BUFFER ACTION 19 



that the sodium acetate solution retards in some way changes in pH value 

 upon the addition of an acid while a solution of sodium chloride does not. 



Solutions of such compounds as sodium acetate which are relatively re- 

 sistant to changes in pH due to the addition or loss of hydrogen or hydroxy! 

 ions are known as buffer solutionsj and this property of solutions is called 

 buffer action. Solutions such as sodium chloride which show no buffer effect 

 are called unbuffered solutions. 



If I cc. of a normal sodium hydroxide solution be added to lO cc. of a 

 normal solution of sodium chloride, a marked increase in pH will occur. If 

 I cc. of the same solution be added to 10 cc. of a normal solution of sodium 

 acetate, its pH will also increase markedly. In other words the sodium acetate 

 solution is buffered against the addition of an acid, but not against the addi- 

 tion of a base. If, on the other hand, 10 cc. of a normal solution of acetic 

 acid be substituted for the sodium acetate it will be found that this solution 

 is strongly buffered against the introduction of hydroxyl ions into the solu- 

 tion. A mixture of equal volumes of molar sodium acetate solution and molar 

 acetic acid solution will exhibit buffer action against both acids and bases over 

 a considerable range of the pH scale. 



Two important points regarding buffer solutions have been brought out 

 by the foregoing discussion. No one solute as a rule will act as a buffer against 

 both acids and bases. Furthermore no buffer solution will exert a buffer 

 effect over the entire range of the pH scale. Different buffer solutions vary 

 greatly in their effectiveness in maintaining pH stability. Some are strongly 

 buffered ; others weakly. Some are strongly buffered against acids and weakly 

 buffered against bases ; of others the converse is true. The commonest types of 

 buffer systems are those composed of a weak acid plus one of its salts. The 

 sodium acetate-acetic acid buffer system already described is such a system. 

 Practically all of the buffer solutions of importance in living organisms belong 

 to this group. 



Buffer action consists essentially in the tying up of free hydrogen or 

 hydroxyl ions nearly as rapidly as they are introduced into the solution in 

 the formation of compounds which are only slightly dissociated. The ensuing 

 change of pH is therefore relatively small in proportion to the volume of 

 acid or base added. As an illustration let us consider once more a solution 

 consisting of both sodium acetate and acetic acid dissolved in water. 



These two compounds dissociate as follows: 



CHaCOONa ^ CH3COO- + Na+ 

 CH3COOH ^ CH3COO- -f H + 



