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I'm planning an experiment to test the iron content of spinach. I plan to ash spinach, grind it, then dilute it in a solution which would hopefully oxidise the iron ions in it to $\ce{Fe^3+}.$ This is in order to get a reaction with ammonium thiocyanate solution in the end, which will allow me to determine concentration by colorimetry.

What should I dilute my ashed spinach into to oxidise the iron in it? Preferably something safe with the thiocyanate.

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Use ceric ammonium sulfate.[1,2] Quoting from reference 2:

2 Reagents

(i) 0.25 per cent ceric ammonium sulphate in 8.8 N sulphuric acid.
(ii) 40 per cent potassium thiocyanate solution.
(iii) (a) Stock standard ferrous iron solution - 100 μg iron per ml.
(b) Working standard ferrous iron solution - 0.3 to 10 μg iron per ml

2.2 Procedure

To five ml of clear water sample in a test tube 0.5 ml of ceric ammonium sulphate solution is added and mixed thoroughly by shaking. Finally, 0.5 ml of potassium thiocyanate solution is added and again mixed properly. Exactly 5 minutes after addition of thiocyanate solution, the absorbance of ferric thiocyanate complex is measured at 480 nm in a spectrophotometer (Spekol, Carl-Zeiss, Jena).

A series of freshly prepared working standard ferrous iron solutions (upto 10 ppm iron) were treated similarly and the absorbances measured at 480 nm. From the calibration curve, plotted with concentrations of different standard iron solutions against absorbances, the iron content of the unknown sample is evaluated.

References

  1. Willard, H. H.; Young, P. Ceric Sulfate as a Volumetric Oxidizing Agent. Ii. Determination of Iron. J. Am. Chem. Soc. 1928, 50 (5), 1334–1338. DOI: 10.1021/ja01392a013.

  2. Goswami, D. C.; Kalita, H. Rapid Determination of Iron in Water by Modified Thiocyanate Method. DSJ 1988, 38 (2), 177–182. DOI: 10.14429/dsj.38.4835. Also available via CiteSeerx. (The pdf icon has a cached version of the article.)

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  • $\begingroup$ @OscarLanzi Thanks for the answer, unfortunately doesn't look like I have access to ceric ammonium sulfate. $\endgroup$ Sep 6 '20 at 23:04
  • $\begingroup$ That is too bad, this is a pretty standard choice. To get a better answer I think you will need to provide more input. I assume you have some chemical knowledge and thus can identify some oxidizing agents to which you do have access. We can then pick the best one. $\endgroup$ Sep 6 '20 at 23:28
  • $\begingroup$ sigmaaldrich.com/catalog/substance/… $\endgroup$ Sep 7 '20 at 1:03
  • $\begingroup$ @OscarLanzi I am thinking maybe sulphuric acid would be a good choice. However it would need to be quite concentrated to work from my understanding. Do you think 1 molar sulphuric acid would be sufficient? Obviously I would later dilute it with de-ionised water since thiocyanate and concentrated acids don't go well. $\endgroup$ Sep 7 '20 at 2:44
  • $\begingroup$ One molar sulfuric acid is far from sufficient. It would be completely dissociated into aqueous hydrogen and bisulfate ions, and neither of the latter will do the job you want. Usually when chemists want sulfuric acid as an oxidant, they don't mess around. They go for the fully concentrated stuff. $\endgroup$ Sep 7 '20 at 9:35

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