# Ions in water - what does ppM actually mean?

bitman
"A Copper test is performed much as you would test for chlorine residue. A level of copper in the range of 0.15 through 0.20 ppM"

The above is an extract from a page on swimming pool ionisers. The ionisers produce copper ions for water cleaning purposes.

0.2ppM (parts per Million) - I don't understand how you can quantify copper ions in this way. Is this a ratio by weight? or does it man 1 atom of ionised copper per 5 million water atoms.

Any thoughts on the actual meaning of this appreciated.

What I'm really looking for is an idea of how much copper I need to electrolyse to prevent algae growth.

I am attempting to construct a simple ioniser for a small decorative fountain we have. Currently we use chlorine tablets but that kills the surrounding plants which get splashed - and more importantly displeases 'she who must be obeyed'.

Bitman

Homework Helper
IIRC from high school chemistry, using Avogadro's number you can convert particle numbers into moles. Then you can turn the ppM quantities into molarities.

lewdtenant
I would imagine that this is based on weight (mass). Rarely have I seen it based on number of atoms.

Then, 0.2 g Copper per (10^6) g H2O = (0.2/1000000)= 2*(10^-7) Wt %

MW Cu = 63.5 g/mol
MW H2O = 18

Then, .0000002 * (18/63.5) = 5.67*(10^-8) Mol %

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bitman
The question is not as trivial as it might at first seem. It could be by volume (tricky for gases with changing pressures), by mass, by atoms as I suggested earlier. As any of these will yield different results it seems a pretty arbitrary thing.

Bitman

lewdtenant
Indeed it can be arbitrary, but usually for liquids it is on a mass basis, and for gases it is on a volumetric basis. Personally, I would go with this assumption first because it seems the most reasonable.