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Reaction of K2CO3 with KH2PO4

  1. Jun 1, 2014 #1

    cmb

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    Could someone offer a pointer to whether I have the right thinking on this, please?

    The way I see it, the K2CO3 would lead to bicarbonate ions, HCO3-, and hydroxides, OH-, upon water dissociation.

    Now when those bicarb ions meet the H2PO4- and HPO4-- phosphate ions, I presume you could see an oxidation of the bicarbonate ions and a release of CO2?
    ( HCO3- + H2PO4- -> HPO4-- + H2O + CO2 )

    The only thing is that with CO2 in solution, would it not re-react with the other K2CO3 and reach an equilibrium, per K2CO3 + CO2 + H2O -> 2KHCO3 ?

    This would suggest K2CO3 + KH2PO4 <-> KHCO3 + K2HPO4?

    Thing is, there's something in the back of my mind that makes me think not all of the CO2 would be reabsorbed by the K2CO3, so what we'd see is some escaping with a quite slow bubbling away of the CO2 that doesn't end up back in a bicarbonate. So over a non-rapid timescale, we'd see all of the CO3-- drift away, but quite slowly, and we could end up with;
    K2CO3 + KH2PO4 -> K2HPO4 + KOH + CO2 (gas)

    ( and also that K2HPO4 + KOH <-> K3PO4 + H2O ? )
     
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  3. Jun 1, 2014 #2

    Borek

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    No oxidation here.

    Actually this is a rather complicated equilibrium, and the final result depends on the mixture composition.

    If there is enough KH2PO4 you will end with no carbonate at all, as CO2 will leave the solution.
     
  4. Jun 1, 2014 #3

    cmb

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    The oxidation state of the carbonate goes from 1- (HCO3-) to 0 (CO2). Is that not reduction?

    Anyhow, whatever the semantics of the chemistry terms (happy to be enlightened, but unimportant in the question) what are you saying is needed to be known in regards the mixture composition to determine the final state of the solution mixture?

    Let's say I start with a 5M solution of K2CO3 and I add a teeny few grains of KH2PO4. Is CO2 liberated, and if so what is the reaction?

    Flip side - if I have a pile of KH2PO4 in a beaker and I start dripping 5M K2CO3 soln on top of it, what is the equation then?
     
    Last edited: Jun 1, 2014
  5. Jun 1, 2014 #4

    Borek

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    Carbonate doesn't have oxidation state. Oxidation state is a property of an atom, not of a molecule (or a composite ion).

    As I said earlier, this is way too complex to be answered easily. You have a mixture of two weak acids, one is diprotic, second is triprotic. What happens depends on their amounts and strengths (plus, to some extent, on partial pressure of carbon dioxide in the air). On some very general level: first case - no observable reaction (doesn't mean nothing happens), second case - evolving carbon dioxide, producing - at least initially - K2HPO4.
     
  6. Jun 1, 2014 #5

    cmb

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    I think we understand the context of a carbonate atom being 'affected' (I don't know what term) to become CO2 so the semantics seem not relevant.

    At the end of the day I'm just trying to make a pH 7 potassium buffer with KH2PO4 if I don't have K2HPO4. I'm going to add K2CO3 to KH2PO4 until it hits ph 7. I'm really only interested in knowing if there will still be CO3-- swimming around in there once I do that. My feeling is that there won't be because the CO2 will come out, albeit slowly, as the bicarbonate ions meet protonated (either 1 or 2) phosphate ions. It'd be good to understand that, but if it is so complicated it is beyond reasonable analysis, then there it is.
     
  7. Jun 1, 2014 #6

    Borek

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    "I know you have to put a liquid into car to ride, what does it matter if we call it water or gasoline?".

    Doable, but see below.

    Yes, there will be some CO32- in the solution.

    But you may have problems related to the instability of the solution - IMHO you risk losing CO2 in which case pH will be drifting up. Shouldn't matter on a short time scale, perhaps it won't matter at all. Difficult to say more without knowing concentrations.

    It will be much better to use KOH. Or NaOH, unless you need to have only potassium in the solution.
     
  8. Jun 1, 2014 #7

    cmb

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    Well, I was presuming ions can be oxidised and reduced like anything else can. The IUPAC include, within their definition of oxidation, "the net removal of one or more electrons" from an ion.

    How does a -ve ion become a neutral gas molecule if there is no net removal of electrons? Net electrons before = 1, net electrons after = 0?

    There's also an example on wiki that looks similar in nature to the bicarbonate example I gave above;

    http://en.wikipedia.org/wiki/Redox

    I'm sorry if I was unclear on the subtlety. I guess I still am. I don't think suggesting my confusion in this regard is so far off the mark in the manner of your suggestion of confusing water for petrol, but I am quite sure I am not familiar with the various ways the various redox terms are used in chemistry.

    Back on topic, yes, exactly, that was what I was thinking and the thrust of the question. If we still have CO3-- hanging around in solution, I can imagine that will outgas CO2 as it bumps into hydrogen phosphate ions, but that'll take a while. As long as they all go, eventually, it does not present a hurdle - simply ensure the molar contributions of K and PO4, overall, match the ratio required for the buffer. I'd guess it'd be measured as a little too acidic to start, and then over time pH will rise a little and stabilise. Would you agree?
     
  9. Jun 1, 2014 #8

    Borek

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    Ions co be oxidized and reduced, but in this case there is no oxidation nor reduction. None of the reactions happening requires charge transfer, in none of these reactions oxidation state of any atom changes.

    You don't have a -ve ion becoming a neutral gas molecule. CO32- and CO2 don't differ just by -2e.

    In the nitrate example nitrogen atom before reaction has oxidation state of +5, and 0 after the reaction, This is a radically different situation, carbon is +4 before and after the reaction.

    CO32- presence is not a problem. Problem is, in pH 7 solution substantial part of carbonate (around 15%) is in the form of carbonic acid, which is equivalent of just dissolved carbon dioxide. CO32- is quite stable, dissolved carbon dioxide will leave the solution as long as it is not in equilibrium with atmospheric CO2.
     
  10. Jun 2, 2014 #9

    cmb

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    OK, I understand what you're saying, but for my clarification, what term, if any, should/could be used in regards what happens to a carbonate ion when its oxygen atom is removed to form CO2? Is it the 'deoxygenation' of a carbonate ion?
     
  11. Jun 2, 2014 #10

    Borek

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    If anything, it is protonation, followed by a simple decomposition:

    CO32- + 2H+ -> H2CO3 -> H2O + CO2
     
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