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Methylene Blue's equivalent weight

  1. May 1, 2013 #1
    I have to prepare a Methylene Blue solution 1ml = 0.01 meq, or 0.01 N, from MB powder. To know the amount of powder required, I need to know MB's equivalent weight, or equivalent number, but haven't been able to find it so far.
    Does anyone know?
    Help appreciated
  2. jcsd
  3. May 1, 2013 #2
    a little help

    Methylene blue is 319.85 g/mol for the dry, non hydrated green powder
    from there you can easily calculate the grams per liter for a 0.01M solution.
    As far as a 0.01N solution you will need to research the number of ion carriers.
    However , my guess is that a "Normal" solution is not meaningful for methylene blue.
  4. May 1, 2013 #3
    Thanks for the reply.

    Do you know where could I find that number of ion carriers? I've looked on the web, but haven't found anything so far.
  5. May 1, 2013 #4
    Equivalents may refer to different phenomena. For instance in acid/base titrations an equivalent is the amount of substance that gives 1 mol of protons. So in a monoprotic acid the terms equivalent and normality the same as mols and molarity. For polyprotic acids such as sulfuric acid, 1 mol of sulfuric acid can give 2 moles of protons. Therefore for sulfuric acid 1equivalent = 0.5mol of sulfuric acid because 1 mol of sulfuric acid will yield, when fully deprotonated, 2 mols of protons.

    This idea is also extended to redox reactions in much the same way as above except instead of protons you think in terms of mols of electrons. So an equivalent of quinone (for instance, off the top of my head) would be 0.5mol because quinone will accept 2 electrons in a redox reaction. Or something like Fe2+ or Fe3+ going to Fe(s) can be treated accordingly.

    Basically you should be able to figure out an equivalent weight given the Molar Mass and a balanced reaction.
  6. May 1, 2013 #5
    Methylene blue will be used to determine the methylene blue index of pet coke, by its addition to a solution of pet coke acidified with sulfuric acid.
    So the reaction is between MB and sulfuric acid.
    Can I say then that the equivalent weight is Molar Mass/2 (since the acid gives 2 protons) ?
  7. May 1, 2013 #6
    I'm not familiar with what you are trying to do and it is not clear whether you are doing a redox titration or an acid/base titration. Regardless what I wrote above is how equivalents are defined and how people work with them, it simplifies language but makes the Math's slightly more difficult.

    Remember equivalent is the amount of substance which will accept/donate 1mol of protons/electrons. Normality is just Eq/L, completely analogous to moles and molarity.

    You must therefore start with a balanced chemical equation for the reaction which you will be studying. Try doing that first and perhaps I can then help, otherwise you should wait for someone more knowledgeable about these things to answer.
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