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KH2PO4 Question???
I'm trying to figure out how KH2PO4 disassociates into aqueous solution. Also, does the K in KH2PO4 disassociate into a usable K for plants? I know
that PO4 becomes available, but I'm not sure about K?
I've been googling and searching.....can't find the information anywhere without trying to figure out disassociation constants, etc.
Please help....I'm writing a fert calculator for something to do, and I want to be accurate!
that PO4 becomes available, but I'm not sure about K?
I've been googling and searching.....can't find the information anywhere without trying to figure out disassociation constants, etc.
Please help....I'm writing a fert calculator for something to do, and I want to be accurate!
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Planted Tank Enthusiast
Quote:
Originally Posted by Lycosa
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I'm trying to figure out how KH2PO4 disassociates into aqueous solution.
I've been googling and searching.....can't find the information anywhere without trying to figure out disassociation constants, etc.
I've been googling and searching.....can't find the information anywhere without trying to figure out disassociation constants, etc.
In solution the K+ ion is immediately freed from the (H2PO4) ion, so yes, the K is plantavailable.
The (H2PO4) ion will then dissociate based on the existing pH and the equilibrium constant to form (HPO4)2 ions and H+ ions.
The (HPO4)2 ions will dissociate based on the pH and the equilibrium constant to form (PO4)3 ions and H+ ions.
BUT  LeChatelier's principle tells us that if you (the plants) remove (PO4)3 ions, more (HPO4)2 and (H2PO4) ions will dissociate to replace it . . .
So, for calculation of available P (as PO4), just use the total KH2PO4  as the plants need the P they will get it  whatever form it is in.
Kevin
Kevin
72g bowfront planted, CO2, 4x  T5HO, Eheim 2213 and 2217, 2 angels, pristella tetras, blue tetras, betta, albino bristlenose pleco, albino cories. Sword, vals, hygros, ludwigias, java moss and fern, anubias
2g Macquarium. Clown gravel, fluorescent plastic plants, and 2 guppies.
72g bowfront planted, CO2, 4x  T5HO, Eheim 2213 and 2217, 2 angels, pristella tetras, blue tetras, betta, albino bristlenose pleco, albino cories. Sword, vals, hygros, ludwigias, java moss and fern, anubias
2g Macquarium. Clown gravel, fluorescent plastic plants, and 2 guppies.
Wannabe Guru
to sum it up i guess, i think someone once told me or i've read somewhere that you should get the/enough K that you need from the KH2PO4 and not have to dose K separately.
"Some of the worst things imaginable have been done with the best intentions."  Jurassic Park
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Thanks, KevinC, again for the information. You've answered my question completely.
Actually, since the recommended ppm for phosphate is small, (1  2 ppm), the available potassium from KH2PO4 is small and not near the recommended dose. I wanted to include the additional K from KH2PO4 into the calculation, however miniscule the amount. If one were dosing excess KH2PO4, since, as KevinC said the K is in usable form, it is a source which should be accounted for in the calculation. I'm basically writing the program in a different format than the available fert calc's that I've seen. More along the lines of choosing target ppms for ferts and having everything else calculate automatically. Anyhow, writing the program has forced me to learn a lot more, and that is never a bad thing!
Quote:
to sum it up i guess, i think someone once told me or i've read somewhere that you should get the/enough K that you need from the KH2PO4 and not have to dose K separately.
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Algae Grower
Wow impressive! KevinC, you should write some articles
Wannabe Guru
Quote:
to sum it up i guess, i think someone once told me or i've read somewhere that you should get the/enough K that you need from the KH2PO4 and not have to dose K separately
I also find out by trying that using phosporic acid instead of KH2PO4 is also as great, but the downside is that it is in liquid form which is troublesome for some people. (Warning : concentrated phosphoric acid is highly corrosive and should be handled with care. Use only diluted acid if you have not trained)
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Wannabe Guru
Quote:
Originally Posted by medicineman
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If I'm right that is from EI method of fertilizing in barr report. Tom reported that K from KNO3 and KH2PO4 if given as per EI is of enough level.
"Some of the worst things imaginable have been done with the best intentions."  Jurassic Park
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Algae Grower
Quote:
Originally Posted by KevinC
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Well, without the dissociation constants and LeChatelier's Principle, it can't be done . . . HOWEVER:
In solution the K+ ion is immediately freed from the (H2PO4) ion, so yes, the K is plantavailable.
The (H2PO4) ion will then dissociate based on the existing pH and the equilibrium constant to form (HPO4)2 ions and H+ ions.
The (HPO4)2 ions will dissociate based on the pH and the equilibrium constant to form (PO4)3 ions and H+ ions.
BUT  LeChatelier's principle tells us that if you (the plants) remove (PO4)3 ions, more (HPO4)2 and (H2PO4) ions will dissociate to replace it . . .
So, for calculation of available P (as PO4), just use the total KH2PO4  as the plants need the P they will get it  whatever form it is in.
Kevin
In solution the K+ ion is immediately freed from the (H2PO4) ion, so yes, the K is plantavailable.
The (H2PO4) ion will then dissociate based on the existing pH and the equilibrium constant to form (HPO4)2 ions and H+ ions.
The (HPO4)2 ions will dissociate based on the pH and the equilibrium constant to form (PO4)3 ions and H+ ions.
BUT  LeChatelier's principle tells us that if you (the plants) remove (PO4)3 ions, more (HPO4)2 and (H2PO4) ions will dissociate to replace it . . .
So, for calculation of available P (as PO4), just use the total KH2PO4  as the plants need the P they will get it  whatever form it is in.
Kevin
90 gal, eheim 2217, pressure co2, 4x54w T5HO.
Eheim limp #432
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Eheim limp #432
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Planted Tank Guru
You can get into the chemistry of it all if you like, but it can be distilled and made much simpler for those of us without degrees in chemistry!
Try reading this  http://www.rexgrigg.com/
Try reading this  http://www.rexgrigg.com/
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 VP & Horticulture Awards Program Chair
I <3 Eheim #195
Everyone seems normal... until you get to know them.
Planted Member
I know this thread is old, but I am going to ask for clarification here rather than start a new one.
So if I were making an excel calculator for the amounts of K and P from a KH2PO4 solution, the amount of K would be whatever K is in the solution, straight up, and the amount of P would be the amount of P in the solution?
I mean all of this in mass using the molecular masses and finding the percent of P and K in the solution I could find the mass of K and P I am adding to the aquarium, which allows me to get the ppm added to the tank right?
So if I were making an excel calculator for the amounts of K and P from a KH2PO4 solution, the amount of K would be whatever K is in the solution, straight up, and the amount of P would be the amount of P in the solution?
I mean all of this in mass using the molecular masses and finding the percent of P and K in the solution I could find the mass of K and P I am adding to the aquarium, which allows me to get the ppm added to the tank right?
Planted Tank Enthusiast
Quote:
Originally Posted by Ishar
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I mean all of this in mass using the molecular masses and finding the percent of P and K in the solution I could find the mass of K and P I am adding to the aquarium, which allows me to get the ppm added to the tank right?
Sergio C.
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Planted Member
awesome thanks for the reply
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