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I read in the local paper that common food products like turmeric can be adulterated. It continued to specify the adulterant which they called metanil yellow. On searching on the Net, I was able to find that metanil yellow is prepared using some basic raw materials like metanilic acid and diphenylamine. The paper continued to lay out procedures on how to find out whether the turmeric you buy from the local market is adulterated. To do this, one must first dissolve half a spoon full of turmeric powder in 20ml of lukewarm water. It then told to add a few drops of hydrochloric acid. Upon doing this, if the aqueous solution turned pink, violet or purple, it indicated the presence of metanil yellow. It continued to state the risks of consuming turmeric adulterated with metanil yellow. One of these risks was that it is a carcinogen. I'm an Indian, so turmeric is included in nearly all of the dishes I eat in a day, so I became very curious. My question is this:

1.) What is the chemical formula for metanil yellow, how is it prepared (in layman's terms), and how does it react with hydrochloric acid to turn pink, purple or violet (a chemical equation would be helpful)?
2.)What are the risks in consuming metanil yellow?

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  • $\begingroup$ If You look for "metanil yellow" in Google, You find ample information on formula, MSDSes and- an article from two Indian scientists on riscs from elongated consumption. All other things You ask for wre either silly (eg synthesis in laymans terms) or I start to think for what You need this information. In general: what is Your issue? $\endgroup$ – Georg Jul 8 '12 at 13:03
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    $\begingroup$ The thing with azo dyes like metanil yellow is that they are broken down within the body to the corresponding aromatic amines used to make them. Thus, any danger one might encounter from internal consumption of an azo dye would depend on the aryl amine... and yes, a good number of aryl amines are potentially carcinogenic. $\endgroup$ – user95 Jul 8 '12 at 14:20
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    $\begingroup$ As for why that test with hydrochloric acid works: metanil yellow is in fact an acid-base indicator. $\endgroup$ – user95 Jul 8 '12 at 14:22
  • $\begingroup$ Hmm, I see a few issues with your question: For one, it looks like you haven't properly researched the issue on your own, a Google search would answer the first part of your first question. Secondly, you have asked too many questions in one post--I suggest you restrict the post to either the $\ce{HCl}$ reaction or the toxicity. (Make two posts if you want). $\endgroup$ – ManishEarth Jul 9 '12 at 4:56
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I hope to keep this short.

Metanil yellow is an azo dye synthesized from the coupling of metanilic acid and diphenylamine, as described here. Here's a sketch:

metanil yellow

Now, the thing with most azo dyes like metanil yellow is that when they are taken internally, liver enzymes or intestinal flora will usually reduce the azo dye to the components that were used in the diazo coupling. (See this article for more details.) For metanil yellow, the metanilic acid is poorly absorbed from the intestinal tract (and is thus easily excreted), and your remaining worry is the diphenylamine, which is a suspected mutagen and carcinogen (apart from the usual toxicity associated with arylamines). (In short, the metanil yellow itself is not your worry, but the metabolite diphenylamine sure is.)

By way of contrast, the natural coloring agent in turmeric is the phenolic compound curcumin:

curcumin

As it turns out, both compounds are in fact acid-base indicators, though they have different transition ranges; for metanil yellow, the pH transition range is 1.2-2.3 (red to yellow), while for curcumin, the transition range is 7.8-9.2 (yellow to red-brown). (Data taken from the Handbook of Acid-Base Indicators.) I'm not quite sure where the purple/violet color would come from, but the key point is that natural turmeric would be reddish in an alkaline environment, while metanil yellow will be reddish in an acidic environment; however, vinegar (pH ~ 2.4) is not sufficiently acidic to display the color change for metanil yellow, and thus a stronger acid like hydrochloric acid is needed to display the color change.

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  • $\begingroup$ It is possible the pink-purple color is a result of a reaction between metanil yellow and curcumin or another component of turmeric. $\endgroup$ – Ben Norris Aug 8 '12 at 23:21

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