August 25, 2019, 02:23:44 PM
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Topic: In a mixture of plant matter, what can I expect to react with KMnO4 and H2SO4?  (Read 304 times)

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Offline Cole

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Firstly, if this is the wrong subforum, please inform me.

I am attempting an experiment to determine the concentration of calcium oxalate contained within a sample of Alocasia Macrorrhizos. I've blended the plant, and then coarsely sieved it to produce a mildly viscous, green, opaque mixture. I'm using a redox titration to determine the CaOx conc, so I need to add sulphuric acid to the mixture. The problem is, when I perform the titration, the mixture loses its green colour and becomes white instead. My knowledge of plant biology (or any biology for that matter) is quite limited, but I think its chlorophyll, hence the loss of the green colour.

So my 2 questions are:
1. If not chlorophyll, then what could be reacting?
2. How do I go about removing chlorophyll from the solution in a way that wouldn't interfere with the CaOx, or the titration reaction?

In case its important, the reaction I am looking for is:
2MnO4- + 16H+ + 5C2O42- → 2Mn2+ + 8H2O + 10CO2

Another potentially important fact, is that the currently observed reaction occurs at room temp (~23C), and the wanted reaction would probably need a higher temp to start (both reactants are negatively charged).

Offline phth

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You can remove the organic soluble molecules by organic extraction e.g. fatty acids and chlorophyll, and what's left are the water soluble organic molecules that can react e.g. nucleotides and amino acids contain reactive functional groups e.g. alcohols and amines etc. For best results search google for alternative procedures. a month in the lab is worth a day in the library

Offline Mitch

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You should be able to crystallize out the oxalate under strong acid conditions. Try the method described in this review:

Oxalate in crop plants
J. Agric. Food Chem. 1987, 35(6), 926-938
DOI: 10.1021/jf00078a019
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Offline Cole

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Thanks for the replies.

Starting with phth's reply, by "organic extraction", do you mean something like adding ethanol and heating the solution at about 80'C? I've tried that, and the solution becomes clearer, with a dark green precipitate forming and sinking to the bottom of the tube. But on analysis of the remaining fluid, it seems like the calcium oxalate is also being removed from the solution.

Now Mitch. That doc you linked contained a substantial amount of information, but didn't really expand on any of it, and I'm having trouble researching the things it says.
"strong-acid solution (e.g hot 1 MOL HCl) leading to the dissolution of crystalline calcium oxalate" sounds promising, and it might be possible that if the crystals are better dissolved into the solution, the ethanol method might not remove it, but I'm not sure.

It also says "extraction of tissues with water", which sounds great but I can't find anything that says what this means.

Finally, it mentions the use of enzymology to determine concentration. Would this entail using oxalate oxidase enzymes as a catalyst to do the reaction:
oxalate + O2 + 2H+ --> 2CO2 + H2O2
And then record the volume of CO2 formed and then use that to determine concentration?

Edit:
Just thought of another method; use the ethanol to produce the dark green solid (which I hope contains the calcium oxalate), remove the liquid and then heat up the remaining solid until it is red hot. This should burn away most impurities and turn the calcium oxalate into calcium oxide, which I can then use to determine concentration.

Thanks for all the help so far  :)

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