# Homework Help: Bromothymol Blue Equilibrium

1. Nov 23, 2015

1. The problem statement, all variables and given/known data
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?

2. Relevant equations

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

3. 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!

2. Nov 24, 2015

### Staff: Mentor

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

3. Nov 25, 2015

### epenguin

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

4. Nov 25, 2015

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

5. Nov 26, 2015

### Staff: Mentor

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.