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Why HCN not KCN kills?

  1. Oct 8, 2012 #1
    Why HCN not KCN kills???

    I have read in wikipedia's article for Potassium cyanide and in there it is written that
    "
    The lethal dose for potassium cyanide is 200-300 mg.[6] The toxicity of potassium cyanide when ingested depends on the acidity of the stomach, because it must react with an acid to become hydrogen cyanide, the deadly form of cyanide. Grigori Rasputin may have survived a potassium cyanide poisoning because his stomach acidity was unusually low.[7]
    ". http://en.wikipedia.org/wiki/Potassium_cyanide
    My question is Why HCN not KCN kills???? Untill now it had been known to me that it is cyanide ion which kills...
     
  2. jcsd
  3. Oct 8, 2012 #2

    Borek

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    Re: Why HCN not KCN kills???

    Don't believe in everything you find in wikipedia. No idea what is the real mechanism of the cyanide poisoning, but whoever wrote the wikipedia article was wrong. At blood pH most of the cyaninde (around 99%) is already protonated. In stomach pH is much lower, so amount of protonated cyanide is even higher.

    Besides, if the pH in Rasputin stomach was "unusually low" it would mean amount of protonated cyanide would be higher, not lower.
     
  4. Oct 8, 2012 #3
    Re: Why HCN not KCN kills???

    why acidic medium be there for protonation???
     
  5. Oct 8, 2012 #4

    Borek

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    Re: Why HCN not KCN kills???

    Where "there"?
     
  6. Oct 8, 2012 #5

    epenguin

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    Re: Why HCN not KCN kills???

    Partly memory, have not info to hand, but certainly the mechanism of cyanide poisoning is that cyanide forms a practically irreversible bond with the terminal oxidising catalyst ('electron acceptor') in the cell called cytochrome a3. That is, oxygen via this catalyst oxidises other cytochromes and things which in a chain of one thing oxidising another oxidise glucose etc. and give us or energy. The cytochromes contain iron-heme groups similar to those in haemoglobin, and it is alternately reduced by cellular electron donors and oxidised by oxygen . The cyanide forms a bond with the iron, I presume the >Fe3+< . It is CN- that reacts. (It may be a minority of total cyanide but it is in rapid equilibrium with the HCN and reacts rapidly and irreversibly.)

    So the effect is that it shuts down practically all the metabolism of the cell.
     
    Last edited: Oct 9, 2012
  7. Oct 9, 2012 #6
    Re: Why HCN not KCN kills???

    I am not sure why the moderators aren't posting my reply. Maybe speculating about Rasputin is against Forum policy. So here goes a shorter reply.
    HCN is less topic than KCN in the stomach because HCN is a gas. The stomach isn't made to absorb gas. However, a salt like KCN is easily absorbed.
    If Mr. R had an acid stomach that day because of something that he ate, the KCN could have turned into HCN. Then, the HCN would have been dissipated before his blood stream could absorb it.
    I am not saying that I believe that theory. I was just discussing the chemistry of that hypothesis.
     
  8. Oct 10, 2012 #7

    Borek

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    Re: Why HCN not KCN kills???

    Sorry, but this is a nonsense that I have already addressed in the very first answer in the thread.

    pKa of HCN is around 9.3. That means already at pH 7.3 - which is more or less pH of the blood - CN- is protonated in 99% (or - in other words - 99% of the cyanide is in the HCN form). Note that that means cyanides in the blood stream are transported mainly in this form, not as CN-.

    pH in stomach is much lower than that - around 2. That means 99.99999% protonation. You can't get substantially higher than that because of the low stomach pH.

    In short: we can safely assume in the stomach ALL cyanide is protonated, regardless of whether someone has an acid stomach or not. This is a high school chemistry.
     
  9. Oct 10, 2012 #8
    Re: Why HCN not KCN kills???

    Thank you. I didn't think to look up the pKa of HCN. In my defense, the article in that link the OP quoted didn't bother to look it up, either. The link says that the acidity would effect the chemical species of the cyanide, and according to this "high school" analysis this couldn't be true.
    I think that answers the OP's question. The pH of the stomach couldn't affect the absorption of cyanide, one way or the other. Therefore, that Wikipedia article and the speculation behind it is wrong.
     
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