K3po4

Chiya

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Jun 23, 2008
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Hi All,

I managed to get my hands on some K3PO4 (Potassium Phosphate) and I'd like to make a reference solution for dosing into my tanks, 50G and 15G respectively.

However, I found that the Potassium Phosphate that's used around here is KH2PO4.

If I followed the reference solution in the sticky, I'm worried that I might be dosing 3x the amount of K into my tanks.

From the EI pages :

(+ or -) 2ppm of K+ is pretty reasonable.
(+ or -) 0.2ppm of PO4 is pretty reasonable (?)

K+ range 10-30ppm
PO4 range 1.0-3.0 ppm


I know that EI has proven that we can dose excess without fear, but my inability to calculate is stopping me (thank god)

Appreciate if anyone could 1) tell me if K3PO4 is usable, 2) how to make a reference solution.

*reference solution will make it easier for me to dose as I have a syringe.
** I only managed to find out that the molecular mass of K3PO4 is 136.

Thanks in advance!!

Regards,
Ryan
 

VaughnH

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Since we don't dose a lot of PO4 the increased K from using K3PO4 wouldn't be a problem. Apparently K3PO4 makes water very alkaline - raises the measured KH, making it inaccurate as a measure of carbonate in the water. Not being a chemist, I don't know exactly what I am saying here. But, now I wonder if this is what water companies add to tap water to raise the pH, and which messes up the KH/pH/CO2 table results.
 

Chiya

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Jun 23, 2008
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Hi Hoppy,

I was told that K3PO4 makes water alkaline too.
But we are adding CO2 to our tanks, which makes it acidic. So will it somehow balance?

Plus, my tank water is about 2-3KH, pH 6.4-6.7 so if we are only adding little amounts of PO4, the alkalinity might not mess up the tables??

I wished I could still remember my college Chemistry :(

So do I use the reference solution as shown in the stickies?

Regards,
Ryan
 

VaughnH

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The KH/pH/CO2 tables are not accurate in any case. For one thing municipal water companies add stuff to the water to keep the pH above 7, in order to eliminate erosion of copper piping. That stuff alone ruins the KH/pH/CO2 relationship.

The amount of potassium in the water is not critical - actually none of the fertilizer amounts is critical, which is why EI works so well. So, if you substitute K3PO4 for KH2PO4 you might want to double the dosage of the phosphate compound because there is a bit less than half the phosphate in the K3PO4 as in the KH2PO4. (K3PO4 usually comes hydrated with a molecule of H2O attached).
 

VaughnH

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aquabillpers;30725 said:
Why does K3PO4 increase the KH?

Bill
I speak as an expert in chemistry, with years of experience.......I wish. According to what I read about K3PO4, it is strongly alkaline in water. KH measurements are really alkalinity measurements.

You know how, when you can't swim hardly at all, and you find yourself in deep water, the best thing is to tread water and hope someone who can swim well will show up. I'm treading water.
 

aquabillpers

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VaughnH;30726 said:
I speak as an expert in chemistry, with years of experience.......I wish. According to what I read about K3PO4, it is strongly alkaline in water. KH measurements are really alkalinity measurements.

You know how, when you can't swim hardly at all, and you find yourself in deep water, the best thing is to tread water and hope someone who can swim well will show up. I'm treading water.

Great advice! :)

I'd think that the there should be some extra C in that to change the KH, but what do I know? Some kind of chemical reaction could occur that would increase the KH, I guess.

I once knew a lot, but memories fade and the sciences change.

We need that swimmer. :)

Bill
 

Chiya

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Jun 23, 2008
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Hi Guys,

Still treading water? I'm going to try sinking to the bottom and see if I step on anything =)

I'm going to try making a reference solution based on the EI pages.

Since K will be 'under-dosed', I might supplement with Seachem's Potassium.

Hopefully I'm able to spot the deficiencies without too many plant deaths.

Regards,
Ryan
 

ceg4048

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Hi Ryan,
It's not clear why you feel that you'll be underdosing K by using K3PO4. First of all, the majority of the K we get in EI dosing comes from the K in KNO3. Just look at the relative amount of KNO3 added versus the amount of KH2PO4. So if you are adding enough KNO3 to satisfy the NO3 requirements then you are automatically adding enough K.

Secondly, as clearly seen in the chemical formulas K3PO4 (212 grams/mole) has 3 K atoms whereas KH2PO4 (136 grams/mole) has only a single K atom. Therefore if you are dosing the same amount of teaspoons of K3PO4 you are by default dosing more K, so why would you need to supplement additional K?

If anything, what I'd be more worried about is that K3PO4 will provide less PO4 for the same volume teaspoon dosage. Roughly, a teaspoon of K3PO4 will be 56% heavier. It will be about 45% PO4. A teaspoon of KH2PO4 is about 70% PO4 (these are just ballpark figures mind you.)

Just to keep life simple it looks like you need to add about 25% more K3PO4 by volume to get similar amounts of PO4 compared to if you were adding KH2PO4 - and you'll not need to add more K, that's for sure.

Cheers,