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**To**:**aquatic-plants at actwin_com****Subject**:**Re: Hardness in ppm****From**:**Paul Sears <psears at nrn1_NRCan.gc.ca>**- Date: Thu, 9 Jul 1998 10:25:30 -0400 (EDT)

Steve P. wrote- > > I propose that we stop using units of hardness to refer to concentration > of magnesium and calcium. The appropriate units for those things are in > ppm or mg/L of Ca++ or Mg++. The usage of mg/L of CaCO3 is somewhat > useful because it allows one to use direct weights of the most commonly > used calcium salt however, test kits should give one and only one result > and that is the concentration of calcium ions (one test) and the > concentration of magnesium ions (another test). Unfortunately, the tests we have don't work like that. We actually measure Ca++ and Mg++ with one test, getting a total molarity, and then measure one of the two alone (Ca++, I think). The other we then get by difference. The molarities can then be converted to ppm results for the individual species, but in practice it's simpler to stop at the "equivalent CaCO3" stage, so we tend to do so. The tests measure the concentrations in terms of numbers of ions per litre (that is what molarities are). If you have (say) apples and grapes and can count the two together, or apples alone, you can easily get the number of grapes by difference. If you want the total _weights_ of each, and can only measure by counting (but know the weights of individual apples and grapes), you must do the subtraction to get the numbers of each _before_ you try to calculate weights. The calculations are not at all difficult, but they must be done. > > For measuring alkalinity, I propose that we should use units of > equivalent CO3-- however there may be a more precise scientific measure. The obvious one to use would be the one actually there: HCO3-. You won't like the next bit though - the obvious _unit_ to use is millimolarity. This goes for Mg++ and Ca++ as well. The good bit about using GH, KH, and ppm CaCO3 equivalent is that they are _all_ directly related to molarities (numbers of ions in solution), and that is what our tests measure. ppm Ca++, ppm Mg++ are derived measures. They are not difficult to get - simply multilply by the atomic ("ionic") weight, but there is no avoiding doing the calculations, as long as some of our tests measure two or more things together. > The issue is NOT: > > "are we analytically-enlightened individuals able to apply conversion > factors?" If you want Mg++ and Ca++ in ppm, you will have to do _some_ calculations after using your test kits. -- Paul Sears Ottawa, Canada

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