Ions in water - what does ppM actually mean?

  • Context: High School 
  • Thread starter Thread starter bitman
  • Start date Start date
  • Tags Tags
    Ions Mean Water
Click For Summary

Discussion Overview

The discussion centers around the meaning of ppM (parts per million) in the context of measuring copper ions in water, particularly for applications in swimming pool ionisers and decorative fountains. Participants explore the implications of this measurement in terms of weight, volume, and atomic ratios.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant questions whether ppM represents a ratio by weight or the number of atoms of ionised copper compared to water atoms.
  • Another participant suggests using Avogadro's number to convert particle numbers into moles and then into molarities, indicating a mathematical approach to understanding ppM.
  • A different participant proposes that ppM is based on weight (mass), providing a calculation to illustrate this perspective.
  • One participant notes that the definition of ppM could vary depending on whether it is measured by volume, mass, or atoms, suggesting that this variability complicates the understanding of the measurement.
  • Another participant agrees that while it can be arbitrary, ppM for liquids is typically based on mass, which they consider a reasonable assumption.

Areas of Agreement / Disagreement

Participants express differing views on the definition of ppM, with some supporting the idea that it is based on mass while others acknowledge the potential for variability in measurement methods. No consensus is reached on a definitive interpretation.

Contextual Notes

The discussion highlights the complexity of defining ppM, with participants noting that different measurement bases (mass, volume, atomic) can yield varying results. There are also unresolved mathematical steps in the calculations presented.

Who May Find This Useful

Individuals interested in water chemistry, particularly in the context of pool maintenance or decorative water features, may find this discussion relevant.

bitman
Messages
15
Reaction score
0
"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
 
Physics news on Phys.org
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.
 
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 %
 
Last edited:
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
 
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.