Temperature and Ionic Strength , Effects on Equilibria Using pC-pH Diagrams

DETERMINATION OF TEMPERATURE AND IONIC STRENGTH EFFECTS ON EQUILIBRIA USING pC-pH DIAGRAMS 

The pC-pH diagrams can be used for systems having temperatures other than 25°C and ionic strengths other than zero. The modification of a 25°C-based pC-pH diagram for temperatures other than 25 C merely entails the use of equilibrium constants for the appropriate temperature in the equilibrium expressions used to construct the diagram. The Van t Hoff relationship (Eq. 3-27) can be used to convert 25°C equilibrium constants to the values at the desired temperature. 

Construction of pC-pH diagrams for ionic strengths other than zero requires the use of activities rather than concentrations in all equilibrium 

TABLE 4-5 Proton Condition Solution for a NH4CI Solution 

As an example of these two types of corrections, let us construct the pC-pH diagram for a 10 M hypochlorous acid solution at 15 C with an ionic strength of 0,1 M. In this example, equilibrium constants and pH values based on concentration measurements will be denoted as and p H while the constants and pH values based on activity will be referred to as K and pH. The constant for HOCl at 15 C is 10 at 15 C is 4.5 X 10 (see Table 4-2), The master variable in the pC-pH diagram is p H, defined as 

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