a method for 
the oxidation of these substances before using the Winkler method. Hale and 
Melia (1913) state that waters containing 0.1 part of nitrite nitrogen per million, 
or considerable organic matter, change the results of the regular Winkler method. 
Very few lake waters contain sufficient nitrite or organic matter to affect the regular 
method, but one of these modifications should be used on river or lake waters that 
are contaminated with sewage. 
Winkler (1913) describes a colorometric method of estimating oxygen. He 
uses adurol, a photodeveloper that is colorless in neutral solutions but turns brown 
in proportion to the oxygen present when made alkaline with ammonia or borax. 
