ALKALI RESERVE 



431 



quantities of other salts sueh as phosphates also present. These 

 latter are, however, quantitatively unimportant. In the regula- 

 tion of the hydrogen ion coneentration, the sodium bicarbonate is 

 the most important plasma constituent. It has been shown 

 practically, and it may be deduced theoretically, by consideration 

 of the law of mass action, that where a solution contains a weak 

 acid, and a salt of that acid, the hydrogen ion concentration of the 



, ^. . J.- 1 J. ^1 i.- [concentration of free acid] 



solution IS proportional to the ratio ^ ; -:, \ 



[concentration of salt] 



Of the crystalloids in solution in the plasma, sodium bicarbonate 

 has the most marked " buffer " effect. In this salt a strong 

 base is united with a weak acid and, therefore, any acid stronger 

 than H2CO3 will take the place of the HCO3 ion in its combination 

 with sodium. 



For instance. 



Na 

 CH3COO 



+ 



= Sodium bicarbonate. 

 = Acetic acid. 



CH3C00Na 



H2CO3 



(Sodium acetate). (Carbonic acid). 

 This reaction appears just to postpone matters because carbonic 

 acid is set free and, although this acid is only slightly dissociated, 

 yet, as an acid, it must be reckoned with. The dissociation of 



NaHC03 is a reversible reaction 



NaHCOg — 

 HoO — 



Strong. Weak. 



This means that the direction of the reaction will, in the main, 

 depend on the relative amount of C02(HoO) present compared 

 with the amount of NaHC03. For plasma, this ratio has been 

 determined experimentally. To give the normal |jH of 7-4, plasma 

 must have 3-75 grams of CO2 bound in NaHCOg present for every 

 gram of uncombined CO2 : or, by volume, one volume of COg 

 remains constant when associated with 20 volumes combined as 

 carbonate. In tabular form this reads : 



Free CO, 1 1 



TS 1 r.A = 771^' by weight, -- by volume. 



Bound to, 3-75 -^ "^ ' 20 -^ 



this ratio is present owing to reduction 

 in the CO., tension some will dissociate to balance the dissociation 



If excess of NaHC03 over 



