Phosphate Dosing

[Scott714]

Hi, I’m looking to create a stock solution of potassium phosphate to dose phosphate into my system via dosing pump.

Am I right that 1g of dipotassium phosphate dissolved in 500ml of RO water would produce a stock solution that when 1ml of stock solution is added to 100L of water it would increase phosphate by 0.01ppm?

Appreciate someone checking my numbers. I think the precise weight is actually .9 something grams in 500ml but I rounded it up to 1g.

 

[Randy Holmes-Farley Verified Community Expert]

Hi, I’m looking to create a stock solution of potassium phosphate to dose phosphate into my system via dosing pump.

Am I right that 1g of dipotassium phosphate dissolved in 500ml of RO water would produce a stock solution that when 1ml of stock solution is added to 100L of water it would increase phosphate by 0.01ppm?

Appreciate someone checking my numbers. I think the precise weight is actually .9 something grams in 500ml but I rounded it up to 1g.

Any reason you picked that over sodium phosphate?

Mostly yes on the math, but there are three complexities:

1. Dipotassium phosphate may be only partly dipotassium phosphate and partly water in the solid as many commercial materials are hydrates. Do you have a picture of the bottle or a web link?

2. Even if it is anhydrous dipotassium phosphate, that material is only 55% phosphate by weight. 1 g of the solid thus contains 550 mg of phosphate. Put that into 500 mL of water, and it contains 1.1 mg/mL. Add 1 ml to 100 L and the addition is 0.011. You understand this from the 0.9 comment.

3. The biggest issues is in usage, much of what you add may bind to rock and sand, and adding, say 0.1 ppm, might give a far lower amount actually left remaining in the water.

FWIW, this calculator is good enough for phosphate dosing:

http://www.theplantedtank.co.uk/calculator.htm

 

[Scott714]

Appreciate the calculation confirmation - chat GPT worked it out for me but I can’t trust it haha.

This is what I bought:
https://allchemical.com.au/shop/dipotassium-phosphate-food-grade/
And the reason I went with it is because I’m currently dosing this:
https://www.coralessentials.com.au/coral-power-phosphate-up.html
Which states on the bottle it’s potassium phosphate, however this costs ~$35 a bottle (100ml) and I reckon I just bought 100 lifetimes worth for $27 (1kg) haha. However maybe I made an error not going with sodium phosphate?

 

[Randy Holmes-Farley Verified Community Expert]

Appreciate the calculation confirmation - chat GPT worked it out for me but I can’t trust it haha.

This is what I bought:
https://allchemical.com.au/shop/dipotassium-phosphate-food-grade/
And the reason I went with it is because I’m currently dosing this:
https://www.coralessentials.com.au/coral-power-phosphate-up.html
Which states on the bottle it’s potassium phosphate, however this costs ~$35 a bottle and I reckon I just bought 100 lifetimes worth for $27 haha. However maybe I made an error not going with sodium phosphate?

The concern is rising potassium, but phosphate doses are low enough to not be a significant concern. Nitrate doses are larger and potassium nitrate is not a good choice.

 

[Scott714]

Yes, I will keep an eye on the potassium through ICPs. My system (1000L) is only 3 months old and with only two dartfish at the moment with a lot of sps frags. The frags are paling out a bit (but all encrusting and showing signs of growth) and my nutrients are undetectable. I’ve been dosing your ammonium bicarbonate solution and need a solution for the phosphate side of things. Been taking it very slow with the dosing and I can just get some pink on the salifert nitrate test now. However both still reading zero on Hanna high range nitrate and Hanna phosphorous testers.

 

[Randy Holmes-Farley Verified Community Expert]

ok, let us know how it goes. :)