VARIABLES OF AQUARIUM WATER 69 



fate readings should be multiplied to give dissolved 

 oxygen in p. p.m. 



TITRATION procedure: In the following pro- 

 cedure a separate pipette is used for each reagent, 

 and each reagent is added below the surface. 



To the 250 ml.-water sample add 1.0 ml. of the 

 manganous sulfate solution. Immediately add 1.0 ml. 

 of the alkaHne potassium iodide sodium azide solution, 

 stopper and invert twice to mix. Permit the resulting 

 precipitate to settle. A white precipitate indicates 

 very low oxygen, and a brown precipitate indicates 

 the presence of oxygen. After the precipitate has set- 

 tled ( one or two minutes ) add 2.0 ml. of concentrated 

 sulfuric acid by letting the acid run down the neck of 

 the bottle into the sample. Again stopper and invert 

 to mix. 



Measure out 200 ml. of the prepared sample 

 and pour this quanity into a clear flask or beaker. By 

 use of a burrette titrate the thiosuHate into the 200-mL 

 sample until the brown color becomes a pale straw 

 color. Then add starch indicator which will turn the 

 sample blue. The end point of the titration is reached 

 when the blue color fades completely and does not 

 return for a period of 30 seconds. The number of milli- 

 liters of sodium thiosulfate used times the correction 

 factor from standardization is equal to the p.p.m. of 

 dissolved oxygen in the sample. 



Free Carbon Dioxide Determination 



Carbon dioxide in aquarium water is produced 

 by all organisms present, but under different condi- 

 tions different organisms are more abundant and 



