how to maintain ionic strength?

WS novalidaddress at
Wed Sep 20 15:00:56 EST 2006

Dear Experts,

I would like to demonstrate that a certain effect is due to the
presence of a certain salt (X++(Y-)2, X is a metal, Y is a monovalent
anion) in an enzymatic reaction.
Of course, I simply might leave it out, but it is there in a fairly
high concentration (lets assume 50mM), so one might say the effect was
due to the ionic strength, i.e. a salt effect.

So, I am thinking of replacing XY2 simply by NaCl, as the latter
already is present in the mix. The replacement cation should be from a
different group of the periodical table (alkali is preferred, so sodium
would be the first choice).

Assuming 100% dissociation, there are 150mM of ions present when 50mM
of XY2 are dissolved. Neglecting the fact that the ratio of positive
and negative charges is distorted, I just might use 75mM NaCl instead.
Of course, I also might run 2 controls: 50mM NaCl (in respect to X) and
100mM NaCl (in respect to 2Y).
Another approach would be to demonstrate first that 50mM XZ (Z is a
bivalent anion) has the same effect as XY2 and then replace XZ by NaCl.
But then someone might object that Z also contributes to the effect.

Are there any generally accepted solutions?


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