# K3PO4 instead of KH2PO4?



## lipadj46 (Apr 3, 2011)

Quick question, I don't have KH2PO4 in the lab but we have a bunch of K3PO4 for some reason that we don't use. Can I use this as a potassium and phosphate source?


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## Newt (Apr 1, 2004)

I dont see why you cant.
Did you look to see if the Fertilator Calculator has it?
You want to make sure you dose it properly.

http://en.wikipedia.org/wiki/Potassium_phosphate


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## wet (Nov 24, 2008)

This fert looks sweet. In case anyone else is looking at it, K3PO4 has been added to http://calc.petalphile.com

Any chance you could take a few trials of the mass of a leveled teaspoon, lipadj46? Right now I am estimating vs the density of KH2PO4 and just guessing at grain size and stuff. (At 6 grams/tsp.)

Thanks!


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## lipadj46 (Apr 3, 2011)

wet said:


> This fert looks sweet. In case anyone else is looking at it, K3PO4 has been added to http://calc.petalphile.com
> 
> Any chance you could take a few trials of the mass of a leveled teaspoon, lipadj46? Right now I am estimating vs the density of KH2PO4 and just guessing at grain size and stuff. (At 6 grams/tsp.)
> 
> Thanks!


Sure I just need to remember to take a teaspoon to school with me


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## ray-the-pilot (May 14, 2008)

lipadj46 said:


> Quick question, I don't have KH2PO4 in the lab but we have a bunch of K3PO4 for some reason that we don't use. Can I use this as a potassium and phosphate source?


I don't recommend using K3PO4 in place of KH2PO4 becausee it is exteremly alkaline and will rapidly increase the pH of your aquarium. This will also increase the total alkalinity of your tank and really mess up the CO2 levels as well.

If you absolutely insist on using K3PO4, you should check your pH and KH frequently.

One of the advantages of KH2PO4 is that it is a buffer arround a pH of 7.0.


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## wet (Nov 24, 2008)

ray-the-pilot said:


> I don't recommend using K3PO4 in place of KH2PO4 becausee it is exteremly alkaline and will rapidly increase the pH of your aquarium. This will also increase the total alkalinity of your tank and really mess up the CO2 levels as well.
> 
> If you absolutely insist on using K3PO4, you should check your pH and KH frequently.
> 
> One of the advantages of KH2PO4 is that it is a buffer arround a pH of 7.0.


Please correct me, ray-the-pilot. But a healthy dose of PO4 from K3PO4 is like 25 umol/L K3PO4, right? I have a hard time believing that this is significant vs KH2PO4, mostly because healthy ("EI+") dosing from KH2PO4 has been fine in my tanks, including some that were RO/DI/softwater tanks. What am I missing?


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## lipadj46 (Apr 3, 2011)

ray-the-pilot said:


> I don't recommend using K3PO4 in place of KH2PO4 becausee it is exteremly alkaline and will rapidly increase the pH of your aquarium. This will also increase the total alkalinity of your tank and really mess up the CO2 levels as well.
> 
> If you absolutely insist on using K3PO4, you should check your pH and KH frequently.
> 
> One of the advantages of KH2PO4 is that it is a buffer arround a pH of 7.0.


I actually found a gigantic tub of KH2PO4.


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## niko (Jan 28, 2004)

Wow! All this information is so interesting! 

The more we know about the chemistry of planted aquarium fertilizers the better our tanks could be.

Why is it that two extra H atoms make such a difference on the Alkalinity?

--Nikolay


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## ray-the-pilot (May 14, 2008)

wet said:


> Please correct me, ray-the-pilot. But a healthy dose of PO4 from K3PO4 is like 25 umol/L K3PO4, right? I have a hard time believing that this is significant vs KH2PO4, mostly because healthy ("EI+") dosing from KH2PO4 has been fine in my tanks, including some that were RO/DI/softwater tanks. What am I missing?


This is chemistry. Those two little K's make a big difference. 
H3PO4 is a weak acid when you add all that KOH to make K3PO4 the result is very basic. So if KH2PO4 in about neutral and you add 2x times more of KOH on a molar basis to make K3PO4 the result is very basic. I wouldn't put my hands in a 10% solution of K3PO4. (I think K3PO4 is used in industrial cleaners).
If you work in a lab check it out. Put a little bit of K3PO4 in some water and test the pH. it is probably about 12. 
Seriously I don't think you should use it at all, especially if you are using presurized CO2!.


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## Newt (Apr 1, 2004)

Tha MSDS for K3PO4 says "highly alkaline"
http://www.jtbaker.com/msds/englishhtml/p6060.htm

Its good to have a chemist on board.


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## wet (Nov 24, 2008)

> H3PO4 is a weak acid when you add all that KOH to make K3PO4 the result is very basic. So if KH2PO4 in about neutral and you add 2x times more of KOH on a molar basis to make K3PO4 the result is very basic. I wouldn't put my hands in a 10% solution of K3PO4. (I think K3PO4 is used in industrial cleaners).


I'm just going to assume I'm not the only gardener that is still confused by this. I get that a weak acid and strong base will make an alkaline solution, but I'm under the impression that at dilution this becomes less important.

Even with high relative PO4 dosing (for the aquarium) we're talking about like 0.0005% K3PO4, with is 20,000 times more diluted than your 10% K3PO4 example.

A practical example: there's plenty of folks who wouldn't stick their hands into Glutaraldehyde but are competent in dissolving it to safe levels in the aquarium. (I'm not trying to build a strawman but rather get an idea of how bad parts per *million* dosing of this stuff is vs the steps gardeners already take with their aquariums.)

Thanks! I understand that it may be boring to you, but showing how the KPa stuff and pH will move in the concentrations of our aquariums would be valuable to non-scientists like me.


