OF VITAL PHENOMENA 51 



dialysis allowed to proceed to approximate equilibrium. The 

 capsule is removed and the right quantity of indicator added. 

 The color is compared with that of a series of tubes of the same 

 bore, containing the same amount of indicator and solution but 

 of known PH. (Such tubes may be obtained, sealed, from 

 Hynson, Westcott and Co., Baltimore.) Or the tube may be 

 compared with the colored chart. One effect of dialysis is to 

 dilute the solutes affecting the PH and causing an error which 

 is less the greater the buffer value of the solution. 



Buffers and Solutions of Standard Hydrogen Ion Concentration 



The PH of water, or solutions of neutral salts, or very dilute 

 solutions of strong acids or alkalis, is changed by minute traces 

 of certain impurities. Absolutely pure water (conductivity 

 water) is neutral but may not remain so. If exposed to the air 

 it becomes acid due to absorption of C0 2 , and if kept in glass 

 it becomes alkaline, due to solution of the glass. "Pyrex" or 

 "nonsol" glass is better in this respect, but any glass is improved 

 by allowing a jet of steam to condense on the surface and drain 

 off, a process known as steaming out. Silica or metal vessels 

 are more reliable. This difficulty in maintaining fixed hydrogen 

 ion concentrations is obviated by the use of buffers. The chief 

 buffer in organisms is NaHCO a . The reaction of a pure solu- 

 tion of NaHCO a is but very slightly alkaline, that of Na 2 CO a 

 somewhat more alkaline and that of C0 2 but slightly acid. If 

 we add a strong acid to a solution of NaHC0 3 the acidity of 

 the mixture will not exceed that of a solution of C0 2 until all 

 of the NaHC0 3 has been decomposed. If we add a strong alkali 

 to a solution of NaHC0 3 the alkalinity of the solution will not 

 exceed that of a Na 2 C0 3 solution until all of the NaHCO s has 

 been decomposed. One drop therefore of n HC1 would acidify 

 a liter of H 2 or neutral salt solution more than a liter of the 

 acid would acidify a liter of m NaHCO a solution — hence the 

 advantage of a relatively high concentration of buffers in stand- 

 ard solutions. The standard solutions used by Sorensen (1912) 

 are shown in the chart in Fig. 20. On the ordinate is given the 

 number of cc of the first solution of a pair to be taken, when 

 the second solution is to be added to give a total volume of 

 10 cc. The stock solutions are as follows : 



