Region I : Before strong base is added to weak acid

This region I titration equation calculates the pH of the solution before the strong base is added to the weak acid. 

Solving the titration problem for Region I: Before base is added. 

Given: Formal concentration of weak acid and strong base, K_a or pKa. 

Since the strong base is not added yet, the solution contains only water and the weak acid. Thus we can treat this as a a typical acid equilibrium problem and solve for the hydronium ion concentration to find the pH.

the equilibrium equation is then 

* Note: we can ignore the x in the denominator, since F>>x, we can assume that the hydronium concentration will not have a significant affect on the formal concentration. 

We then multiply both sides by F and take the square root and get the following equation:

we can then plug this in to the pH equation, then the final equation is 

This is a typical graph for titration of weak acid with a strong base. 

Description

The equation is 

where 

• K_a is the acid equilibrium constant, the constants for all of the acids can be found here.
• F is the formal concentration of the weak acid in units of (mol/L)

References

Harris, 9th Edition.  Pp.236-238