18 THE PROPERTIES OF SOLUTIONS. [CH. I. 



acetate. It must be noted that they do not all disappear, 

 for some of the acetic acid formed is dissociated into H and 



CHg.COO. 



In the animal body the proteins, sodium bicarbonate, 

 and phosphates all function as buffers, and help to maintain 

 a constancy in the hydrogen ion concentration of the tissue 

 fluids. 



The effect o! dilution on (H). With a weak acid of 

 the type HA, the extent of dissociation is governed by the 

 equation 



K ..... < 



(HA) is the concentration of the undissociated mole- 

 cules per litre, and K is the " Dissociation constant " of 

 the acid. 



Since (H) = (A), we can write this 



(H) 2 = K(HA) or (H) = \/K(HA). 



This indicates that if a solution of a weak acid be diluted 

 four times (H) is halved; if it be diluted 16 times it is 

 reduced to one-fourth. 



In the presence of any considerable amount of the 

 sodium salt, the effect of dilution is quite different. 



We can write equation (i) in the following form : 



__ K(HA) 



~~ 



The sodium salts of weak acids are very freely dis- 

 sociated, so that there is a relatively high concentration of 

 A ions in the solution of the mixture. From equation (2) 

 it will be seen that an increase of (A) must cause a decrease 

 in (H). The dissociation of the weak acid being thus 

 depressed it follows that practically all the acid is present 

 in the undissociated form, so that we can assume 

 that (HA) = (acid). Further, practically all the free ions 

 arise from the dissociation of the sodium salt, so that (A) =*. 

 (Sodium salt.) 



