Usually, the color change is not instantaneous at the pKa or pKb value, but a pH range exists where a mixture of colors is present. This pH range varies between indicators, but as a rule of thumb, it falls between the pKa or pKb value plus or minus one. This assumes that solutions retain their color as long as at least 10% of the other species persists. For example, if the concentration of the conjugate base is 10 times greater than the concentration of the acid, their ratio is 10:1, and consequently the pH is pKa + 1 or pKb + 1. Conversely, if a 10-fold excess of the acid occurs with respect to the base, the ratio is 1:10 and the pH is pKa − 1 or pKb − 1.
Usually, the color change is not instantaneous at the pKa or pKb value, but a pH range exists where a mixture of colors is present. This pH ra,nge varies between indicators, but as a rule of thumb, it falls between the pKa or pKb value plus or minus one. This assumes that solutions retain their color as long as at least 10% of the other species persists. For example, if the concentration of the conjugate base is 10 times greater than the concentration of the acid their ratio is 10:1, and consequently the pH is pKa + 1 or pKb + 1. Conversely, if a 10-fold excess of the acid occurs with respect to the base, the ratio is 1:10 and the pH is pKa − 1 or pKb − 1.