80 PHOTOSYNTHESIS 



plants were able to liberate oxygen in the light when grown in solutions 

 of sodium bicarbonate and Hassak ^^ carried out quantitative experiments 

 with potassium bicarbonate to demonstrate this fact. In this process the 

 water naturally becomes alkaline. 



If a solution of sodium bicarbonate is allowed to stand in contact 

 with air containing carbon dioxide, an equilibrium will be established 

 between the hydrolyzed bicarbonate solution and the carbon dioxide in 

 the air. This equilibrium will depend upon the partial pressure of the 

 carbon dioxide in the air and will consist of a solution containing a high 

 concentration of HCO3- and relatively small amounts of normal car- 

 bonate. The concentration of HCO3- will be higher with increased 

 partial pressure of COo in the air and will be decreased by the addition 

 of normal sodium carbonate or alkali to the solution. These relations 

 have been clearly expressed by Warburg.-^ From the equations of the 

 electrolytic dissociation of carbonic acid : 



H^ X HCO3- _ /, ^,_ ,,:_„„;„,:._„ ..,„.,, ^/ HXO3 



CO2 



H^ X COs 



/ . . HXOsN 



:l kj = the dissociation constant X ^^ — j 



= K 



HCO3- 



through division : 



(HC03-)^ =^'-K (1) 



CO3-XCO, k„ 



If in mixtures of NaHCOo and Na2C03 the values of HCO3- and 

 CO3— are known we can calculate the COo concentration in each case if 

 K is known. If NaHCOs and NaoCOg represent the total concentra- 

 tion of bicarbonate and carbonate in moles per liter, a the dissociation of 



(T-TCO ~ \ 

 ,, ,, ' I and j5 the dissociation of the carbonate 

 NaHCOs / 



-^-—- I . we get from ( 1 ) 



NaoCOa/ 



aHNaHCOo)- 



pNaXOa X COo 



— K 



(NaHC03)^_^^P_^^, ^2) 



NaoCOs X CO3 a 



In solutions which are not too dilute, i.e. where the hydrolysis can be 

 neglected this equation agrees with that of McCoy ^° and Seyler and 



* Hassak, Unters. hot. Inst. Tiihingni. 2, 465-477 (1888). Pringsheim, N., Jalirb. 

 wiss. Bol., 19, 138 (1888). 



"Warburg, Riochcm. Zcit.. 100, 238 (1919). 

 *'McCov, Amcr. Chem. Jour., 29. 437 (1903). 



