156 PHYSIOLOGICAL CHEMISTRY 



true acidity of a solution, that is, its hydrogen ion concentration. In the 

 case of strong acids and bases very accurate results for the true acidity 

 may be obtained in this way. In the case of weak acids or bases the 

 titration values may give but slight information as to the true acidity. 

 Thus in the case of a slightly dissociated acid, such as acetic acid, as 

 fast as the acidity due to its dissociated hydrogen ions is neutralized 

 the undissociated acid ionizes further and the titration value finally 

 obtained represents the total acid present at the beginning both ionized 

 and unionized. Salts of strong acids and very weak bases and vice 

 versa also hydrolyze during the course of the titration and the values 

 obtained in no sense represent the true acidity. 



Hydrogen ion concentrations may be determined through a certain 

 range by means of indicators. The unknown solution is treated with 

 a few drops of indicator and the color obtained compared with that 

 produced with the same amount of indicator and a solution of known 

 hydrogen ion concentration. If the same tint is produced in both 

 cases the two acidities are the same. This is of course only true when 

 the indicator chosen is a suitable one, that is, one that shows definite 

 color changes in hydrogen ion concentrations in the neighborhood oi 

 that of the unknown. The choice of indicators for this purpose is 

 somewhat different than that for titration purposes. For use in the 

 determination of the hydrogen ion concentration of a solution we need 

 an indicator showing a very gradual change in color through a given 

 range, one which is not readily affected by the presence of neutral salts 

 or other substances likely to be present, and the color of which does 

 not fade too rapidly. The ranges through which a number of indicators 

 may be used with satisfactory results for the determination of hydrogen 

 ion concentrations is indicated in the chart (Fig. 44, page 157). Those 

 surrounded by the heavy lines are the most satisfactory. 



The chart also indicates how standard solutions of definite hydrogen 

 ion concentrations may be made up from a series of stock solutions, 

 by mixing in various proportions. The stock solutions indicated on the 

 chart were suggested by Sorensen and are as follows: o.ioNHCl; 

 o.ioNNaOH; 7.505 g. glycocoll plus 5.85 gm. NaCl per liter; 11.876 

 g. Na 2 HPO 4 .2H 2 O per liter; 9.078 g. KH 2 PO 4 per liter; 21.008 g. 

 citric acid in i liter of 0.20 N NaOH; 12.404 g. boric acid in i liter of 

 o.io N NaOH. The other solutions are 0.20 N sodium acetate and 

 0.20 N acetic acid. Solutions of known hydrogen ion concentration are 

 prepared from these by mixing in the proportions indicated on the 

 chart, the abscissae representing parts of the more alkaline or less acid 

 constituent. Thus a mixture of seven volumes of the sodium acetate 

 stock solution with three volumes of the stock acetic acid solution 



