Bromophenol Blue - Dye

Dye

Bromophenol blue is also used as a dye. At neutral pH, the dye absorbs red light most strongly and transmits blue light. Solutions of the dye therefore are blue. At low pH, the dye absorbs ultraviolet and blue light most strongly and appears yellow in solution. In solution at pH 3.6 (in the middle of the transition range of this pH indicator) obtained by dissolution in water without any pH adjustment, bromophenol blue has a characteristic green red color. This phenomenon is called dichromatic color. Bromophenol blue is the substance with the highest known value of Kreft's dichromaticity index. This means that it has the largest change in color hue, when the thickness or concentration of observed sample increases or decreases.