REACTION AND RESISTANCE OF FISHES. 229 



hut when the water flows out of the tap, the pressure is removed 

 and the carbonic acid at once begins to dissociate into CO 2 and 

 water. The CO 2 passes off into the air and the dissociation 

 of the acid continues until equilibrium with the CO 2 in the 

 atmosphere is established. Parallel with the dissociation of 

 the carbonic acid there goes an increasing tendency for the bicar- 

 bonates to break up to form the normal carbonate, and by the 

 time the acidity from the carbonic acid has diminished to ap- 

 proximate neutrality, the bicarbonates are producing a sufficient 

 quantity of the normal carbonate to give the water an alkaline 

 reaction to phenolphthalein. Thus by regulating the amount 

 of aeration, the water can be left acid, made neutral or even al- 

 kaline. 



Biologists speak of the carbonates, bicarbonates, and car- 

 bonic acid, as fixed, half bound and free CO 2 , respectively. The 

 fixed is that existing as simple carbonates, the half bound that 

 necessary to convert the carbonates into bicarbonates, and the 

 free that remaining in excess (Seyler, '94, p. 104). It, will be 

 seen that the bicarbonates contain both fixed and half bound 

 CO 2 , i. e., CO 2 which is to become half bound is added to CO 2 , 

 that is already fixed, to form the bicarbonates. Failure to 

 recognize this fact often leads to confusion when these terms 

 are used. 



The amounts of the three kinds of CO 2 can be determined 

 accurately by titration, using two indicators, phenolphthalein 

 and methyl orange. Methyl orange is unaffected by H 2 COs 

 and hence the bases present as carbonates or bicarbonates can 

 at once be titrated with acid. Carbonates are alkaline to phe- 

 nolphthalein, bicarbonates are neutral, and free CO 2 is acid. 

 A carbonate titrated with acid, therefore, becomes neutral to 

 phenolphthalein (if titrated under conditions which prevent 

 loss of CO 2 ) when the carbonates have all been converted into 

 bicarbonates. 



Methyl orange is not affected by H 2 CO 3 because this acid 

 does not produce a high enough concentration of H ion. The 

 indicator is however very sensitive to OH ion and reacts to the 

 minute amounts that are present in a bicarbonate solution. 

 Phenolphthalein, on the other hand, is very sensitive to H ion 



