Ionic Strength?

Tom Anderson via (by ucgatan from
Wed Jan 16 15:53:42 EST 2008

On Wed, 16 Jan 2008, Dr Engelbert Buxbaum wrote:

> Am 02.01.2008, 05:03 Uhr, schrieb <M.Akramraza from>:
> > I need a such programme which help me to calculate ionic strength of a
> > solution
> You can easily do that by hand, simply add the concentration of all the
> components.

The concentration multiplied by half the square of their charge, i think
you meant to say.

> Note that we need the concentration of particles, that is, 1 M
> NaCl is 2 osm (1 osm Na+ and 1 osm Cl-).

Osmoles? Hang on, are we calculating ionic strength or osmolarity here?

Whichever, it's not quite as simple as just adding things up if there are
weak acids and bases involved. If you know the pH of the solution, you
first have to calculate the fraction of ionisation of any such species
(and if they're polyprotic or polybasic, the fraction of each of the
ions), so that you have the actual concentrations (including of H+/OH-!)
to do the adding up. If you don't know the pH, then you first need to
calculate that from the concentrations and pKas of the ingredients, and
that can be a real pain. Especially since the effective pKa varies
according to the ionic strength ... Simpler just to make the solution up,
measure the pH, and go from there!

Hmm. I know you can measure osmolarity directly, but is there an
instrument which measures ionic strength?


Tom Anderson, MRC Laboratory for Molecular Cell Biology, UCL, London WC1E 6BT
(t) +44 (20) 76797264   (f) +44 (20) 76797805   (e) thomas.anderson from

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