After thoroughly searching and asking senior researchers, nobody has been able to provide an answer for this. I am performing in vitro digestion to study iron dialyzability following Kapsokefalou and Miller 1991 work, in which PIPES and HEPES are used as buffers. Concentrations values are provided in normality, and I do not know what is the equivalence in molarity, as they are big molecules with several groups that can play a role in the buffering capacity.
It may also be of help to know that PIPES is a disodium salt, and HEPES a monosodium salt.
Thanks in advance
Hi Rodrigo, the theoretical definition of normality is equivalent concentration, which means the molar concentration (molarity) multiplied by the equivalent factor. Now, it is true that the definition of equivalent factor is very ambiguous and reaction dependent. For acid/base reactivity, which is what you are talking about when you mention a buffer (capacity to absorb pH variations produced by release of small or moderate amounts of protons or hydroxide ions by whatever substance is added to the buffer), normality would be the molarity multiplied by the number of protons or hydroxide ions that can be released by the acid or base, respectively, you are working with only if the reaction is complete. If you have any pKd value the normality will have to be calculated in a more complex way, taking into account the dissociation constant.
However I don't think in your case one should be so complicated, I bet the authors you cite were not considering all this... I guess only chemists are picky with these things... One can imagine that the sulfonic acids in the presence of a strong base will dissociate completely (or vice-versa with the sulfonates in the presence of a strong acid), so I would just go for the simple conversion. In practice you answered yourself in the last sentence of your question: HEPES molarity is equivalent to its normality, while PIPES molarity is half than its normality.
In simple words, it would mean, for example, that the same buffering effect given by a 20 mM HEPES solution you can obtain with a 10 mM PIPES solution, because both are 20 mN.
It might be worth considering that PIPES is slightly more acidic (as sulfonic acid) than HEPES (pKa 6.8 vs 7.5), so the buffering ranges are slightly shifted. Also, HEPES is dissociating less at lower temperatures, so it buffers better.
Hi Rodrigo, the theoretical definition of normality is equivalent concentration, which means the molar concentration (molarity) multiplied by the equivalent factor. Now, it is true that the definition of equivalent factor is very ambiguous and reaction dependent. For acid/base reactivity, which is what you are talking about when you mention a buffer (capacity to absorb pH variations produced by release of small or moderate amounts of protons or hydroxide ions by whatever substance is added to the buffer), normality would be the molarity multiplied by the number of protons or hydroxide ions that can be released by the acid or base, respectively, you are working with only if the reaction is complete. If you have any pKd value the normality will have to be calculated in a more complex way, taking into account the dissociation constant.
However I don't think in your case one should be so complicated, I bet the authors you cite were not considering all this... I guess only chemists are picky with these things... One can imagine that the sulfonic acids in the presence of a strong base will dissociate completely (or vice-versa with the sulfonates in the presence of a strong acid), so I would just go for the simple conversion. In practice you answered yourself in the last sentence of your question: HEPES molarity is equivalent to its normality, while PIPES molarity is half than its normality.
In simple words, it would mean, for example, that the same buffering effect given by a 20 mM HEPES solution you can obtain with a 10 mM PIPES solution, because both are 20 mN.
It might be worth considering that PIPES is slightly more acidic (as sulfonic acid) than HEPES (pKa 6.8 vs 7.5), so the buffering ranges are slightly shifted. Also, HEPES is dissociating less at lower temperatures, so it buffers better.
Normality is process dependent. So 1M H2SO4 is 2N if we're referring to it donating 2 mol of H+, but it is considered 1N if we're talking about the involvement of SO42- since there is only 1 mol of it. Since, we're referring to the ability of these buffering agents to exchange H+, as Pietro stated "HEPES molarity is equivalent to its normality, while PIPES molarity is half than its normality".
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