[Enrik]

Hello.
I was asked to prepare a solution 1 M of sodium nitrate from one open bottle in my lab, in order to produce standard for the determination of nitrate in water sample and compare them with EPA standard.

My values came out pretty messed up. So I went back to check my stock reagent and I've noticed that it is quite hygroscopic (i.e. water). I've confirmed on the manufacturer website that NaNO3 it is hygroscopic.

Would I be able to remove the water from it? Perhaps using:

a) a desiccator
2) a 105C oven for 24h.

which option would be better?
I am afraid that fumes from very warm NaNO3 may be toxic...

thanks for your help

Enrik

[Zyklonb]

I think either will work, but I doubt only 105 degree's C will work, even for 24 hours.
industrially, it is heated to over 350 degree's C for just a few minutes.
It will decompose slowly and evolve oxygen as thermally it can be reduced to sodium nitrite (NaNO₂).
As for a desiccator, conc. sulfuric acid or sodium hydroxide should work [not at the same time ].
As for knowing when it reaches anhydrous state, first evaluate the %/mass of water in the crystals.
Then using simple math, calculate how much it will weigh after the water is gone.
Then use whatever method you want to get rid of the water - until it weighs as much as you calculated.

[Arkcon]

I'm afraid I can't find a definitive reference on the topic, so I'll have to take guess, then give you some general help.

A desiccator only sets up an equilibrium between the desiccant and the sample.  It might help keep things dry, for a while, but only very fairly hygroscopic things can dry that way, off hand I think that salicylic acid is one.  I doubt it will work for sodium nitrate.

Drying in the oven at 105 ° C seems like it should work, and I don't think it will be detrimental to the sodium nitrate, it doesn't decompose until much higher temp.

However, generally, for assay purposes, when we have a standard to adhere to, you're using EPA, I often use FDA or USP, we're not permitted to do just what you're doing.  You can't simply build a standard, as carefully as you can, from the best material you can come up with, and call it a standard.  Instead, you must purchase standardized solutions, or if you make them yourself, standardize the solution against a known standard.

[Borek]

However, generally, for assay purposes, when we have a standard to adhere to, you're using EPA I often use FDA or USP, we're not permitted to do just what you're doing.  You can't simply build a standard, as carefully as you can, from the best material you can come up with, and call it a standard.  Instead, you must purchase standardized solutions, or if you make them yourself, standardize the solution against a known standard.

Exactly. NaNO₃ is not a standard substance, so it should be not used for direct preparation of a standard solution.