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Thread: PPMs

  1. #1
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    PPMs

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    Hi all,

    just wondering when we talk about ppm (eg, 10~20ppm for KNO3, etc), are we refering to the entire tank's volumn or just the part after water change...because if its the latter, how would we know how much is left in the tank (after water change) to determine the desired ppm level?

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    We are referring to the entire tank's volume of water

  3. #3
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    ppm stands for part per million and generally refers to mg/L.

    If you have 100 ppm Nitrate and you change 25% of the water and replace it with 0 ppm Nitrate water you will have 75 ppm Nitrate in your tank. [The math: ppm_intank - (%water change x ppm_intank) = new ppm]

    Now for something I posted on The All Wet Thumb Forums a while back that may be of interest:
    1 ppm NH3 equates to 0.058/1000 mol* NH3 (Mr = 17 g/mol)
    1 ppm NO2- equates to 0.022/1000 mol NO2- (Mr = 46 g/mol)
    1 ppm NO3- equates to 0.016/1000 mol NO3- (Mr = 62 g/mol)

    Now for the real nasty math...

    ...so 1 ppm NH3 will yield 0.058/1000 mol of NO2- which equartes to 2.7 ppm (0.058/1000 mol x 46 g/mol) which equates to 3.6 ppm NO3- (0.058/1000 mol x 62 g/mol). So NO2- is 2.7x heavier than NH3 and NO3- is 3.6x heavier than NH3. NO3- is 1.4x heavier than NO2-.

    35 ppm of NO3- you give 35 ppm x (17 g/mol/62 g/mol) = 9.7 ppm NH3 or 35 x 46/62 = 26 ppm NO2-. This is far more than the 5 ppm you have found so far... Where did the remainder go? N2 perhaps?

    tt4n

    * ppm is mg/L. Molecular concentrations are normally worked out in g/L so the amounts here are in mol/1000 as 1 g is 1000x more than 1 mg. 1 mol of any substance contains 6.02 x 10^23 molecules. The mass of 1 mol of any substance is called its molecular mass (Mr). The amount of mol = mass in gram divided by molecular mass (N = g/Mr)

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    Hi Selwyn,

    Long time no see. It depends on what you are trying to measure and your objective.

    For measuring the nutrient content of the tank, obviously, the water sample from the tank is what you would consider. That's you are measuring the complete tank's water. I doubt this is what you are after.

    For fertilizing, I guess that's your question, it depends on the frequency of your dosage. If you dose once/twice a week and have CO2 plus modreate to high light, you should consider the whole tank water. This is because under this conditions, most nutrients would have been exhausted. If you dose 2~3 times a week, target only the water you changed.

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    Thanks guys!!

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