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I am working on finding the calcium content lost upon cooking kale, using complexometric titration. For my indicator, I am using Eriochrome Black T indicator (EBT; solid form, not the liquid).

According to my research:

To determine the calcium content of kale when cooked at different temperatures, complexometric titration will be used. This technique involves adding a titrant of known concentration to a sample until a color change signaling the endpoint, indicating the reaction is complete.

Setup and Process:

Titrant: Ethylenediaminetetraacetic acid (EDTA) is used as the titrant due to its ability to form stable complexes with metal ions like calcium $(\ce{Ca^2+})$ and magnesium $(\ce{Mg^2+})$.

Reactions:

$$\ce{Ca^2+(aq) + EDTA^4-(aq) → [Ca-EDTA]^2-(aq)}$$ $$\ce{Mg^2+(aq) + EDTA^4-(aq) → [Mg-EDTA]^2-(aq)}$$

Excess EDTA and Indicator: Excess EDTA is added to the sample to ensure all metal ions are complexed. EBT indicator is then added, by which the solution initially turns to dark blue color.

Back Titration with Magnesium Chloride: Magnesium chloride is added to react with the remaining EDTA. The EBT indicator reacts with magnesium ions to form $\ce{MgIn(aq)}$, turning the solution to pink color. However, EDTA displaces $\ce{Mg^2+}$ from $\ce{MgIn(aq)}$ due to forming more stable complexes, eventually returning the solution to blue.

Endpoint Detection: The endpoint is reached when all EDTA has reacted, and EBT forms permanent complexes with magnesium ions, turning the solution permanently pink.

In summary, complexometric titration with EDTA and EBT indicator is used to determine calcium content by observing the color change that signals the titration's endpoint.

But now here is the tricky part. Instead of turning black, my kale solution turns darkish-greenish, and upon titrating, the end point is a faint brownish-green colour. There is barely any pink! Why would something like this happen! As needed, the $\mathrm{pH}$ of the solution is also maintained. Does this mean that my indicator is not compatible with the kale samples? My predictions are that the small tiny particles of kale are causing the solution to have a strong green colour, because of which the indicator is not working. If so, could you guys please suggest another indicator for me?

The following is an image of before titration. Instead of black it is bluish-greenish-black!

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    $\begingroup$ To the close voter, what's not clear? $\endgroup$
    – Nilay Ghosh
    Commented Jun 1 at 13:09
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    $\begingroup$ For mineralization, search for methods applicable to your resources. // What is the color of the solution with the adjusted pH but without added indicator? // When a solid indicator is used in complexometry, it is usually mixed with powdered inert salt like KCl in ratio 1 : 100, for easier dosage of small amounts. // There should be generally used the minimum amount of indicator where color transition is still clearly visible. More indicator means slower transition, as you titrate the indicator complex. You should see lightly colored white background behind the solution. $\endgroup$
    – Poutnik
    Commented Jun 1 at 16:58
  • $\begingroup$ @Poutnik I appreciate the response. I will talk to my lab senior about it and try that out! $\endgroup$
    – user146547
    Commented Jun 2 at 4:41

2 Answers 2

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Classical volumetric titrations are meant for relatively clean samples free from organic stuff. Dyes can adsorb on particles, that can interfere. One has to destroy the organic components/colored compounds by acid digestion of the extract. This serves to remove the color and release bound metals. One should not proceed directly to titration. Note that Mg in plants is bound by another complexing agent-chlorophyll. This is why kale and other leaves are green.

Ask your supervising teacher about digestion process (post extraction) and make sure the indicator is prepared as follows. The reference is Advances Practical Inorganic Chemistry, by Gurdeep Raj, pg.394.

The colour at different pH given by Eriochrome black T are:

  • Red - below pH 5.5
  • Blue - between pH range 7-11
  • Yellowish orange - above pH 11.5

Between pH range 7-11, when metallic salts are added, the colour of the indicator changes from blue to red. At the end point in EDTA titration, the indicator will be set free and the colour change will be from red to blue.

Preparation of Eriochrome Black T as Indicator:

  1. The solution of the indicator is prepared by dissolving 1 g of Eriochrome Black T in 15 ml of triethanolamine and adding 5 ml of absolute alcohol. It can also be prepared by dissolving 0.2 g of Eriochrome Black T in 20 ml of absolute alcohol. Use about 3 to 4 drops of the indicator solution.
  2. 1 g of the indicator is ground with 100 g of NaCl to a very fine powder. This mixture is stored in a well-stoppered bottle. Use about 0.2 g of the indicator in each titration.
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  • $\begingroup$ Hey @AChem , I was told to simply dilute the Kale sample, and account for it in my calculations. Do you think that makes sense? $\endgroup$
    – user146547
    Commented Jun 2 at 4:42
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    $\begingroup$ @Jermyyyy, No your original procedure is problematic. The organic material from kale extract should be destroyed by oxidation before titration. The pH has to be adjusted and Eriochrome B T must be prepared accordingly. Talk to your teacher about it. $\endgroup$
    – ACR
    Commented Jun 2 at 5:11
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I've been trying to do the same experiment for spinach and its calcium content determination upon boiling at different temperatures. My solution upon titration is not turning the colour red at all in fact it turns red the moment I add the EBT indicator way before the actual titration (I'm using EDTA solution as the titrant and the titre is CaCl2 solution)

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  • $\begingroup$ At doubts, It is good to perform a parallel procedures with a pure Ca salt solution and pure water. Organic acids from the plant matrix may interfere. $\endgroup$
    – Poutnik
    Commented Sep 27 at 10:14

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