392 



SCIENCE 



[N. 8. Vol. XXXVII. No. 950 



Acidity 



(H) 1 10-1 10-2 10-3 10-4 10-5 10-6 

 — =— I 1 1 ( 1 ) 1 



(OH) 10-" 10-" 10-'2 10-" 10-10 10-9 10-8 



ents of the ocean, blood, protoplasm, etc., 

 provided the acid be in excess, it is a 

 simple matter to determine the reaction, 

 which can best be measured by the values 

 of (H) and (OH), following the consid- 

 erations above. 



For this we possess a host of reliable 

 data and a tried and well-seasoned theory 

 • — the mass law. Now there is, in connec- 

 tion with the application of the mass law 

 to ionization, a certain characteristic prop- 

 erty of an acid, its ionization constant, k, 

 which measures its tendency to dissociate 

 in aqueous solution, thereby to produce 

 hydrogen ions, and hence to increase the 

 intensity of acidity. Strong acids have 

 ionization constants which are of the order 

 of magnitude of 1.0, weak acids of the 

 order of magnitude of 0.0001, the weakest 

 acids, 0.00000001, or less. 



TABLE OP IONIZATION CONSTANTS 



HCl, HNO3, etc 1 



H3PO4 0.011 



HsAsO, 0.005 



HNO2 0.0005 



H2CO3 0.0000003 



NaHjPOi 0.0000002 



H2S 0.000000091 



H3BO3 0.0000000007 



Na^HPOi 0.00000000000036 



It has been discovered that in the gen- 

 eral case above discussed of weak acid and 

 salt, the concentration of ionized hydrogen 

 is always almost exactly proportional to 

 the ratio of free acid to salt, and is equal, 

 in very close approximation, to the prod- 

 uct of this ratio by a number slightly 

 greater than the ionization constant of the 

 acid. That is to say, representing free 

 acid by HA and salt by BA. 



XT 4 



10-' 



10-8 10-9 



Alkalinity 

 10-10 10-11 



-I- 



10-12 10-13 lOU 



-I 1 1 



10-' 10-« 10-' lO-* 10-3 10-2 10-1 



1. 



whence, if fc= (H) 

 HA 



ba' 



From this relationship therefore follows 

 the conclusion, fully established by experi- 

 ment, that whenever in such a solution the 

 excess of acid, HA, is chemically equiva- 

 lent to the quantity of salt, BA, the hydro- 

 gen ion concentration is almost exactly 

 equal to the ionization constant of the 

 acid, and this is one of the very best meth- 

 ods quickly to detect and characterize an 

 acid. But the ionization constant of car- 

 bonic acid (first hydrogen atom) at room 

 temperature is 0.0000003. Hence, in a so- 

 lution containing exactly equivalent quan- 

 tities of free carbonic acid, for example, 

 sodiiim bicarbonate, the hydrogen ion 

 concentration must be approximately 

 0.0000003 N. Further, since 



HA_{H) 

 BA~ k ' 



if the amount of acid be ten times the 

 amount of salt 



the hydrogen ion concentration must be 

 about 0.000003 N, and if the reverse be the 

 case 



\ BA 10 / 



the value must be nearly 0.00000003 N. 



The range of variation of concentration of 

 hydrogen ions in the usual solutions of the 

 chemical laboratory considerably surpasses 

 the limits of 1.0 N and 0.00000000000001 N. 

 In comparison with such enormous differ- 

 ences those between 0.000003 N and 

 0.00000003 N are almost negligible (1/100: 

 1/100,000,000,000,000). Hence ordinarily 

 it is quite accurate enough to speak of any 



