Balanced NaNO3 and Na3PO4 solution(s)? Can they be mixed?

[Reefinmike]

I’m manually dosing the two of these and would like to automate it. Since they share the same cation, is there any reason I cant mix the two so i can use one doser? I realize bacterial or algal growth may be an issue but i have no problem limiting my mix to 2 or 4 weeks worth. My nitrate solution is 137gr NaNO3 and filled to the 1000ml mark. 1ppm should raise 100L tank water 1ppm. I wanted to create a phosphate solution that when dosed in equal parts would maintain the 16:1 redfield ratio. My previous solution used 3.76 gram and 1ml supposedly raises 100l tank water 0.02ppm. I want 0.0625ppm per ml. Do i multiple the 3.76 gram by 3.125? 11.75gr? I tried working it out using the molar weights and figured up 10.69gr. I’m no chemist and pretty certain I got lost in the conversions somewhere.

Thanks for any help!

 

[Randy Holmes-Farley]

You can mix them just fine, but the optimal ratio to mix will depend on the tank needs. Since you are already dosing you may know the relative amounts.

The redfield ratio is neither a goal in the tank (IMO) nor an optimal ratio in a supplement.

 

[Reefinmike]

You can mix them just fine, but the optimal ratio to mix will depend on the tank needs. Since you are already dosing you may know the relative amounts.

The redfield ratio is neither a goal in the tank (IMO) nor an optimal ratio in a supplement.

Thanks Randy! I know the redfield thing always stirs up some controversy. I’m not diehard set on it but i feel it cant hurt to start there. I have found that phosphate depletes at a higher rate as I started down the redfield path last week. Phosphate always bottoms out before nitrate.

I just dosed some of my stronger 11.75gr Na3PO4 solution and noticed it clouded up when it hit the tank water. My 3.76gr solution didn’t do this. I forgot where i found the initial mixing instructions but perhaps it’s so diluted to avoid whatever precipitation may be going on. Any idea what could be happening? Since you say I can mix the two, I will probably end up diluting everything a good bit to avoid this.

 

[Randy Holmes-Farley]

Thanks Randy! I know the redfield thing always stirs up some controversy. I’m not diehard set on it but i feel it cant hurt to start there. I have found that phosphate depletes at a higher rate as I started down the redfield path last week. Phosphate always bottoms out before nitrate.

I just dosed some of my stronger 11.75gr Na3PO4 solution and noticed it clouded up when it hit the tank water. My 3.76gr solution didn’t do this. I forgot where i found the initial mixing instructions but perhaps it’s so diluted to avoid whatever precipitation may be going on. Any idea what could be happening? Since you say I can mix the two, I will probably end up diluting everything a good bit to avoid this.

Na3PO4 is a high pH solution, and your may get some magnesium hydroxide precipitation where it first hits the water. That will redissolve, but dilution, slower dosing, or using a different phosphate salt, such as Na2HPO4, will lower the pH. Note too that Na3PO4 adds a small amount of alkalinity.

 

[Reefinmike]

Thank you so much Randy! I diluted both of my solutions down with 9 parts water , mixed the two and now let my apex do all the work for me. I’m sure i’d be fine without the dilution but it makes programming round the clock dosing easier.

 

[Randy Holmes-Farley]

You’re welcome.

Happy Reefing!

 

[ReeferBud]

I’m also dosing NO3 and PO4 and have been for the past year or so. While I was initially interested, like you, in mixing the NO3 and PO4 to dose a single solution, what I’ve found is that I’ve needed to adjust the NO3 and PO4 doses independent of each other to stay in the parameter range I’m targetting over time. If I had mixed them, I would have lost this flexibility and would probably have ended up mixing individual solutions anyway and doing a bunch of manual doses.

 

[Dkeller_nc]

OP: I'm a little confused about what precisely you wanted for a concentration for the phosphate ion in your solution. Are you trying to back-calculate the amount of Na3PO4 you'd need in 1 liter of solution such that 1 mL of this solution raises the PO4 concentration in 100L of tank water by 62.5 ppb (0.0625 ppm)?

To expand on Randy's comment about the precipitation when a solution of Na3PO4 is added to tank water, sodium phosphate is available as monosodium phosphate (NaH2PO4), disodium phosphate (Na2HPO4), and trisodium phosphate (Na3PO4). The pKa's of these salts are about 7.0, 9.5, and 12, respectively. You can expect a dilute solution of these compounds in water to have approximately the pH listed.

If you choose to do this, you can alter the pH of a tri, di or mono sodium phosphate solution by the addition of hydrochloric acid or sodium hydroxide. Technically, in a laboratory one should add phosphoric acid to a trisodium phosphate solution to preserve it's identity as an (only) sodium phosphate solution, since adding HCl adds chloride ions in addition to protons (H+), but for reef tank dosing purposes, we don't really care whether the solution has some excess chloride ions.

Another way to produce a sodium phosphate solution of a particular pH is to mix various ratios of mono and di sodium phosphate solutions to produce a final solution of a particular pH between 7.0 and 9.0 - there are lots of tables on-line for making phosphate buffers in this manner.

 

[Reefinmike]

OP: I'm a little confused about what precisely you wanted for a concentration for the phosphate ion in your solution. Are you trying to back-calculate the amount of Na3PO4 you'd need in 1 liter of solution such that 1 mL of this solution raises the PO4 concentration in 100L of tank water by 62.5 ppb (0.0625 ppm)?

Thank you so much for the informative reply. You interpreted it correctly, I wanted a stock phosphate solution that adds .0625ppm/ml to 100l. I’m not certain if my 10.7g/L or 11.75g/L solution is correct but I’ve found that doesn’t really matter. As @ReeferBud mentioned, i’m finding my tank’s usage isn’t aligning with the 16:1 red field ratio. Phosphate seemed to be drawn up faster so I bumped up the concentration 30% last week. Daily testing showed .5ppm nitrate and 0.01 or 0.00 phosphate. If my math is correct, I’m adding .5ppm no3 and .04ppm PO4 daily. I dosed an additional .06ppm PO4 a couple days ago and it was back to near undetectable a day later. I’ve increased the concentration of my solution yet again. I massively diluted the combined solution I’m dosing so precipitation hasn’t been an issue.

 

[Dkeller_nc]

One reason that you might interpret your tank as consuming phosphate a good deal faster than nitrate actually doesn't have anything to do with biological processes. Unlike nitrate, phosphate can abiotically precipitate with calcium to form calcium phosphate, which has a very low solubility. This tends to bind to new dry rock or sand to coat the surface. When that process is complete, you may see your phosphate level rise a good bit more with the same additions than you do now. In fact, this process sometimes causes aquarists grief when they've never monitored or controlled phosphate, and they can't understand why GFO is immediately exhausted in their filtration train and the tank still has about the same level of phosphate in the water as when they put the new GFO into the tank.

 

[ReeferBud]

PO4 can be tricky and you’ll observe its consumption vary due to different factors. For example, if you become PO4 limited and have available NO3, once you start to increase PO4 dosage, it can take some time and require quite a bit of PO4 dosing until the nutrients achieve equilibrium. It can also bind and precipitate, further causing “swings”.

Like you said and completely agree, the exact solution concentration doesn’t really matter. It’s more about finding the dosing level that brings your tank’s parameters into balance.

I find NO3 to be much more straightforward to maintain in a target range through regular dosing and will rarely ever need to adjust the dosing amount.