Making phosphate and nitrate reference solutions

[jccaclimber]

Generic case: It is good practice to check the calibration of your measurement tools to ensure their validity.
Specific case: My Red Sea phosphate test says my tank is at 0.1 PPM. My friend's Hanna checker says 0.46 PPM. Both tests repeat.

I've found reference solution online, but at 15 Euros for 100 mL shipped from the EU I'm inclined to make my own in bulk.

I have KH2PO4 and I have KNO3. I also have access to a vacuum desiccator, an analytical scale, distilled water (grocery store, I don't trust my RO at the moment), graduated cylinders, and a combination of glass pipettes and plastic syringes with markings ranging from 0.01 mL to 0.1 ML.

1) Is there a concern with KH2PO4 and KNO3 absorbing water from the air? My plan is to weigh out a portion, leave it in a vacuum desiccator for 5-10 minutes, then weigh it again and see if the weight changes. If it stops changing I'll assume it's dry.
2) I'm thinking 40 PPM for the nitrate and 1 PPM for the phosphate. I can dilute these down later fairly easily.
3) I haven't tried these tests (Red Sea, Hanna) with fresh water, but would it be a good idea to make these solutions out of salt water rather than fresh (thinking 1.026 SG)? The salt on hand is Red Sea Coral Pro. I think (without much evidence admittedly) that the nitrate and phosphate in the salt mix isn't going to cause much error based on the concentrations I'm planning. I can then dilute them 10:1 (or whatever is needed) for a low range solution.

Thoughts, advice, obvious errors?

Edit: I guess I never signed up here. I've been lurking for years while frequenting RC.

 

[Randy Holmes-Farley Verified Community Expert]

This would be my plan:

1. Dry the salts first in vacuum. That may not be needed, but cannot hurt.
2. Make a stock solution of, say 100 ppm nitrate and 1 ppm phosphate in the distilled water.
3. Spike the stock solution into new salt water and get a value that you want to test, say 10 ppm nitrate and 0.1 ppm phosphate by adding 9 parts salt water to 1 part stock solution.

 

[PatW]

Yeah, Randy’s method sounds good. I just did a by guess and by gosh method for nitrates. I add half a cup of salt like sodium nitrate, to a gallon of RODI. A couple of tablespoons of that solution into my 300 gallon display keeps my tank at 2 ppm.

 

[jccaclimber]

Randy,
Why put the stock solution in to salt water at the time of use, vs. in advance? Are you concerned making a salt solution in advance will enable growth that binds up some of the nitrate/phosphate, or something else? I can see the benefit of being able to use my solutions in fresh or marine tanks, although I'm generally far less concerned with accuracy in my planted fresh water tanks.

 

[Randy Holmes-Farley Verified Community Expert]

It may be fine to make it in salt water, but any precipitation of calcium carbonate that is known to happen in new salt water may deplete phosphate.

 

[StubbyAcro]

2. Make a stock solution of, say 100 ppm nitrate and 1 ppm phosphate in the distilled water.

Is using potassium nitrate and trisodium phosphate for the reference solution ok? Will this mixed solution be stable?

 

[Dan_P]

Generic case: It is good practice to check the calibration of your measurement tools to ensure their validity.
Specific case: My Red Sea phosphate test says my tank is at 0.1 PPM. My friend's Hanna checker says 0.46 PPM. Both tests repeat.

I've found reference solution online, but at 15 Euros for 100 mL shipped from the EU I'm inclined to make my own in bulk.

I have KH2PO4 and I have KNO3. I also have access to a vacuum desiccator, an analytical scale, distilled water (grocery store, I don't trust my RO at the moment), graduated cylinders, and a combination of glass pipettes and plastic syringes with markings ranging from 0.01 mL to 0.1 ML.

1) Is there a concern with KH2PO4 and KNO3 absorbing water from the air? My plan is to weigh out a portion, leave it in a vacuum desiccator for 5-10 minutes, then weigh it again and see if the weight changes. If it stops changing I'll assume it's dry.
2) I'm thinking 40 PPM for the nitrate and 1 PPM for the phosphate. I can dilute these down later fairly easily.
3) I haven't tried these tests (Red Sea, Hanna) with fresh water, but would it be a good idea to make these solutions out of salt water rather than fresh (thinking 1.026 SG)? The salt on hand is Red Sea Coral Pro. I think (without much evidence admittedly) that the nitrate and phosphate in the salt mix isn't going to cause much error based on the concentrations I'm planning. I can then dilute them 10:1 (or whatever is needed) for a low range solution.

Thoughts, advice, obvious errors?

Edit: I guess I never signed up here. I've been lurking for years while frequenting RC.

If you can afford it, just by the reference standard. If you enjoy lab work, use Randy’s scheme.