Region II: Before the equivalence point

The region II titration equation calculates the pH of the solution before the equivalence point is reached. 

Solving a titration problem, Region II: Before the equivalence point. 

Given: Formal concentration of the weak acid and strong base, the amount of strong base is added to the acid, pK_a and or K_a. 

Frist solve for equivalence volume.

*Note: equivalence volume (V_e) is volume at which the acid and base are in equilibrium. The dilution formula can be used to solve for Ve. 

Since we have started titrating with strong base, the weak acid is going to react with the strong base creating water and strong acid. We do not know the concentration of the strong acid, the ice table is used. The ice table will look as following

*Note: We ignore the concentration of water since it does not contribute to the pH. We are also assuming that since this is before the equivalence point there is more weak acid in the solution than strong base, thus weak acid is 1.

The pH equation is 

We can then plug in the concentrations from the ice table, the pH equation then looks as following 

The graph represents a typical titration graph of a weak acid with strong base, with region II highlighted in red.

Description

The equation is 

where 

• V_add is the volume added of the strong acid in units of (mL)
• V_eq is the equivalence volume in units of (mL)
• pK_a is the log of acid equilibrium constant, can be found here.

References

Harris, 9th Edition.  Pp.236-238