Chemical Forums
Chemistry Forums for Students => Undergraduate General Chemistry Forum => Topic started by: grast_girl on July 17, 2008, 07:39:12 AM
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I know this might seem like a simple question, but I've spent a couple of days trying to get an answer that will satisfy my thesis examiner.
I have extracted some plant cell wall material in a series of solutions of KOH with increasing concentrations (0.5M, 1.0M and 4.0M), the resulting residue was then extracted in 4.0M NaOH. I measured the concentration of certain alkali-releasable compounds in my extracts and found that although the residue had already been extracted in 4.0M KOH the 4.0M NaOH released some as well. Previously I had assumed NaOH and KOH were interchangeable strong bases, but it appears not, so now I'm assuming NaOH is a stronger base. I have a reference that says both NaOH and KOH are "strong" bases, ie pKb less than zero, but I've also seen the Chem buddy thing that says NaOH (pKb=0.2) is stronger than KOH (pKb=0.5). I was hoping someone might be able to give me a more reliable reference that states this, as NaOH being stronger would seem to explain my results nicely.
If anyone can think of a better reason, shoot.
Thanks
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Sodium is less electronegative than Potassium, so NaOH is more willing to release the hydroxy group and is the stronger base.
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Difference in base strength is small, I doubt its importance here. I would rather think in terms of possible additional effects - Na+ is smaller then K+, could be solution viscosity is different and so on.
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It's a saponification reaction removing phenolic acids from polysaccharide chains, so it's possible that ion size is important, not so sure about solution viscosity, although I suppose it might have an effect because of it being a solid/liquid system.
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Perhaps the 1st extraction is not long enough to release all of the compounds. Try doing your second extraction with KOH and to see whether you observe the same effect as with NaOH.
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I'm a little puzzled, as I'd always thought it was the other way around. KOH carries out a transesterification reaction better than NaOH at the same concentration (in biodiesel manufacture).
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Mark, I'm only going by what my results give me.
Yggdrasil, I've finished all the experimental work for my thesis, so I can't do as you suggest. I might have some suitable data that could provide the answer though. Having looked at a few references where they've done the same thing, this is not an isolated observation, although none of them give any reason why it should be so.