[APD] PPM Concentrations

["Steven Pituch" <spituch at ev1_net>]

Hi all,

I have been helping Art Giacosa at AquaticPlantCentral develop a nutrient
calculator for planted aquaria.   I have run into a technical question and
need some expert advice.  I have been calculating the ppm concentration of a
solution by dividing the mass of the compound in milligrams (say NO3) by the
amount of liters of water the compound is added to.  So 1 gram of KNO3 in
100 gallons of water would yield (1gm x 1000 x .61)/(100 gal x 3.77) = 1.6
mg/l or 1.6 ppm of NO3.

It seems that this equation is OK for small concentrations but for larger
concentrations I need to add the volume of the compound to the volume of the
water.  In other words the solution volume equals the solvent volume plus
the solute volume (this is the wet volume of the compound).  I therefore
need the density of the compound.  There are published bulk densities, but
these are for the dry powdered compound and include the void space between
granules.   Therefore, the bulk densities vary a lot based on type of
manufacture.  However, I need the density of the compound after it is added
to water, as I need to determine the volume of the compound when it is in
solution.  The Merck Chemdat database has a density value which is quite
consistent.  For example, for KNO3  it says the density is 2.11 gm/cc.  I
suspect that this is a value of density of KNO3 after it is put into
solution, but have found no supporting documentation on the Internet.

So my question is: Is the density at the Merck Chemdat website at
 http://www.chemdat.de/cdrl/catalog/standard/en/index.html
the density I need to calculate the concentration of a compound in a
solution?
If not, what should I do.

I also realize that ppm is actually a weight computation, but since most
people dispense by volume I feel determining mg/L is better way to represent
this.

Regards,

Steve Pituch

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