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## lipadj46 (Apr 3, 2011)

+1 we are about a half teaspoon in 55 gallons, I'm too lazy to do a Kb calculation but I wonder if you would even see a change in pH.


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## ray-the-pilot (May 14, 2008)

wet said:


> I'm just going to assume I'm not the only gardener that is still confused by this. I get that a weak acid and strong base will make an alkaline solution, but I'm under the impression that at dilution this becomes less important.
> 
> Even with high relative PO4 dosing (for the aquarium) we're talking about like 0.0005% K3PO4, with is 20,000 times more diluted than your 10% K3PO4 example.
> 
> ...


OK say your first dose is 1 ppm of PO4. That is .001g/L or .001/92 = 1.0x 10 ^(-5) Mol/L
When you use K3 you add 2 times this number in extra KOH or 2 x 10^(-5)Mol/L of KOH which fully ionized so produces 2 x 10^(-5)Mol/L OH ions. 
pOH = -log(2 x 10^(-5)Mol/L) = 4.6
pH = 14 - pOH =14 -4.6 = 9.4.
That is the initial pH change. The problem is also that you are adding extra K that is not being used. This will result in a gradual build up in alkalinity. Resulting in increased pH and problems with your CO2 regulation. 
Now really this is scientific mumbo jumbo. Try using K3PO4 for a couple of months and keep track of your pH and KH. If they stay OK hurray! If not you learned something. My prediction is that you will stop using it by the first week but I could be wrong.


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## ray-the-pilot (May 14, 2008)

lipadj46 said:


> +1 we are about a half teaspoon in 55 gallons, I'm too lazy to do a Kb calculation but I wonder if you would even see a change in pH.


Then try it! 1/2 teaspoons * 7 days = 3.5 teaspoons


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## wet (Nov 24, 2008)

(thanks ray-the-pilot! Added a direct link to this thread via your post above if anyone uses K3PO4 on the calculator.)


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## Jeffww (May 25, 2010)

The assumption of K3PO4 making an alkaline solution is in the absence of other buffers. I think with the organic acids present in an average aquarium, you should be fine. Additionally, what excess K+ there is should be easily taken up by plants. A free roaming potassium is less work to take in than a K+ that has affinities for a free floating SO4. Of course they're all ionized anyways....I think that it'll buffer the water but not enough to notice, especially at these concentrations. 

edit: I mean an alkaline solution in an aquarium


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## ray-the-pilot (May 14, 2008)

This whole thread is a typical no facts discussion!

Nobody yet has said that they actually have used K3PO4 in their aquarium and knows for a “fact” how it works. So everything anyone has said is just opinion. (Including my own statements).

If I had some K3PO4, I'd test it on a sample of my aquarium water and find some facts about the efficacy of the stuff. But I'm not about to buy it just to prove something. 
If anyone can send me some I can do some tests on a sample level that could help in getting to the “facts” of this discussion.
Or if anyone (especially those who think it is OK) wants to try it, please do and give us the facts! If not then it doesn’t seem prudent to tell people it is OK based on an opinion. 

BTW the fact that no one in the hobby is using it tends to make me think that it is probably not so good.
BTW2 The pKa of HPO4-2 is 12.3 that means that it buffers your water at this pH. The pH of even dilute solutions of K3PO4 is over 13.0. Because the pH scale is a log scale this means that a dilute solution of K3PO4 is over 1,000,000 times more alkaline than an equivalent KH2PO4 solution. This is a very powerfully alkaline solution and I would be careful using it. 

I would not tell someone who doesn’t have experience working with chemicals to use it! It would be too easy to mess up and kill off everything in your tank!


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## lipadj46 (Apr 3, 2011)

Wow I gave not checked this thread in a while. So for kicks I added 10 grams teaspoons to the 55 gallon tank we have in the bio room here and pH did not change (well at least accoring to the cheapo pH pen I had handy). 

As far as the rest of the chemistry mentioned above... Sure if you make a solution of H3PO4 with a pH of 12.3 it will act as a buffer but just adding K3PO4 to water will not automatically make the pH=13. A 0.04 M solution of NaOH has a pH of around 12.6. Now that is nothing to just scoff at but its not very dangerous unless you get it in your eyes or drink it or dump enough of it in a fish tank to kill all your fish. 

And just to note I use KH2PO4 in my fish tank at home.


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## ray-the-pilot (May 14, 2008)

lipadj46 said:


> I added 10 grams teaspoons to the 55 gallon tank we have in the bio room here and pH did not change (well at least accoring to the cheapo pH pen I had handy).


The first fact! But I'm a little confused about just how much you used in this test. I would need a little more data before I'd tell an inexperienced person that this stuff is OK to use.
Since you have some K3PO4 try using it as a substitute for KH2PO4. You will need to use about 56% more to keep the PO4-3 level constant. Try it for a few weeks. You will need to check the pH and the total alkalinity (carbonate hardness). If these two tank parameters stay stable then it is a go.

Here is some pH chemistry.

The pKa of HPO4-2 is 12.3. To make a buffer at this pH you need to add equal molar amounts of the acid and salt to get a pH of 12.3. That means if you have the salt (ie K3PO4) you will have to add acid to the solution to bring the pH down to 12.3. The pH does not depend on the concentration of the acid/salt solution only the relative amounts of each. 
The buffer capacity does depend on the dissolved material and their concentration. The buffer capacity is usually measured as total alkalinity (or carbonate hardness).
On an equal molar basis, a 0.04 M solution of K3PO4 has 3 times the total alkalinity of a 0.04 M solution of NaOH. And yes the pH will be higher! 
Because K is heavier than Na and there is a lot of it in K3PO4, on a gram basis K3PO4 is only about 70% as alkaline as NaOH.

OK someone try it out for a few week and report the facts!


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