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KNO3 (mis)measurement

Discussion in 'Aquatic Plant Fertilization' started by NanoPlantGuy, Apr 4, 2015.

  1. NanoPlantGuy

    NanoPlantGuy Subscriber

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    Wikipedia says KNO3 has a density of 2.1 g/cm^3, which is equal to 2.1 g/mL. So I weighed 5 mL of KNO3 on a digital scale expecting about 10.2 g but instead I see 5.2-5.5 g.


    I checked my scale by measuring 5 mL of water and I get 4.64 g, not exactly right but ballpark.


    What am I to make of this? Am I misinterpreting something? I need to get my measurements right so I can dose properly...
     
  2. jrneuzil

    jrneuzil Member

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    Its called Estimated Index. It does not need to be perfect. Just, well, estimate.
     
  3. Solcielo lawrencia

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    Volume is misleading because air does not count. ;)
     
  4. NanoPlantGuy

    NanoPlantGuy Subscriber

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    Jrneuzil, I'm not asking how much to dose. I'm asking is my KNO3 actually KNO3 as its not weighing what I think it should.


    SL, I do not believe its air...


    To clarify, I assume I have KNO3 and missing something simple. I would like to know what that is.


    Thanks and here are pics.
     
  5. Solcielo lawrencia

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    Did you compact it as tightly as possible so that there is no air in the volume? That's probably the problem, but I don't think it's humanly possible to compact it so that it's pure KNO3 without any air. Also, those API test tubes are not accurately marked at 5ml. All of mine are a bit less than true 5ml.
     
  6. NanoPlantGuy

    NanoPlantGuy Subscriber

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    If the weight is off by a factor of two, then that would mean almost half of the volume up to the 5 mL line would have to be air, it just doesn't look like that to me.


    Similarly, to. account for the difference in measured weight (5.5 g) and expected weight (10.2), I'd have to conclude that the 5 mL is really about 2.5 mL. That seems unlikely also.


    Combining your ideas (air volume plus underestating test tube volume), I still think we're leaving something unexplained.
     
  7. Christophe

    Christophe Subscriber
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    That density figure is for solid crystal KNO3, not for the granulated form. All bets are off once granulated, varies somewhat with humidity. Best guess from http://calc.petalphile.com is about 1.05 g/ml.
     
  8. NanoPlantGuy

    NanoPlantGuy Subscriber

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    Christophe, Interesting point. I'm by no means at all a chemistry buff. I've just been googling KNO3 and following where that leads me. So, is the granulated form granulated crystal or something else? If its just broken up crystal then the density you cite from http://calc.petalphile.com would only be due to air pockets between the granules, no?


    I'm still a bit skeptical about the 1.05 g/mL number. Where did you see that on there?


    1.05 g/mL sounds more like the density of KNO3 dissolved in water (fyi, the density of pure water is approximately 1.00 g/mL at 0 C and decreases as temperature rises). Anyway, I found a table here (http://terpconnect.umd.edu/~nsw/chbe301/chap02.pdf, table 2-77) that shows the density of KN03 in dissolved in water at various concentrations and temperatures. 1.05 g/mL comes at up at various combinations...


    Any other thoughts on why my ~5 mL of KNO3 only weighs ~5.5 g?


    Thanks!!!
     
  9. Christophe

    Christophe Subscriber
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    Nope, once it's dissolved, it pretty much fully ionizes, then we only talk of ppm of K+ or NO3-. To get that figure from the calculator site I did the 'result of my dose' calculation for 1 tsp (4.929 ml), found what concentration of NO3 that gave, then did a 'dose to reach target' calculation to meet that same concentration. That gives a result in grams, 5.2g.


    That's really only an estimate again, mass per teaspoon will vary by how fine or coarse the granules are, and with humidity, etc.


    The main point is, we aren't doing anything terribly exact here. All you need is to make sure the nutrients are present and available in reasonable concentrations.
     
  10. Solcielo lawrencia

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    Again, using volume as a measurement of mass is subject to extreme variances due to air.
     
  11. NanoPlantGuy

    NanoPlantGuy Subscriber

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    So you think the variation I'm seeing in KNO3 is mostly explained by the coarseness of grain size and humidity?


    Perhaps that's true, I'm just surprised (and not completely convinced) that that's it. Can you elaborate on etc.?


    P.S. I get the main point. We want to supply a non-limiting amount of nutrients and its EI so just E. But if I want to target 20-30 ppm NO3 and I'm off by a factor of two I wind up with 40-60 ppm NO3. I'm running a high tech planted tank in a Fluval Spec 2 with lots of shrimp. Off by a factor of two matters I think.
     
  12. Solcielo lawrencia

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    That's why you use weight (not volume) for granulated minerals.
     
  13. NanoPlantGuy

    NanoPlantGuy Subscriber

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    So I'll trust you and my scale then. I suppose another way to settle the issue would be to mix two solutions, one by weight and the other by volume, then test for ppm and see which is closest to target.
     
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