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MANUAL OF MICROBIOLOGICAL METHODS 



in which brackets represent concentrations, and pK' (= — log K') is 

 called the apparent ionization exponent of the indicator's proton donor- 

 acceptor system. Simple calculations, using, for example, 0.8, 0.5, and 

 0.3 as values for the ratio [proton acceptor]/[proton donor] at each of the 

 pK' values 3, 6, and 9, will show that indicators with different pK' values 

 cover different ranges of pH (see Fig. 1). For 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 dolor 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 determine 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 6. All 

 but three of the compounds are sulfonphthaleins which are particularly 

 useful in bacteriological work because of their high tinctorial power, low 

 or moderate salt and protein errors, and relative resistance to bacterial 

 action. Table 6 gives the pK' values of the indicators and their sensitive 



" Stock solutions in 95 per cent ethanol for the indicator acids, or in water for the 

 indicator salts, unless otherwise specified. 



^ 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. 



" Do not use with phthalate buffers. 



^ Dissolve 50 mg in 100 ml of water. 



• Dissolve 20 mg in 60 ml of 95 per cent ethanol, and add 40 ml of water. 



^ Dissolve 100 mg in 65 ml of 95 per cent ethanol, and add 35 ml of water. 



Source: See Clark (1948) and Kolthoff and Rosenblum (1937). 



