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Milling Yellow 3G

Milling Yellow 3G

Class: Azo
Ionisation: Acid

Chemical structure of Milling Yellow 3G

Common Name: Milling yellow 3G
Suggested Name: Acid yellow 40
Other Names:

Numerous others

C.I. Number: 18950
C.I. Name: Acid yellow 40
Color: Yellow
Solubility Aqueous: 3%
Solubility Ethanol: 3%
Absorption Maximum: 412
Empirical Formula: C23H18ClN4NaO7S2
Formula Weight: 584.99


A dye named milling yellow 3G was specified in Slidders’ fuchsin-miller technique for fibrin. It is not possible to identify the dye with certainty as there are three dyes with that name recorded in the second edition of the Colour Index. Those dyes are acid yellow 40 (CI 18950), acid yellow 42 (CI 22910), and acid yellow 56 (CI 24825).

The commercial name milling yellow 3G was listed as a designation for acid yellow 40. However, it was noted in the addendum for additions and alterations that the name milling yellow 3G was to be removed as a designation for acid yellow 56. It was also noted that it was to be added to the list of names for acid yellow 42.

To complicate the issue, Gurr lists milling yellow 3G but gives the formula for acid yellow 56, despite the Colour Index specifying that the commercial designation of milling yellow 3G should not be applied to that dye (it should be milling yellow H3G).

To complicate it even further, the commercial name milling yellow is applied to a large number of other dyes. Milling is a dyeing process for wool and yellow is a popular color, so it is not surprising. The letter designations following the name vary considerably for different dyes, but occasionally dye manufacturers coincidentally use the same designation. This is perhaps a case where the CI name should have been used.

So, which dye is the one meant? The most likely choice is acid yellow 40. This is noted to be soluble in both water and cellosolve (2-ethoxethanol), the common solvent used in techniques similar to the fuchsin miller. It is also very similar in structure to tartrazine, another dye which is commonly used for a similar purpose in a technique from which the fuchsin-miller is derived. The formula and data above are for acid yellow 40. Information about the other dyes is below.

The other dyes

Acid yellow 42, CI 22910

Acid yellow 56, CI 24825


  1. Edward Gurr, 1971
    Synthetic dyes in biology, medicine and chemistry
    Academic Press, London, England.
  2. Colour Index
    Society of dyers and colourists, UK.
  3. Green, F (1990)
    The Sigma-Aldrich handbook of stains, dyes and indicators
    Aldrich Chemical Company, Milwaukee, Wisconsin, USA