1. Indicators are weak acids that change color when they become bases.
Indicators are frequently written as: HIn
HIn ⇌ H+ + In-
color color 2
The indicator’s equilibrium is controlled by pH
End point - when the indicator changes color.
The equivalence point has to fall in the range of the end point.
Choose indicators based on the calculated pH at the equivalence point.

2. Indicators
Since it is an equilibrium the color change is gradual except when the ratio of
[ 𝐼𝑛 − ] [𝐻𝐼𝑛] 𝑜𝑟 [𝐻𝐼𝑛] [ 𝐼𝑛 − ] = 10 1
Since the Indicator is a weak acid, it has a Ka.
The pH at which the indicator changes is calculated using the Henderson-Hasselbalch equation.
pH = pKa +log(10)
or pH=pKa +log(0.10)

3. Indicators
To determine which indicator to use for any particular titration, use the pH at the equivalence point and pick an indicator with a pKa ± 1 of the pH.
Because the pH changes so drastically around the equivalence point, the indicator should change color.
The end point is the point in the titration when the indicator changes color – it is not necessarily the equivalence point, although it should be close.
The equivalence point is the point at which the moles of the acid = moles of the base.