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For my chemistry experiment, I want to boil spinach in water for different timings. Using the water that is left after boiling it, I was to determine the amount of iron content in the water solution. However, I am very confused on what method to use, etheir through titration (which I find easier to do) or through using a spectrometer. I am not sure if one will cause difficulty when I am doing the experiment. I need to decide this very soon, and I am very confused.

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  • $\begingroup$ Spectrophotometric method is reliable. What method do you have in mind? $\endgroup$
    – ACR
    Commented Feb 10, 2022 at 7:57
  • $\begingroup$ Mineralisation of the sample and than inorganic iron measurement by AAS or by colorimetry, i.e. with SCN- complex. $\endgroup$
    – Poutnik
    Commented Feb 11, 2022 at 19:45

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There are plenty of difficulties in your project. 1) Iron atom is probably included in one or several colored complexes whose formula are not obvious. 2) The amount of iron in spinach is not huge : $3.4$ mg in $100$ g fresh spinach, according to CIBA-Geigy Scientific Tables, Basel 1972. This small amount is difficult to measure whatever the method used. 3) Iron in plant is usually Fe(II). But in air, it is oxidized into Fe(III) and the spectrum of both ions are rather different. If you want to extract iron out of its complex, you would have to dry the spinach sheet and burn them, then extract iron out of the ashes. In the ashes, iron is probably under the form $\ce{Fe2O3}$, and, if heated to a high temperature $\ce{Fe2O3}$ is not soluble any more in concentrated acids like $\ce{HCl}$ or $\ce{H2SO4}$. It has to be dissolved in melted $\ce{NaHSO4}$ in a platinum crucible at $400°$C. Not easy !

The amount of iron present in vegetables does not vary much. Per $100$ g vegetables, one finds $0.7$ mg iron in carrots, $1.3$ mg in artichokes, $0.9$ mg in celery, $1.1$ mg in cabbages, $1.1$ mg in cauliflower, $3.1$ mg in dandelion, $3.1$ mg in spinach, $6.11$ mg in beans, $8.6$ mg in lentils, $6.2$ mg in parsley, $8.4$ mg in soya. Whatever your choice, these values are rather small.

Look ! $ 8.4$ mg iron is $0.15$ mmol, and needs $\ce{0.03 mmol KMnO4}$ to be titrated. If this $\ce{KMnO4}$ is contained in a $0.01 M$ solution, these $0.03$ mmol are contained in a volume $\ce{V = n/c = 3 10^{-5} mol/0.01 mol/L = 3.00 mL}$. And remember that this value is a maximum ! Most of the time, you will obtain volumes lower than $1$ mL.

Furthermore, as stated by M. Farooq, titrating a vegetable extract will above all determine the amount of organic substances. For example, sulphuric acid will hydrolyse cellulose into reducible sugars like glucose, which reacts with permanganate.

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  • $\begingroup$ Hi So I changed my experiment a bit, instead of looking at the boiling time of the spinach, I will be using different vegetables to find the one with the most iron concentration. My process will be to boil the vegetables, extract the liquid and mix it with sulphuric acid (H2S04). After doing so, I will titrate with potassium permanganate. Is this better? Please let me know if I need to change anything else. $\endgroup$
    – Arpit S
    Commented Feb 11, 2022 at 15:48
  • $\begingroup$ KMnO4 is not going to work as it will lead to misleading results. KMnO4 will react with all the organic matter first. For trace amounts of iron, spectrophotometric methods are reliable. This is why I was asking which spectrophotometric you had in mind? $\endgroup$
    – ACR
    Commented Feb 11, 2022 at 16:03
  • $\begingroup$ The answer to your question given one hour ago has been edited in my answer sent yesterday. $\endgroup$
    – Maurice
    Commented Feb 11, 2022 at 17:21

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