Used as a dye, color marker and pH indicator, bromophenol blue is produced by adding bromine to a heated phenolsulfonphthalein and glacial acetic acid solution. Structurally similar to phenolphthalein, bromophenol blue changes to yellow at a pH level of 3.0 and to blue at a 4.6 pH level. This reaction can be reversed. When applied as a color marker, bromophenol blue regulates polyacrylamide gel electrophoresis and agarose gel electrophoresis processes. Carrying a slightly negative charge at moderate pH levels, bromophenol blue migrates in the same direction as proteins or DNA in gels. How quickly it migrates depends on the buffer composition and gel density. As a dye, bromophenol readily absorbs red light and transmits a bluish light. Therefore, dye solutions containing bromophenol are blue. At lower pH levels, bromophenol blue dye absorbs blue and ultraviolet light but appears yellow in solutions. Because bromophenol blue possess the largest change factor when turning a difference color due to solution thickness, it has the highest value listed in Kreft's dichromaticity index.