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I am looking for a way to measure the iron content of spinach for a chemistry experiment. Unfortunately I am not sure that thiocyanate will be available and I'm therefore trying to find an alternate method. The most common one seem to be a titration with potassium permanganate. The issue with this method is that other much more abundant metals in spinach would also react with the permanganate.

I had the idea to ash the spinach, dissolve the ashes in water. Then use a displacement reaction since iron is the least reactive metal in the chemical makeup of spinach. I am not sure exactly how but I thought it might be possible to use a compound to precipitate the more reactive ions out of solutions.

Do any of you have some ideas regarding this experiment?

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  • $\begingroup$ I suppose you need to dissolve ash in acid, otherwise iron could end as insoluble oxides ( or metal if highly heated with carbonized organics ). The iron oxidation state after dissolution can be uncertain, as oxides tend to be non-stoichiometric and reducting environment during burning undefined. $\endgroup$
    – Poutnik
    Aug 25 '20 at 13:06
  • $\begingroup$ Through spectrometer: chemistry.stackexchange.com/questions/18435/… $\endgroup$ Aug 25 '20 at 13:16
  • $\begingroup$ Or through titration: chemistry.stackexchange.com/questions/42866/… $\endgroup$ Aug 25 '20 at 13:17
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    $\begingroup$ @NilayGhosh yes I am looking for a method other than the spectrometer since I may not have access to KSCN. The problem with that titration is that many other metals will react with the permanganate. $\endgroup$ Aug 25 '20 at 14:32
  • $\begingroup$ @Poutnik How could I proceed from them to determine the amount of iron in the solution? $\endgroup$ Aug 25 '20 at 14:33
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There are iron testing kits in water which are available at an affordable price. I have not used them but read more about them here Testing iron in well water. You can test your spinach extracts for a yes no type answer if iron is present in significant quantities or not. Just keep in mind that PopEye's spinach is really not that rich in iron. Too much iron is a liver toxin.

I am afraid to share the bad news for you that with the resources you have, you cannot reliably determine iron content in spinach. Micronutrients or minerals cannot be determined by classical methods because they are present in a complex matrix and in very small quantities (parts per million range). They need instrumental analysis (absorption by molecular complexes, atomic emission or atomic absorption). I saw a couple of papers from ResearchGate which talk about the titration of spinach extract with permanganate. Nothing can be more wrong than this procedure in analytical chemistry. They are basically titrating all the organic stuff with it. Those methods are horribly wrong. Someone on SE Chemistry posted a query on how to determine iron in spinach by titration Iron in spinach...that is all incorrect. Yes, you will generate some numbers with those burette readings but those numbers have no significance.

Regarding your ashing method, that will not work either, because you need at several kgs of spinach to get a small amount (milligram) of iron. What should be the ashing temperature? How would you isolate iron from the ashes? Selective precipitation to get rid of other minerals? Nope, that is a bigger headache. For that you would need more reagents.

If this is a school project, make sure you have the resources after searching the literature. If this is an home experiment- classical methods will not work.

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As you are looking for inventive ways to possibly measure Fe presence for a school presentation, may I suggest a crude (but likely interesting) visual aid in possibly determining relative iron concentration.

One idea is that Fe ions in the presence of acidic H2O2 will engage in radical production. The latter reaction in the presence of suitable light over time could result in a cyclic reaction system (for example, photo-Fenton). The latter is a conceivable path to magnify even small transition metals presence.

The latter radicals (especially, the powerful hydroxyl radical) can breakdown organic matter, including, for example, a strongly colored dye over time, resulting in eventual de-colorization. The relative periodic time evolution for the latter could be a manner to simply display/document apparent differences in even small relative iron (also copper, manganese,..) concentrations.

Here is a citation from the literature as an example, Effect of iron salt on the color removal of water containing the azo-dye reactive blue 69 using photo-assisted Fe(II)/H2O2 and Fe(III)/H2O2 systems.

So, spinach grown in iron-rich soil, reduced to ash added to acidic H2O2 (plus perhaps more acid added 1st to the spinach, further lowering pH) in the presence of a photo-sensitive dye subject to photolysis, should display more rapid de-colorization (as captured by periodic snapshots) than one grown in iron-poor soil.

You may experiment with various ways of numerically quantifying the tentative iron content, as deemed viable given limited resources (including funds which alternatively, could purchase an iron testing kit, available here, for example, at $99, or less expensive water testing strips here).

Note: I am assuming one does not have the time to actually grow the same variety of spinach. Otherwise, I would simply speculate that iron-richer spinach may possess different coloration (and perhaps other discernable quantifiable attributes) for the purpose of a simple relative classification as to iron content.

This would conceptually be based, relatedly to above, on naturally occurring photosynthesis. To quote from a supporting source:

Iron, one of the structural elements of organic components that play an essential role in photosynthesis and nitrogen assimilation of plants, is available at extremely low concentrations in large parts of the Southern Ocean's surface waters. We tested the hypothesis that photosynthesis is the primary target of iron stress in phytoplankton living in this specific environment, resulting in a reduced carbohydrate production.

On the suggested coloration argument relating to iron content, also a confirming reference, to quote:

Leafy vegetables featuring a deep, dark green color, such as spinach and collard greens, are usually high in iron.

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