An important point on any speciation plot is the point at which the two lines intersect. At the intersection point, the acid and its conjugate base have equal concentrations. According to the equation,

if [HA] = [A^-], then the ratio [HA]/[A^-] = 1. Therefore, the ratio [H₃O⁺]/Ka must also equal 1 and [H₃O⁺] = K_a. If these values are converted to their logarithmic equivalents, then pH = pK_a at this point.

Therefore, at the intersection point of a speciation plot, where the concentration of an acid and its conjugate base are equal, pH = pK_a. If pH < pK_a, the concentration of acid is greater than the concentration of conjugate base; if pH > pK_a, the concentration of conjugate base is greater than the concentration of acid.

Why Should We Care?

Since pH = pK_a at the intersection point of a speciation graph, the pK_a value of an acid can be determined quickly from this plot. Also, if the speciation plot of an unknown acid is determined, the pK_a value indicated by the intersection point on the graph can be used to determine the identity of the acid.