Orange G or orange gelb is a synthetic azo dye used in histology in many staining formulations. It usually comes as a disodium salt. It has the appearance of orange crystals or powder.
The main use of Orange G is in the OG-6 Papanicolaou stain, to stain keratin, however it is also an important component of the Alexander test for staining pollen.
It is often combined with other yellow dyes and used to stain erythrocytes in trichrome methods.
Orange G can be used as a color marker to monitor the process of agarose gel electrophoresis, which runs approximately the size of a 50 base pair (bp) DNA molecule, and polyacrylamide gel electrophoresis. Bromophenol blue and xylene cyanol can also be used for this purpose. (However, the apparent "size" of all of these dyes varies according to the concentration of agarose in the gel and the buffer system used, so the appropriate reference should be sought before using the dyes to determine how much a gel has run.)
Despite its two ionizable groups, it displays only two colors in aqueous solution, bright orange at neutral and acidic pH or red at pH greater than 9.
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