Page "PH indicator" Paragraph 7
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Usually, the color change is not instantaneous at the pKa value, but there is a pH range where a mixture of colors is present.
This pH range varies between indicators, but as a rule of thumb, it falls between the pKa 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 ten times greater than the concentration of the acid, their ratio is 10: 1, and consequently the pH is pKa + 1.
Conversely, if there is a tenfold excess of the acid with respect to the base, the ratio is 1: 10 and the pH is pKa – 1.
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