March 14, 1913] 



SCIENCE 



393 



solution containing both free carbonic acid 

 and a bicarbonate, when the disparity be- 

 tween the concentrations of the two sub- 

 stances is not very great, as of constant 

 reaction. For, obviously, the neutral point, 

 which at a temperature of 25° amounts to 

 a concentration of hydrogen and hydroxyl 

 ions 0.00000001 N, falls well within the 

 narrow range of reaction of such solutions, 

 being characterized by a ratio of carbonic 

 acid to bicarbonate of about 1 : 3. 



Thus carbonic acid, like the almost 

 equally weak acid, phosphoric acid (after 

 its first hydrogen has been neutralized by 

 base), has the remarkable property of 

 preserving a neutral reaction whenever it 

 exists in solution with its salts, provided 

 there be an excess of acid. All acids whose 

 strength is even a little either greater or 

 less than carbonic acid lack the property. 

 There is nothing mysterious about this 

 fact; any other weak acid will hold con- 

 stant the reaction in its own range of re- 

 action; thus acetic acid in the neighbor- 

 hood of a hydrogen ion concentration 

 N/100,000, etc. 



This characteristic of carbonic acid is 

 of the utmost significance, first by regula- 

 ting one of the most fundamental of phys- 

 ico-chemical conditions, and secondly, by 

 preserving throughout nature the char- 

 acteristic chemical inactivity of water, 

 which disappears whenever the reaction 

 becomes either appreciably acid or ap- 

 preciably alkaline. Almost the only case 

 of important geological action due to acid- 

 ity or alkalinity of water is the action of 

 fresh water, containing carbonic acid 

 itself, to weather the rocks. This process 

 is however self-limited, for the dissolved 

 material forms bicarbonates, and thus at 

 once provides permanently balanced solu- 

 tions.' 



' L. J. Henderson, ' ' The Fitness of the Environ- 

 ment, " Chapters IV. and V. New York, The 

 Macmillan Company, 1913. 



Elsewhere, within and without the or- 

 ganism, carbonic acid is almost always ac- 

 companied by bicarbonates, and a close 

 approach to neutrality is the result. In 

 the organism the variation in ratio of 

 phosphates is similar to the case of the car- 

 bonates, as may readily be illustrated by 

 experiment. Thus a solution consisting of 

 equal parts of monosodium phosphate and 

 disodium phosphate will be found to give 

 a neutral reaction with both methyl orange 

 and phenol phthalein, and the neutrality, 

 thus indicated, will not be disturbed by 

 the addition of relatively large amounts of 

 either acid or alkali. 



We may next consider the equilibrium 

 within the organism, where the concentra- 

 tion of ionized hydrogen can undoubt- 

 edly vary between 5 N/100,000,000 and 

 N/10,000,000, but during life probably not 

 much more widely, in the body at large. 

 At body temperature the most probable 

 values of the ionization constants of the 

 acids in question yield the equations: 



NaH,PO, 



(H) =2.1X10-' xj^-po;- 



If 



(H)=0.5X10-'N 



H;C03 1_ NaH,PO^ 1 



NaHCOj ~ 1 3. 8 ' NajHPO^ ~ 4.2 ' 

 and if 



(H) = 1.0X10~'N 



H,C03 _ J^ NaH jPO, _ 1^ 

 NaHC03~6.9' NajHP0i~2.1' 



In short, in order to bring about this seem- 

 ingly insignificant change in reaction, the 

 relative quantities of acid and base in the 

 body must undergo very great changes; 

 or, otherwise stated, until very large quan- 

 titative changes in the amount of acid or 

 base in the body have come about, there 

 can be no appreciable change in the reac- 

 tion. 



