Finally it is time for some serious thinking about buffer solutions. As with the common ion effect when we discussed solubility equilibrium, an important disclaimer needs to be repeated before looking at examples of buffer calculations.
Our simple calculations assume that solutions behave ideally. At the most basic level that means we pretend that all particles in the solution behave independently of one another. This is not true, but it is convenient. However, it gives answers that do not exactly agree with what actually happens in the lab and that is perhaps most clear in the case of buffers. The particles in a solution which interact most strongly are the ions. One effect of this interaction which concerns us is the tendency for ions to group together, making their effective concentration smaller.
In a buffer solution this tends to make the measured pH of acid buffers lower than the calculated pH and the measured pH of base buffers higher than the calculated pH.