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Ppm Interpretation?

Discussion in 'Rotala Butterfly Support & Feedback Forum' started by fkoki, Apr 30, 2020.

  1. fkoki

    fkoki New Member

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    Hi everyone, first time posting here looking for some clarification on the ppm measurement unit.

    I was doing some calculations on my own trying to figure out if the calculator is actually outputting correct numbers and my own results kept disagreeing with the calculator's. I think the problem may be that I'm misinterpreting the ppm measurement unit. I'm a third year student of biology but I've never encountered ppm being used for anything other than gasses in air (eg. CO2). As far as I can tell ppm is a pretty loosely defined unit of measurement so could anyone tell me how the calculator defines it? Is it weight over weight, weight over volume, weight over moles, moles over moles etc.? (cant write the abbreviations because it thinks I'm submitting a URL :p)
     
  2. Koen

    Koen New Member

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    Hi fkoki, the ppm measuring unit in solutions refers to weight over volume, and is defined as mg/L.
    I hope this will help you to verify your numbers with the calculator.
     
  3. Allwissend

    Allwissend Article Editor
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    I was just thinking about writing a short article on this today so here goes the draft :)

    For most people who didn't study in the US PPM might be a new way of expressing concentration of nutrients in the aquarium. I know it was for me when I started out reading about nutrients. Probably because the US doesn't use the decimal-based metric system, it lacks quick ways of converting. This has resulted in popular use of % ppt ( parts per thousand) ppm (parts per million) ppb ( parts per billion). And unfortunately PPM, is not a unit anymore than % is a unit. PPM is a dimensionless quantity.

    In the strictest sense ppm is a ratio of two quantities of the same unit.

    How is PPM used in aquarium keeping ?
    Short answer, it generally means mg/L. And as you may notice they are actually a unit for mass divided by a unit for volume... so you need not wonder why this use of ppm is generally discouraged in scientific publications.

    So how did they arrive at '1ppm=1mg/L' ?
    Given that most solutions are largely water (big assumption) and that the density of water is ~1 kg/L at most common temperatures (an approximation) you get the following:

    1 mg/L = 1 mg / 1 L ~= 1 mg / (1 L*1 kg/L) = 1 mg / 1 kg = 1 mg / 1000000 mg = 1 ppm by mass

    And so, with an assumption and an approximation we ended up using ppm as a de facto unit of mass concentration in aquarium keeping. I would encourage you to write mg/L instead of ppm for clarity.

    For scientific papers most require a molar concentration which would add a further complication for a lot of people in the hobby. Hope this clarifies the issue for you :)
     
  4. fkoki

    fkoki New Member

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    Thank you both, that really cleared it up! A combination of me not knowing what ppm means and forgetting to take into account that not all of a compound is nitrates/phosphates etc... (I haven't had a chemistry related subject in a while:confused:).

    Allwissend, I'd agree that mass concentration is a good compromise between ppm and molar concentration. Ultimately you're gonna dose the same amount whichever you use with the added benefit that you're not confusing us non-americans XD.
     
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