Most health-food stores carry supplements supposedly containing iodine for people with iodine deficiency. I recently bought some kelp tablets that are supposed to have iodine but as a consumer how can I be sure that there is any iodine in them at all, let alone the claimed minimum daily requirement.

How do I know that I'm not just taking green-colored sawdust? Is there a cheap way to test a kelp tablet for its iodine content? If not are there testing labs that I could send some kelp to for testing that isn't going to cost me thousands of dollars?


Take some potato starch dissolved in water (you can easily get it from the water when you boil potatoes), and put a tablet into the water. If the water becomes bluish black, there is iodine in the tablet.

Yes, there are testing labs that will determine the amount of iodine in the tablets which will cost somewhere around \$100 to \$200.

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    $\begingroup$ The deeply coloured iodine-starch complex actually relies on triiodide, which itself comes from the reaction between iodide anions and molecular iodine. I imagine the iodine compounds present in the tablet are in the form of iodide or iodate, with no molecular iodine. Therefore, without the addition of a mildly oxidizing or reducing agent, there will probably be no formation of triiodide ions, and the starch mixture could remain colourless/white even in the presence of iodine compounds. $\endgroup$ – Nicolau Saker Neto Apr 27 '14 at 21:17
  • $\begingroup$ @NicolauSakerNeto Actually, any halide (e.g. $Cl^-$ from table salt) will complex with $I_2$, it doesn't have to be $I^-$. $\endgroup$ – LDC3 Apr 27 '14 at 22:36
  • $\begingroup$ Ah indeed, there are several interhalogen species which share some similarity. But still, the starch colour test should require the presence of a trihalogen anion (iso or hetero), which shouldn't be present in the tablet, and their formation requires the presence of a molecular halogen, which would also likely have to be added or produced in situ. $\endgroup$ – Nicolau Saker Neto Apr 27 '14 at 22:56
  • $\begingroup$ @NicolauSakerNeto Did you noticed that the OP stated kelp tablets. When kelp grows, it accumulates $I_2$ from the seawater and there is about 0.4% $I_2$ present in dried kelp. Since $I_2$ is volatile, the processing of kelp is done in a manner to minimize the lost of $I_2$. $\endgroup$ – LDC3 Apr 27 '14 at 23:02
  • $\begingroup$ Oh I knew seaweed was rich in iodine, but was unaware it was stored in the form of molecular iodine; I figured it was present as an anion such as those used to iodize table salt. Please excuse my comments then! $\endgroup$ – Nicolau Saker Neto Apr 27 '14 at 23:09

Just for reference there are at least other two methods, not so safe and easy to perform, but that can help you:

  • treat with a little bit of $\ce{HNO3}$ 1 ml of your dissolved sample add 0.5 ml of $\ce{AgNO3}$ 0.5 M. If $\ce{I-}$ is present, you should see a yellow precipitate: $\ce{AgI}$.

  • more dangerous!: put in your test tube 50 mg of your substance add 1 mL of concentrated sulfuric acid. Heat the test tube. Violet vapors will appear if $\ce{I-}$ is present.


Kelp is exceptionally good at scavenging the Periodic Table, as in Fukushima radwaste now bathing the US West Coast. Iodized salt takes care of your iodine requirement, and generic multivitamins, and seafood and seaweed (sushi) overall. You would need work at iodine deficiency unless you live in the Midwest with an idiosyncratic diet. You can buy iodide pills at a pharmacy. If a food supplement bottle does not have USP or NF in bold letters on its label, you should not put it into your mouth - no certified assay of contents, amounts, or bioavailability.

Faith-based engineering is rife with tests of faith.

  • $\begingroup$ I'm a vegan and I don't eat fish, I have palmar and plantar fibrimatosis and according to some holistic healing guru's, these diseases are caused by lack of iodine, (and in my case, 35 years of alcohol abuse) :-( $\endgroup$ – John Williams Apr 28 '14 at 1:11

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