An indicator with a lower pka will have a color change interval that is base on the ph scale.
Since indicators are weak acids or bases, they change form as the pH fluctuates. When the indicator's pKa and pH are numerically equal, the indicator will be composed of 50% acid and 50% basic forms. One unit higher in pH results in 10% acid form and 90% basic form. As a result, there occurs a progressive switch from almost exclusively acid to almost exclusively base form.
As the ratio of acid to base changes, the colour of the indicator also changes because indicators come in both acid and basic forms. For our eyes to see a colour as pure red or pure yellow, for example, a ratio of roughly 10:1 is typically required.
The precise ratio will depend on the compound's absorbance and how sensitive our eyes are to a particular colour. However, we will typically observe a change over a pH range of about 2 from pKa -1 (90% acid) to pKa +1 (90% basic).
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