KNO3 and K2SO4 Combined Solubility - The Planted Tank Forum
 
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post #1 of 5 (permalink) Old 05-18-2017, 01:24 AM Thread Starter
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KNO3 and K2SO4 Combined Solubility

So I understand that when you put KNO3 and K2SO4 in solution they each have their own solubility. I think that since they share a K+ ion, the total solubility for both of them in the same solution is less than the individual solubilities - but I can't seem to find a reference to figure out the theoretical amounts.

If I want to put 34g of KNO3 into 1L of DI water, how much K2SO4 could I theoretically dissolve in that same solution?
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post #2 of 5 (permalink) Old 05-18-2017, 08:59 AM
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You'll have to find the Ksp for each chemical and then calculate using common-ion effect

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post #3 of 5 (permalink) Old 05-18-2017, 11:56 AM Thread Starter
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Thank you.

So I think 34g KNO3 in 1L of water is .337M concentration of K. Adding K2SO4 would be

Ksp = [K]^2[SO4]

or
1.32 = (.337+2x)^2 * x

solving i get
x=.583

So I think that means that K2SO4 could dissolve at a concentration of .583M in to my .337M KNO3 solution. That would be ~101g of K2SO4? So up to 101g of K2SO4 can theoretically be dissolved in 1 L of DI with 34g KNO3?

The whole question is because I've mixed 34g KNO3 and 22g K2SO4 into 1 L DI water and I'm seeing some precipitate. 22g would have me at 1/5 of the theoretical limit, so wondering if my math is wrong?
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post #4 of 5 (permalink) Old 05-18-2017, 03:35 PM
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Quote:
Originally Posted by DMtankd View Post

The whole question is because I've mixed 34g KNO3 and 22g K2SO4 into 1 L DI water and I'm seeing some precipitate. 22g would have me at 1/5 of the theoretical limit, so wondering if my math is wrong?
That 1lt should be 23020ppm of K, 20852ppm of NO3, and 4048ppm of S.
These low amounts should not show precipitate unless there is junk in the compounds used.


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post #5 of 5 (permalink) Old 05-19-2017, 01:16 AM Thread Starter
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There are a few grains that don't dissolve no matter what, assume that is the junk. Whatever this other precipitate is though, it dissolves quickly upon shaking the bottle, but then reappears again after a day or so.
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