# Re: Metric units

```Hi Forrest,

You say (snip) > In aligning with the scientific community, we should
all present our > data in terms of meq  (Meq?). This makes calculations
ever so much 		> simpler.

I agree with the first part of your statement, that we should be
"aligning" or "conforming" with the world scientific community. There
ARE some arguments for the traditional avoirdupois units, but that does
not apply to water hardness. The "degrees" of hardness are just an
archaic METRIC system, if one defines *metric* as using grams, liters,
etc. The German "degree" of hardness is defined as the hardness cause by
10 mg of calcium oxide (CaO) in 1 liter of water. Everybody knows, that
CaO will instantly *react* with water transforming into another
compound, but never mind, that's how water hardness is measured in odH.

The German chemists were not completely crazy in selecting CaO, but you
don't really want to hear about it!

You must have never used milliequivalents (meq) in general calculations,
if you recommend them. (Meq = megaequivalent). They are extremely useful
to describe concentrations of *known* species in *known* reactions.
There they can not be beat!

But what is the equivalent weight of manganese, or of iron, or of
oxygen-bound chlorine? That question can not be answered, unless a
particular formula/reaction is specified. How many aquarium keepers, or
chemists for that matter, know that with certainty?

> 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

You are both right and wrong on that count! You are right, that 1 atom
of X in 1 000 000 atoms of Y is 1 ppm, but not necessarily is equal to 1
mg X per 1 Liter Y. However, in WATER solution, water at room
temperature having a density darn near 1.0, the difference usually is
below the resolution of analytical methods. So, one can safely say, that
in *water* 1 mg/L = 1 ppm.  (By the way, it is always ppm, not PPM.)

> Also, isn't there some difference of opinion between some countries as to which is larger - million or billion?  This may lead to misunderstanding when using PPM.

No, there is not. The difference is that in Britain, Germany, etc  one
billion is 1 000 000 000 000. Our "billion", 1 000 000 000, is called a
"milliard". But the Brits are changing, albeit slowly. Their scientists