Trisodium Phosphate calculation

[nkkm]

I purchased Loudwolf Trisodium phosphate as recommended in other threads and using Jame's planted tank calculator I created a stock solution using 0.5 tsp of Na3PO4 to 1000ml of RODI. I expected (based on the Potassium Phosphate in the calculator) that 1ml of the solution would raise 50gal by 0.01ppm.

I have been dosing around 4-6ml daily and still reading 0.00 on a HI713 Phosphate checker.

I decided to try to test the stock solution by adding 0.5ml of the solution to 1000ml or RODI and then testing that water. If my calculations are correct I would have expected a 1.15ppm reading with some margin of error. The result was 0.25 on the Hanna. I believe the checker can test to 2.5 ppm so can someone help where I have gone wrong in my calculations? The stock solution appears to be roughly 25% of what I thought it was.

 

[ichthyogeek]

So.....when making stock solutions, try to use weight-based as opposed to volumetric based measurements when dealing with chemicals like trisodium phosphate. Water is ok with volumetric, since it's approx a 1:1 ratio of grams to mL I believe. 1/2 tsp can weigh much differently based off of how large the triphosphate crystals are. I think I have my chemistry/stoich right, but somebody feel free to check me, as I haven't done it in 4 years or so. Let's work backwards here:

You are trying to raise 50 gallons (189.3L) by .01 ppm (ppm = mg/L) of phosphate, with 1 mL of stock solution. You want to add (.01 mg/L solution) * (189.3 L) = 1.893 mg phosphate dissolved in 1 mL of stock solution. This translates to (1.893 mg)/(1 mL) * (1000/1000) = 1893 mg phosphate. That is equivalent to 1.893 grams of phosphate. Note, that this is phosphate only, not sodium phosphate. To do this, we convert to moles. 1.893 grams phosphate translates over to .019932 moles phosphate. Which means you want that same amount of phosphate in the sodium phosphate you add, or more plainly, you want .019332 moles of sodium triphosphate. Converting that back to grams, you get 3.2676 grams. So you need to add 3.2676 grams of pure sodium phosphate in order to get the stock solution you want. This doesn't take into account anything that might screw with the purity, like contaminants (i.e. that .1% of the rest of the stuff)

Working from the problem you gave, your stock solution, when adding .5 mL Stock to 1000 mL of RODI resulted in .25 ppm (mg/L). Again, working backwards, this tells us that there were (.25 mg/L) * (1000.5 mL) = 250.125 mg phosphate in the stock solution for every .5 mL, or 500.25 mg phosphate for every mL present. If you wanted ~1.15 ppm (mg/L), you would have needed to add (1.15mg/L)*(1000.5 mL) = 1150.575 mg phosphate. Which is ~ 1.9864 grams sodium phosphate. Interesting.

 

[Randy Holmes-Farley Verified Community Expert]

I wouldn't assume there is any huge problem. It is normal to need a lot of dosing to get detectable levels. Much of it binds to rock and sand.

I also wouldn't assume that a phosphate solution can be tested directly as the pH and alkalinity are very different than aquaria.

 

[nkkm]

So.....when making stock solutions, try to use weight-based as opposed to volumetric based measurements when dealing with chemicals like trisodium phosphate. Water is ok with volumetric, since it's approx a 1:1 ratio of grams to mL I believe. 1/2 tsp can weigh much differently based off of how large the triphosphate crystals are. I think I have my chemistry/stoich right, but somebody feel free to check me, as I haven't done it in 4 years or so. Let's work backwards here:

You are trying to raise 50 gallons (189.3L) by .01 ppm (ppm = mg/L) of phosphate, with 1 mL of stock solution. You want to add (.01 mg/L solution) * (189.3 L) = 1.893 mg phosphate dissolved in 1 mL of stock solution. This translates to (1.893 mg)/(1 mL) * (1000/1000) = 1893 mg phosphate. That is equivalent to 1.893 grams of phosphate. Note, that this is phosphate only, not sodium phosphate. To do this, we convert to moles. 1.893 grams phosphate translates over to .019932 moles phosphate. Which means you want that same amount of phosphate in the sodium phosphate you add, or more plainly, you want .019332 moles of sodium triphosphate. Converting that back to grams, you get 3.2676 grams. So you need to add 3.2676 grams of pure sodium phosphate in order to get the stock solution you want. This doesn't take into account anything that might screw with the purity, like contaminants (i.e. that .1% of the rest of the stuff)

Working from the problem you gave, your stock solution, when adding .5 mL Stock to 1000 mL of RODI resulted in .25 ppm (mg/L). Again, working backwards, this tells us that there were (.25 mg/L) * (1000.5 mL) = 250.125 mg phosphate in the stock solution for every .5 mL, or 500.25 mg phosphate for every mL present. If you wanted ~1.15 ppm (mg/L), you would have needed to add (1.15mg/L)*(1000.5 mL) = 1150.575 mg phosphate. Which is ~ 1.9864 grams sodium phosphate. Interesting.

Thanks for the thorough response. I hoped to do by weight but realized the scales I had were not accurate enough. I will need to order some scales for future and redo the stock solution.

 

[nkkm]

I wouldn't assume there is any huge problem. It is normal to need a lot of dosing to get detectable levels. Much of it binds to rock and sand.

I also wouldn't assume that a phosphate solution can be tested directly as the pH and alkalinity are very different than aquaria.

Thanks Randy, I will keep raising the dosage and see where I get to. Its in a bare bottom frag tank but there is some rock in the sump.

 

[Randy Holmes-Farley Verified Community Expert]

There's really no reason to be super accurate since the majority of what you dose may end up on rock and sand anyway. It is not possible to control the final level except by ongoing trial and error.