[JuliaP]

Hi,
hope someone can help me solve this problem, my physical-chemistry Prof. recently passed away and I am lost without him now. I will try to be simple. I had some animal tissue homogenate in NaOH, then I dried the solution and added concentrated HNO3, which is what we normaly do with our tissue homogenates. I did not think about chemistry, I am more a biologist. Now I have a nice white salt precipitate and the supernatant, which I should use to quantify the metals (As, Cu and Zn basically). Can I be sure that none of these metals I am interested in are in the salt precipitate, I mean, are any of these metals capable of displace Na from forming NaNO3?
Hope that makes sense for you, and that you could help me, I cannot believe how stupid I was 

[stewie griffin]

Well the acid-base reaction that you did should produce water and NaNO₃, as you say. Sodium nitrate is very soluble in water. I'm not sure what you did procedurally after adding the nitric acid, but since nitric acid is an aq. solution, I would have expected the sodium nitrate to dissolve. Thus if you filtered off the water layer or removed the water somehow, I would think that the sodium nitrate went with it. Maybe not, but that's my guess.
Copper nitrate is a bright blue powder (or blue solution if dissolved in water) so as long as you don't see any blue I'm quite sure you don't have copper nitrate. I'm not sure about the nitrates of the other two metals.

[JuliaP]

Thanks for your answer. I know NaNO3 is very soluble in water at room temperature,so any idea of what can be the think layer of white salt that is formed. I wonder if it has something to do with the drying of the initial NaOH solution on a hot plate before adding the HNO3, I had the tubes on the hot plate as well after adding the HNO3 for digestion.