# Re: measures

```> From: Adele and Davis Gailitis <adele_davis at yahoo_com>
> Subject: Measures
>
> Ok so call me a little slow BUT........what the !\$@!
> is meq/l ???????????? this has been used extensivly in
> the thread: "Equilibrium, Ca and Mg levels"

That's because the chemists became involved!  :)
When I started keeping aquaria, I was asking myself what the !\$@! are
various other units (KH, GH)?  Working it all out was not helped by the
errors in the books.  I finally found out what a degree KH was by working
backwards from a KH/pH/CO2 table and the equilibrium constant for the
first dissociation of carbonic acid.
An "equivalent" of an ion is the ionic weight of the ion in grams
divided by its charge.  Strictly we should call it gram-equivalent, but
the "gram" usually gets dropped.  So, for calcium (ionic weight 40 and
two charges) the equivalent weight is 20 grams.  That means 20 milligrams
of Ca++ in one litre is a 1 meq/L solution.
You might ask why one would do it this way.  The point is that
a 1 meq/L solution of any ion has the same total electric charge, and
if the ions in question have the same charge number, the same number
of ions per unit volume.  You are using this unit already.  A pH is
minus the log of a hydrogen ion concentration in equivalents per litre.
A pH 7 solution has 10^-7 eqivalents/L of hydrogen ions.
Calculations for equilibria for ions in solution are easy if you use
this type of unit (or the related "molar"), and horrible if you use
ppm or just about anything else.

> Dazed and Confused

That's because of the "GH/KH" stuff.  I'm glad to hear that
Greg Morin puts both units on the labels now.

--