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The reaction of pyridine, acetic anhydride, water and citric acid is used as a colorimetric identifier of citric acid. What reactions occur and why does the color change from colorless to greenish? I have been looking everywhere for the answer and have, so far, come up with nothing.

The reaction is made from 1 ml of water containing citric acid, 1.3 ml of pyridine which is then mixed and then 5.7 ml of acetic anhydride is added. Then it is left in a water bath with a temperature of 32 degrees celsius.

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    $\begingroup$ @PhMgBr - I am not so sure. If it were a homework question, I should have been able to learn more about it using popular search engines. I cannot. This sounds like chemistry that happens in a proprietary colorimetic assay kit, and the chemistry in these assay kits is generally obfuscated by the vendor. $\endgroup$ – Ben Norris Apr 19 '16 at 16:06
  • $\begingroup$ Right @Ben, my bad. Weren't paying enough attention. $\endgroup$ – M.A.R. Apr 19 '16 at 16:11
  • $\begingroup$ @BenNorris The chemistry here was reported in J. Bio. Chem. in the late 40s. It's not proprietary anymore. The reaction with citric acid, from what I see, gives a red color, not green. Green, supposedly, comes from tartaric acid. [This article] is probably behind a paywall for those not hooked up to the proper IP address, but it's all I've really found so far. $\endgroup$ – SendersReagent Apr 19 '16 at 16:18
  • $\begingroup$ Okay, then it might be behind a paywall for the OP. Regardless the question can be improved. $\endgroup$ – Ben Norris Apr 19 '16 at 16:19
  • $\begingroup$ Sorry. This article. Should be able to see an abstract (or something, not sure if SD does first page for old articles). $\endgroup$ – SendersReagent Apr 19 '16 at 16:24

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