Anthocyanins as pH-indicators: stop the titration at red, purple or blue? If you titrate some hydrogen bromide solution (0.1 M) to some sodium hydroxide in order to determine the exact concentration of hydrogen bromide (0.1 M), and you use anthocyanins as a pH indicator. This anthocyanin will be red if you have a low pH value and purple if it is slightly acidic until it is slightly basic. It will be blue if the solution is basic. 

Do you have to titrate until the color of the solution is blue or purple?

Equilibrium from red to purple: $$\ce{H2X+ &lt;=&gt; HX}$$

$K_a = 1.047*10^{-3}$

Equilibrium from purple to blue: $$\ce{HX &lt;=&gt;X^{−}}$$
$K_a = 3.16 * 10^{-8}$
If you titrate some hydrogen bromide solution to some sodium hydroxide [...]


It's probably a bad idea to start with the anthocyane in strongly alkaline solution. Like other pyrylium compounds, flavylium cations do not tolerate (strongly) akaline conditions very well.

The nucleophilic opening of the pyrylium ring to a chalcone is irreversible under the conditions of the titration.