IX4S-8 



MANUAL OF METHODS FOR PURE CULTURE STUDY 



a full discussion of the properties and uses of pH indicators, see Clark 

 (1928), and Kolthoff and Rosenblum (1937). 



Within a short range on the pH scale on each side of the pK' value, 

 every color gradation of the indicator corresponds to a definite pH 

 number; this zone may be called the sensitive range of the indicator. 

 Throughout its sensitive range, an indicator can be used to deter- 

 mine the pH of a solution by comparing its color in the solution with 

 that produced in standard solutions representing known pH numbers. 



The indicators. A selection of indicators is presented in Table 

 2. All but three of the compounds are sulfonphthaleins which are 

 particularly useful in bacteriological work because of their high tinc- 

 torial power, low or moderate salt and protein errors, and relative 

 resistance to bacterial action. Table 2 gives the pK' values of the 

 indicators and their sensitive ranges. The last column, and footnote 

 b of the table give specifications for the preparation of stock solutions 

 of the mono-sodium salt of each of the sulfonphthaleins. 



*See Clark (1948), and Kolthofif and Rosenblum (1937). 



(a) Stock solutions in 95% ethanol for the indicator acids, or in water for the indica- 

 tor salts, unless otherwise specified. 



(b) Grind 100 mg. of the pure indicator acid with the amount of NaOH specified, 

 and when solution is complete dilute with water to a volume that will yield the con- 

 centration recommended in column 4. 



(c) Do not use with phthalate buffers. 



(d) Dissolve 50 mg. in 100 ml. water. 



(e) Dissolve 20 mg. in 60 ml. 95% ethanol, and add 40 ml. water. 



(f) Dissolve 100 mg. in 65 ml. 95% ethanol, and add 35 ml. water. 



It will be noted from footnote a that ethanolic solutions are 

 ordinarily satisfactory. For precise work, however, aqueous solu- 

 tions of the indicator salts are preferable to the alcoholic solutions of 

 the free acids. To obviate the labor of preparing the neutralized 

 solutions, some makers now offer the soluble salts of the sulfonph- 

 thaleins. They are ammonium, sodium, or possibly other salts of 

 these compounds. In ordinary use, the indicator salts contribute 

 negligibly to the total ions present in a test solution, and the nature 

 of the cation may be of no consequence. However, in some studies 



