Question

Bromothymol blue is a weakly acidic (pK, is 7.10) colorimetric pH indicator. The dissociation of this indicator (HIn) and the colors that correspond with each species can be described by the equation

HIn(aq) ⇄ H⁺(aq) In⁻(aq)
acid, yellow conjugate base, blue


Determine what color a solution containing bromothymol blue will be at pH 2.


O blue
O green
O yellow

Answer 1

Bromothymol blue is a colorimetric pH indicator that changes from yellow to green at around a neutral pH, making it suitable for weak acid-strong base titrations.

Step-by-step explanation:

Bromothymol blue is a colorimetric pH indicator that is useful for determining the pH level of a solution. As a weak acid with a pKa of 7.10, it changes color depending on the pH of the environment. In acidic conditions, where the concentration of hydronium ions [H3O+] is greater, bromothymol blue will appear yellow.

In more neutral or slightly basic conditions, it transitions to green. For titrations, choosing the correct indicator is crucial; bromothymol blue would be suitable for titrations where the equivalence point is around neutral pH, such as a weak acid-strong base titration. For stronger acids or bases, indicators with a different range of pKa values would be more appropriate, like methyl red for weak bases or phenolphthalein for weak acids.