170 ACIDITY 



Example. Compare pH 6.6 and 5.1. 



The difference between pH 6.6 and 5.1 is 1.5 units, which can be broken down into 

 units found in the table, namely, 1.0, 0.3, and 0.2. 



A difference of 1.0 equals 10 times. 



A difference of 0.3 equals 2 times. 



A difference of 0.2 equals 1.6 times. 

 Hence a difference of 1.5 equals 10 X 2 X 1.6 = 32. 

 Therefore pH 5.1 is 32 times as acid as pH 6.6. 



Buffers 



Although addition of a minute amount of hydrochloric acid, or any 

 other strong acid, to water produces relatively a great change in pH, 

 biological fluids, in general, do not undergo a comparable change when 

 strong acid or base is added to them, because of the presence of certain 

 compounds in these fluids. Such compounds which resist change in 

 acidity or basicity are known as buffers. 



In general, a buffer consists of a weak acid (or base) and its salt. 

 The buffer is the mixture of the two substances. Examples are acetic 

 acid — sodium acetate, carbonic acid — sodium bicarbonate, ammonium 

 hydroxide — ammonium chloride. Frequently the second hydrogen of a 

 di- or tri-basic acid serves as the weak acid, as in the buffer NaHoPOi — 

 Na^HPOi. Other metals, such as potassium, are equally satisfactory 

 in buffers provided they form water-soluble salts with the acids concerned. 



Buffers exert their effect through chemical reactions that use up most 

 of the hydrogen or hydroxyl ions that are added. This action depends, 

 fundamentally, on the fact that the weak acid (or base) is only slightly 

 ionized. A weak acid HA ionizes according to the equation: 



HA?^H++A- 



The A~ here represents the acid radical. Since this ionization is a 

 reversible process, the addition of extra hydrogen ions shifts the reaction 

 back to the left (law of mass action) and thereby converts most of 

 the added H+ into undissociated HA molecules. On the other hand, if 

 a strong base is added to the buffer, the 0H~ ions react with H+ to form 

 water, and more of the HA molecules ionize to replace most of the H + 

 ions used. In either case the pH remains relatively constant. 



The exact pH of any individual buffer solution and the pH change 

 resulting from the addition to it of a certain quantity of strong acid 

 or alkali may be calculated readily from a knowledge of the dissociation 

 constant of the weak acid or base in the buffer. The mathematical 

 expression for the dissociation constant Ka of a weak acid is based on the 

 equation for its ionization. It is: 



r. _ [H+] ' [A-] 



[HA] 



