Bromothymol Blue sodium salt is a pH indicator. In Vitro: A first characterization and comparison is done by developing an easy and direct measurement method based on a pH indicator system using Bromothymol Blue (BTB) as the indicator and potassium phosphate as the buffer. A pH-shift assay is developed on the basis of Bromothymol Blue as the pH indicator and potassium phosphate as the buffer component^[1]. Three pH indicators are tested for the direct determination of 2- 2-keto-L-gulonic acid (2-KLG) production on a plate. The results show that Bromothymol Blue is superior to the other two indicators in terms of the obvious color change and a suitable pH range (blue to yellow at pH 6.5-7.5). Upon the addition of a Bromothymol Blue solution (0.1%, w/v) to an agar plate, zones surrounding colonies of K. vulgare 07 mutants change their color from blue to yellow because K. vulgare 07 mutants release 2-KLG on agar plates, thereby acidifying surrounding areas around colonies^[2].