32 HYDROGEN ION CONCENTRATION 



In different acids the value of k varies within wide limits. It 

 could be supposed that every H-atom of a molecule has the ten- 

 dency to become dissociated into a H-ion. But this tendency 

 depends upon the chemical configuration of the compounds, and 

 under certain conditions it may become extremely small. Thus, 

 for instance, sugar and also alcohol may be conceived of as acids 

 with extraordinarily small dissociation constants, which in the 

 case of sugar can barely be measured, and in the case of alcohol it 

 is so small as to completely escape measurement by any means at 

 our disposal. It may be stated, however, that wherever the methods 

 for the determination of the constant are still sufficient, the constant 

 has a well defined value, in spite of its small order of magnitude. 



The upper limit of the dissociation constants cannot be discussed 

 at this point without additional elucidation. The study of the 

 strong acids, such as HCl, HBr, HI, HNO3, H3PO4 (the first stage), 

 does not yield uniform values for k, and they will be discussed later. 

 On the other hand, the weaker and the weakest acids follow well 

 in their dissociation the above stated laws. Table 8 gives the 

 dissociation constants of some of the weak acids at 18°. The 

 table also shows the effect of temperature upon k in the case of a 

 few acids. 



10. True and apparent dissociation constants of acids. 



Pseudo-acids 



In the case of some acids the free acid is not known to exist in 

 the free state, and only its anhydride is known. Such is the case 

 with H2CO3 which is not known in its free state, and only its an- 

 hydride, CO2, is known. When CO2 is dissolved in water the 

 resulting solution has an acid reaction. From this circumstance it 

 is deduced that H+ and HCO3"" ions were formed in the solution. 

 The mode of formation of these ions can be explained in two ways: 

 It may be either assumed that the CO2 molecule acquires by addi- 

 tion an 0H~ ion: 



CO2 + H+ + OH- =± HCO3- + H+ 



or, since the H+ ions play no part in the reaction, as the following: 



CO2 + OH- ^ HCO3- 



