(a) Identity.

1. (1) The color additive D&C Blue No. 4 is principally the diammonium salt of ethyl[4-[p[ethyl(m- sulfobenzyl)ami-no]-α-(o- sulfophenyl)benzylidene]-2,5-cyclo-hexadien-1-ylidene] (m- sulfobenzyl) ammonium hydroxide inner salt with smaller amounts of the isomeric diammonium salts of ethyl [4-[p-[ethyl(p- sulfobenzyl) amino]-α-(o- sulfophenyl) benzylidene]-2,5-cyclohexadien - 1-ylidene](p- sulfobenzyl) ammonium hydroxide inner salt and ethyl[4-[p-[ethyl (o- sulfobenzyl)amino]-α-(o- sulfophenyl) benzylidene]-2,5-cyclohexadien-1-ylidene] (o- sulfobenzyl) ammonium hydroxide inner salt.
2. (2) Color additive mixtures for use in externally applied drugs made with D&C Blue No. 4 may contain only those diluents that are suitable and that are listed in part 73 of this chapter for use in color additive mixtures for coloring externally applied drugs.

(b) Specifications. D&C Blue No. 4 shall conform to the following specifications and shall be free from impurities other than those named to the extent that such impurities may be avoided by good manufacturing practice:

Sum of volatile matter (at 135 °C) and chlorides and sulfates (calculated as sodium salts), not more than 15 percent. Water-insoluble matter, not more than 0.2 percent. Leuco base, not more than 5 percent. Sum of o-, m, and p- sulfobenzaldehydes, ammonium salt, not more than 1.5 percent. N-ethyl, N-(m- sulfobenzyl) sulfanilic acid ammonium salt, not more than 0.3 percent. Subsidiary colors, not more than 6 percent. Chromium (as Cr), not more than 50 parts per million. Lead (as Pb), not more than 20 parts per million. Arsenic (as As), not more than 3 parts per million. Mercury (as Hg), not more than 1 part per million. Total color, not less than 85 percent.