The pH at which bromothymol blue be a perfect green color is; pH = 6.8
For Bromothymol, as HX is yellow and X⁻ is blue, the indicator will be a perfect green color when HX = X⁻.
From Henderson-Hasselbalch formula, we known that:
pH = pka + log [X⁻]/[HX] (1)
Where pka = -log ka = -log(1.6 × 10⁻⁷) = 6.8
Since HX = X⁻, [HX]/[X⁻] = 1
Thus;
pH = 6.8 + log 1
pH = 6.8 + 0
pH = 6.8
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