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ionic strength (fwd)
Sender: "David G. Rhodes" <rhodes@model.phr.utexas.edu>
Subject: WSN: ionic strength
The following is a question based on one that I recently posted to
biophysics@net.bio.net. If you find the question naieve, please
consider replying anyhow. The disparity of responses suggests that
the question is not as trivial as I had once feared.
I'm looking at a system in which the interaction between two charged species is
affected by ionic strength, at least as judged by increasing the concentration
of a uni/univalent electrolyte (NaCl) in the buffer. I wish to evaluate the
possibility that an effect observed upon switching from a (sometimes) cationic
buffer to a (sometimes) zwitterionic buffer is due to difference in the ionic
strength. Therefore, I wish to calculate (even roughly) the ionic strength of
the two systems.
> Ionic strength is defined (in nearly every text on my bookshelf) in terms of
> the ionization of a simple uni/univalent or uni/multivalent ionization of
> mobile ions. The typical example calculations would be for systems like
> NaCl <===> Na+ + Cl- and CaCl2 <===> Ca++ + 2 Cl-
> That's fine for most things, BUT what if I want to calculate the ionic
> strength of a buffer solution with a Good buffer, in which the equilibrium is
> +/- <===> 0/-
> While the net charge on the HEPES zwitterion is 0, I'd guess that the stuff
> doesn't tend to dimerize, and is supplied as a Na salt. The main question is
-
> Should I count the "ends" of the HEPES molecule separately,
> treat it as neutral, or
> treat it as a divalent ion?
> By intuition (cw?) I lean towards the first choice, but have some hesitation
> based on formal definitions of ionic strength. I'd like a better reason for
> making my choice, if someone can offer it.
>
> Thanks 10^6.
--
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