34 HYDROGEN ION CONCENTRATION 



with the following state of equilibrium 



[H+] [HCO 



[H2CO3] 



= k6 (4) 



^e^'- 



in which ks is the true dissociation constant of the real carbonic 

 acid, H2CO3. But the value of [H2CO3] is not known, and only 

 that of [CO2] is known. The latter is in fact the concentration of 

 total free carbonic acid in solution, for the molecular species H2CO3 

 is present in extremely minute concentration as compared with that 



A/^i AtC?^ - of the molecular species CO2 in solution. By substituting for H2CO3 



•217 /L.U in (4) its value in (3) above, we arrive at 



T>tc6 O^ [H+] [HCO3-] ^ ks ^ 



^:{: i\\' [CO2] k4 ' 



L£>r4 ^tS I fc^'This expression is identical (reciprocal) with equation (1). There- 



O/ "*y^ ^T^ fore, ke is also only an apparent dissociation constant. 



fi^ C44A1^A6T^'^^^^ apparent "dissociation" constant is the only one which one 



, ^_< can measure. The true dissociation constant of carbonic acid, 



\ Z ^ < H2CO3, remains an unknown quantity. But this apparent constant 



6X^ ^-pPAjZ'iiiay be employed as well as a true constant in formal calculations 



^ and operations. 



According to Thiel and Strohecker-" the true dissociation constant 

 of the true H2CO3 is very large, the minimum probable value 

 being 5 X lO—*, according to Strohecker-^ it is 4.4 X 10~^, which 

 is a much greater value than those of acetic and formic acids. 

 This is quite plausible, since H2CO3 is in fact oxyformic acid. 

 An idea of the relative strength of true carbonic acid may be 

 gathered from the following simple experiment: A few drops of 

 neutral red are added to some of a 0.1 N NaHCOs solution. 

 The indicator shows a yellow color in the alkaline solution. 

 Just enough of 0.1 iV HCl is added to change the color of the 

 solution to red, thus indicating an acid reaction. On waiting a 

 few seconds it is observed that the color changes back to yellow. 

 Upon a further addition of HCl the color becomes red again and 

 then slowly returns to a yellower stage. The reason for this phe- 

 nomenon lies not in the gradual escape of gaseous CO2 but in the 



26 Thiel, A. and Strohecker, R., Ber. d. Dtsch. Chem. Ges. 47, 945 (1914). 

 "Strohecker, Zeitschr. f. Unters. d. Nahr-u. Genussm. 31, 121 (1916). 



