What Side of the Bromothymol Blue Equilibrium is Favored?

[Badgeray]

Homework Statement

Bromothymol blue is an acid-base indicator. When in solution, it forms the following equilibrium:
HInd_(aq) ⇌ H⁺_(aq) + Ind^-_(aq)
What side of the equilibrium is favored while the solution is in the bromothymol blue bottle?

Homework Equations

K_eq = [(C)^c(D)^d]/[(A)^a(B)^b]

The Attempt at a Solution

I think the equilibrium favors the reactants because since it is blue, then bromothymol blue is a base so there is small amounts of H⁺ ions so the K expressions is a small number divided by a bigger number which results in a small number.

Can anyone confirm that my thinking is correct? Thanks!

 

[Borek]

Yes. Bromothymol blue is a weak acid, with pKa around 7.

 

[epenguin]

The equation you give is not the relevant one. The right one would also suggest answer.

 

[Badgeray]

Thank you, but why is Bromothymol Blue an acid even though it is blue?

 

[Borek]

Thank you, but why is Bromothymol Blue an acid even though it is blue?

You are mistaking several things at the same time. Being an acid doesn't mean its solutions have low pH. First, it is a weak acid, second, its solutions - as used in the analytical chemistry - have very low concentration.

What is important is that - as a weak acid - it dissociates, it has a conjugate base (which has a different color than the acid itself), and the ratio of concentrations of both forms depends on the solution pH.