KNO3 (mis)measurement

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...

 

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.

 

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.

 

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!!!

 

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.

 

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.