 Bromothymol blue is green at pH below 6.9, and then turns blue at a pH of 7.6 or greater.Although it could be used in other formats (e.g., Petri plates), Simmons' citrate agar is often used in slants.Organisms growing on Simmons' citrate agar are capable of using citrate as the sole carbon source and they can metabolize the ammonium salt in the medium (serving as a sole nitrogen source for growth).