More Titrations and Buffered Solutions

The titration of a weak acid with a strong base looks slightly different than the curve above, and is shown below. The equivalence point is also not as predictable. Of course, if the base is stronger than the add, the equivalence point will be above 7, and if the acid is stronger than the base, the equivalence point will be below 7. 

Notice the half equivalence point. This is probably more likely be tested by the MCAT than the equivalence point. The half equivalence point is the point where exactly one half of the acid has been neutralized, by the base. In other words, the concentration of the acid is equal to the concentration of its conjugate base. Notice that the half equivalence point occurs at the midpoint of the section of the graph that most represents a horizontal line. This is the spot where we could add the largest amount of base or acid with the least amount of change in pH. Such a solution is considered to be buffered. The half equivalence point shows the point in the titration where the solution is the most well buffered. 

The Hendefson-Hssselbalch equation is simply a form of the equilibrium expression for K  

Notice also that, at the half equivalence point, the pH of the solution is equal to the pffs of the acid. This is predicted by the Henderson-Hasselbalch equation  

If we were to make a buffer solution, we would start with an acid whose pKa is closest to the pH at which we want to buffer our solution. Next we would mix equal amounts of that acid with its conjugate base. We would want the concentration of our buffer solution to greatly exceed the concentration of outside acid or base affecting our solution. So, a buffer solution is made from equal and copious amounts of a weak acid and its conjugate base. 

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