ACIDITY 307 



terms of molarity, grams per liter for hydrogen. The 0H~ ion 

 concentration is also expressed in molecules per liter, or 

 equivalent concentrations; the weight in grams would then 

 be obtained by multiplying by 17, the weight of the 

 OH- ion. 



Buffers. — Pure water is neutral with a pH of 7. If 1 cc. of 

 O.OIA^ HCl is added to a liter of water of pH 7, the resulting pH 

 will be 5 ; but if this amount of acid is added to a beef infusion of 

 pH 7, the change in pH will be hardly noticeable. This power 

 of solutions to resist a change in pH was likened to a tampon, 

 or swab, which soaks up a fluid — not an especially good analogy. 

 Puffer in German and "buffer" in English are both from Old 

 French buffe. (The French use the word tampon for buffer.) A 

 better analogy is that between a pH buffer and a mechanical 

 buffer (a blow or bump) both of which ease a shock. Any sub- 

 stance that prevents a sudden or pronounced change in acidity 

 is a buffer. All physiological solutions such as blood, milk, and 

 sea water are buffered. Buffers are extensively used in the 

 laboratory to maintain solutions at a constant pH value. There 

 are a variety of buffer salts, a well-known one being acetate. 

 The acetate itself is nearly neutral. If strong hydrochloric acid 

 is added to it, weak acetic acid and neutral sodium chloride 

 are produced. Sodium bicarbonate is another buffer salt and 

 one of great biological importance. Blood is well buffered by 

 bicarbonate. It acts in the following manner: 



NaHCOs + HCl = H2CO3 + NaCl 



in which the strongly dissociated hydrochloric acid has been 

 converted by the buffer into the weakly dissociated carbonic 

 acid, the pH change from that of the original buffered solution 

 being small. 



The behavior of sodium phosphate salts is a good example of 

 buffer action. If to a highly alkaline solution of trisodium 

 phosphate of pH 10.5 an equivalent amount of strong hydro- 

 chloric acid of pH 1 is added, then alkaline disodium phosphate 

 and neutral sodium chloride result. More strong acid of pH 1 

 will bring the solution down to only pH 4.5. Still more acid of 

 equivalent amount will finally bring the mixture to pH 2.5. The 

 sodium chloride formed each time is neutral. 



