# Re: Hardness

```>Date: Thu, 25 Jun 1998 15:24:29 EDT
>From: FKing46 at aol_com
>Subject: Re: Hardness
>
>For our purposes, we use the terms "mg/l" and "PPM" interchangeably. But in
>absolute terms, this is not true, is it?

You are correct, it is not true in general.  When dealing with solutions,
ppm is typically calculated by mass, so the values of mg/L and ppm are
equivalent iff[*] the density of the solution in question is exactly 1
g/mL.  Since the density of aquarium water is sufficiently close to 1 g/mL,
using them interchangeably in this context is usually "safe".  Especially
given the precision of most test kits.

>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.

Hmm, never heard of such a thing.

>And finally, we toss about the terms mg/l, PPM and Metric system as if they
>were all consistent.  One would think that mg/l would be a Metric system
>measure but isn't "parts per million" rather system independent?

Yeah, I suppose you could say that.  Strictly speaking, a quantity
expressed as ppm is dimensionless (eg. mass/mass).  Parts per million and
parts per billion, etc., are analogous to percent, where percent is just a
special name for parts per hundred.

>Just a semantic nit, really.

And for the truly nit picking types among us, according to IUPAC
recommendations the preferred symbol for the liter is 'L' rather than 'l'. :-)

[*] iff == if and only if

--Bill
```