QA my chemistry, please? - The Planted Tank Forum
 
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post #1 of 2 (permalink) Old 12-06-2006, 07:48 PM Thread Starter
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QA my chemistry, please?

When calculating concentration (in ppm) for a given element that occurs within a compound, do we actually use the molecular weight for the individual element within the compound, or the entire molecular weight of the compound itself?

e.g., CaCl2: 100 g in 100 ml water = 50% sol'n (500k ppm CaCl2). So if I add 1 ml of this solution to 100 L of water, am I adding 5 ppm Ca2+ (as CaCl2) or am I adding 1.8 ppm Ca2+ (because mol.wt. Ca = 40, Cl2 = 71; 40/111 = 36%; 5 ppm x 36% = 1.8 ppm) and 3.2 ppm Cl-?

Extra question regarding elements within complex compounds e.g., phosphorous in phosphate. When we say "dose X ppm P" do we mean actual ppm P or ppm PO4(3-)?

The reason why I'm confused is I tried to double-check using a formula for raising Ca2+ ppm found on the bottle on my CaCl2. It states that a pool volume of 2,500 gal will experience a 100 ppm increase of Ca2+ by the addition of 3 lbs. of CaCl2. Some simple math, 2,500 gal = 20,000 lbs. Mass% CaCl2: 3/(3+20,000) = 0.015%; x 10^4 = 150 ppm. Now, either that is +50% off claimed ppm increase if one states that it is Ca2+ as CaCl2, or -50% if one uses the elemental weight ratio (150 ppm x 36% = 50 ppm).

Any help appreciated!
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post #2 of 2 (permalink) Old 12-06-2006, 08:29 PM
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Quote:
Originally Posted by Tsigoloeg View Post
e.g., CaCl2: 100 g in 100 ml water = 50% sol'n (500k ppm CaCl2). So if I add 1 ml of this solution to 100 L of water, adding 1.8 ppm Ca2+ (because mol.wt. Ca = 40, Cl2 = 71; 40/111 = 36%; 5 ppm x 36% = 1.8 ppm) and 3.2 ppm Cl-?

Extra question regarding elements within complex compounds e.g., phosphorous in phosphate. When we say "dose X ppm P" do we mean actual ppm P or ppm PO4(3-)?
I left the correct part above - use the % composition to determine the Ca.

On the board here people refer to ppm PO4 (mg PO4/L) and ppm NO3 (mg NO3/L). However, scientific papers and other sources will often use ppm P as PO4 and ppm N as NO3 (some test kits report this way as well). This is easier for example when thinking about total N: you could add the ppm N as NO3, the ppm N as NH3 and the ppm N as NO2 . . .

Kevin

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