From: David G. Rhodes <rhodes@model.phr.utexas.edu>
  To  : rasmb@bbri.eri.harvard.edu
  Date: Mon, 24 Jul 1995 10:48:33 -0600

(Fwd) ionic strength

CROSS-POSTED FROM BIOPHYS.
*********************************
((( Part of posting questions to the net is some combination of
- having the confidence that the information is not readily available
- having no pride and asking stupid questions
- being enough of a slacker that you don't want to dig all day for the answer
With that as a prelude, let me say that if I'm overlooking something painfully
obvious that I should know better than to ask, please excuse the question...)))


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.



--- End of forwarded mail from <rhodes> ("David G. Rhodes")


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