Bromophenol blue (3′,3″,5′,5″-tetrabromophenolsulfonphthalein, BPB),[2] albutest[3] is used as a pH indicator, an electrophoretic color marker, and a dye.

It can be prepared by slowly adding excess bromine to a hot solution of phenolsulfonphthalein in glacial acetic acid.

Since bromophenol blue carries a slight negative charge at moderate pH, it will migrate in the same direction as DNA or protein in a gel; the rate at which it migrates varies according to gel density and buffer composition, but in a typical 1% agarose gel in a 1X TAE buffer or TBE buffer, bromophenol blue migrates at the same rate as a DNA fragment of about 300 base pairs, in 2% agarose as 150 bp.

At low pH, the dye absorbs ultraviolet and blue light most strongly and appears yellow in solution.

[8] This means it has the largest change in colour hue, when the thickness or concentration of observed sample increases or decreases.