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# Re: Aquatic Plants Digest V3 #344

```Forrest King writes:

> For our purposes, we use the terms "mg/l" and "PPM" interchangeably. But in
>  absolute terms, this is not true, is it?  I haven't "run the numbers" but
> one
>  would have to guess that 1 mg of nitrogen in 1 liter of lead (to use a
> rather
>  extreme and silly example) would be different than 1 atom of nitrogen in 1
>  million atoms of lead?

First, Forrest, let's clarify this, just for the record.  I don't keep my fish
or my aquatic plants in lead.

Second- 1ppm is not equal to one molecule per million molecules.  The values
are in weight, ie, 1g of solute per million grams of the solvent (in my tanks,
the solvent is water).

Now, follow this, please.  I asked a couple weeks back if my thinking on this
were incorrect, but I got no response.  By definition, 1g of distilled water
takes up 1 cc of volume.  This was the standard set when the metric system was
"contrived".  1 cc is also one ml, whether it is water, lead, or whatever your
fish happen to prefer.  so then, one mg of water is one thousandth of a cc, or
one millionth of a liter.  Since ppm is by weight, not volume or number of
molecules, then ppm and mg/l are completely equivalent for any solute that has
the same molecular weight as its solvent (again, just 'cuz most of us are
using water, permit me to now assume for argument's sake that the terms
<solvent> and <water> are interchangeable).  Of course, the majority of what
we are worried about in our water (are we clear on that choice of terms?) is
significantly heavier than water, but when you are dealing with solute levels
of less than 1,000 (American- for Europeans on the list-1.000) ppm, we are
talking less than one tenth of one percent, and in most cases less than one
hundredth of one percent.  Sure, hard water weighs more than distilled, just
like ocean water weighs more than fresh, but for hobbyist level calculations,
I think it is safe to ignore this very small levelof difference.  It is
nowwhere near the 2.8% difference between fresh and ocean water.  (why do I
suddenly feel like I'm on the kiilie list instead of the APD?)

As far as my preference for standardizing, I would go with ppm, so I can do
the calculations necessary to figure out formulas like how much of some given
acid at a given concentration will neutralize a given amount of carbonate, or
how much CaCl do I need in my PMDD to achieve the level of calcium I want in
my tank.  But if we choose mg/l, that's okay, too.  I'll just pretend that
mg/l is indeed ppm, and my calculations will be close enough for the girls I
date.  I am not even sure what I should guess a milliequivalent is supposed to
be.

Bob Dixon
```