OXYGEN CONSUMPTION IN ACIDS. 315 



much carbon dioxide at the same pH as does hydrochloric acid. 

 The lack of depressing action of butyric acid therefore remains 

 at present inexplicable. 



Nearly all of the carbonate of the water is liberated as carbon 

 dioxide at a pH of 5.0 and all of it between pH 5.0 and 4.0. 

 The amount of free carbon dioxide present at these acidities is 

 about 30 cc. per liter. At first the fact that all of the available 

 carbonate in the water is decomposed at pH 5.0 to 4.0 appeared 

 to explain the result that the maximum percentage of depression 

 is obtained at such acidity and further acidification does not 

 increase the depression. But the additional fact discussed in 

 the next paragraph that increasing the amount of carbon dioxide 

 by adding the gas is not effective in intensifying the depression 

 invalidates the suggested explanation. 



Analyses were also made of the amount of carbon dioxide gas 

 required to produce a certain pH. It was found that more of 

 the gas is present in the water at a given pH than is set free 

 in the water by adding other acids to the same pH. This result 

 would be expected, since it is highly probable that when the 

 water is acidified, considerable time must elapse before all of the 

 acid reacts with the carbonate of the water. Consequently 

 immediately after acidification part of the free hydrogen ions 

 present are derived from the acid added, and not from the reaction 

 of carbon dioxide with water. The difference in carbon dioxide 

 content between water acidified with carbon dioxide gas and 

 water acidified by other acids is small at pH 7.0 or even 6.5 but 

 at pH 6.0 there is three times and at pH 5.0 nearly ten times as 

 much carbon dioxide gas in the water as is liberated by mineral 

 acids at those same hydrogen ion concentrations. These results 

 are shown in Fig. 2. 



We are thus faced with the question why is carbon dioxide 

 not more effective as a depressing agent than any other acid, 

 particularly at the higher acidities? There was to be sure 

 evidence that carbon dioxide at pH 5.0 for instance has a greater 

 depressing action than any other acid tried. Yet the difference 

 between it and other acids is not as great as might be expected. 

 It becomes necessary to assume, in view of the facts at hand, 

 that the amount of depression of the respiratory rate which can 



