20 PROPERTTES OF SOLUTIONS 



The sodium acetate is strongly dissociated, but the acetic acid will dis- 

 sociate only slightly, being a weak acid. 



Suppose now that a little HCl be added to this solution. This is equiva- 

 lent to adding H+ and Cl~ ions and HCl molecules; the latter, however, 

 will dissociate, forming additional ions as rapidly as the H+ and CI" ions 

 already present are bound up in chemical combination. H+ and CHsCOO"* 

 ions cannot exist side by side in the same solution in appreciable concentra- 

 tions since CH3COOH is a poorly dissociated compound. Hence the added 

 H+ ions are almost all tied up in the formation of CH3COOH. The Cl~ 

 ions form NaCl with the Na+ ions which dissociates in the usual way. The 

 result is that there is only a slight increase in the concentration of hydrogen 

 ions in the solution, and hence only a very slight reduction in pH value. 



Suppose now that instead of HCl, a little NaOH be added to this solution. 

 This is equivalent to adding Na+ and OPI~ ions and NaOH molecules; the 

 latter, however, will produce additional ions by dissociating as rapidly as the 

 Na+ and OH~ ions already present are tied up in chemical combination. 

 But OH" and H+ ions cannot exist side by side in the same solution in 

 appreciable concentrations since HoO is only slightly dissociated. Most of the 

 added OH~ ions therefore combine with the H+ ions produced by the 

 CH3COOH and form HoO. More of the CH3COOH hydrolyzes produc- 

 ing more H+ ions, which in turn unite with more of the OH~ ions. This 

 continues until practically all of the added OH~ ions are tied up. The Na+ 

 ions form CHsCOONa with the CHgCOO" ions which dissociates in the 

 usual way. The final result is that there is only a very slight decrease in 

 the concentration of H+ ions in the system, and hence only a very slight in- 

 crease in its pH value. 



Any mechanism which will act in such a way as to remove hydrogen or 

 hydroxyl ions from a solution may operate as a buffer system. Other types 

 of buffering are known but they are relatively of much less importance in 

 living organisms than the type of chemical mechanism which has just been 

 considered. 



Hydration of Solutes. — Molecules of water adhere to the particles of 

 many solutes. Water thus associated with the particles of a solute is called 

 •water of hydration. For example, each molecule of dissolved sucrose ap- 

 parently has six molecules of water associated with it. This means that if a 

 mol of sucrose (342.2 g.) is dissolved in water there will be bound to the 

 sucrose molecules a total of six mols of water (i 08.096 g.). Ions also are 

 hydrated. Different species of ions appear to have different numbers of water 

 molecules associated with them. Although exact values have been assigned 

 by several experimenters for the number of molecules of water of hydration 



