308 



PROTOPLASM 



pH 10.5 

 pH 8.5 

 pH 4.5 



Na3P04 + HCl 

 Na2HP04 + HCl 

 NaH2P04 + HCl 



NaCl + Na2HP04 

 NaCl + NaH2P04 

 NaCl + H3PO4 - pH 2.5 



Buffers are usually prepared as mixtures of two salts, a well 



known buffer pair being acetic 



Per Cent Acid in Mixture 

 90 80 70 60 50 AO 50 20 \0 



acid and sodium acetate. If 

 these two salts are mixed in 

 different proportions, and the 

 pH values of the mixtures 

 plotted, the curve shown in 

 Fig. 144 is obtained. If hydro- 

 chloric acid in not too great 

 quantity is added to any of 

 these mixtures, the pH remains 

 fairly constant. The reason is 

 as follows. The dissociation of 

 acetic acid is slight (CHs-COOH 

 ^ CH3COO- + H+). The 

 dissociation of sodium acetate 

 is great (CHsCOONa ^ 

 CH3COO- + Na+), as is also 

 that of hydrochloric acid 

 (HCl ^ H+ + C1-). The fol- 

 lowing ions, therefore, occur 

 in a solution of these three 

 substances: CH3COO-, Na+, 

 H+, and CI". The fact that 

 acetic acid ionizes to only a 

 very slight degree means that 

 CHsCOO- and H+ ions cannot 

 exist together in solution to a 

 very great extent and, therefore, that the larger part of the H+ 

 ions from the hydrochloric acid added will immediately combine 

 with CH3COO" ions to form undissociated molecules of acetic acid. 

 The concentration of acetic acid has thus been somewhat increased, 

 but as acetic acid dissociates to such a slight degree, a small increase 

 in its concentration does not change the hydrogen-ion concen- 

 tration of the solution very much. While a pH determination 

 would indicate little change in hydrogen-ion concentration, 

 titration would show the total amount of acid added (total 



10 20 50 40 50 60 10 60 90 

 Per Cent Acetate in Mixture 

 Fig. 144. — The hydrogen-ion con- 

 centration of mixtures of acetic acid and 

 sodium acetate. 



