VARIABLES OF AQUARIUM WATER 7! 



correction factor from standardizing gives the p.p.m. 

 of free carbon dioxide. 



Chlorine Determination 



Chlorine is ahnost always added to municipal 

 water supplies. Tap water may contain a residual con- 

 centration of chlorine as high as 7.0 p.p.m. (1 to 2 

 p.p.m. is fatal to fishes). Since chlorine is so highly 

 toxic to fishes, the existence of any chlorine in aquar- 

 ium water is undesirable. On this basis a qualitative 

 test for chlorine seems as satisfactory as a quantita- 

 tive one. Thus the simple test given here is quaHtative. 



TAKING WATER SAMPLE: Water samples for 

 chlorine determination need not be handled with spe- 

 cial care. They may be dipped from the tank or drawn 

 from the faucet into a graduate. One should bear in 

 mind, however, that chlorine content of tap water 

 varies throughout the day, with the season, with the 

 length of time the water stands in the plumbing sys- 

 tem, and with variations in the treatment poHcies at 

 the water plant. 



REAGENTS REQUIRED: (1) Acid solution (di- 

 lute 500 ml. of glacial acetic acid to 1 L). (2) Potas- 

 sium iodide solution ( dissolve 75 g. of reagent grade, 

 iodate and chloride-free KI and dilute to 1 L). (3) 

 Starch indicator (2 g. soluble starch in 350 ml. hot 

 water ) . 



TESTING PROCEDURE: Using a pipette, put 10 

 ml. of acetic acid solution in a flask, add the same 

 quantity of potassium iodide solution, then pour in 



