how can I avoid NO2 from over heating oil bath?

[GuyDeWhitney]

I have a nice apparatus/system for making white fuming nitric acid. The problem is that using the fuming sulphuric and NaCl in a water bath stinks and is very toxic to breathe even in "man, that stinks" amounts... As well as being dangerous to organic material, non-living as well as your flesh.
The other simple process I use involves an oil bath and two solid constituents - sodium bisuphate and potassium nitrate - and the higher the temp, the faster the production of HNO3 and, an unfortunate amount of NO2.
Is there a "peak" temp where it produces a reasonable reaction with a reasonable expenditure of power/gas AND the least possible amount of NO2?
Both reactions produce white fuming purity but, one requires an extra step to prepare the sulphuric and is more "messy" not to mention hazardous, the other produces white and red fuming nitric.

 

[butcher]

Unless you give better details of what you are doing and how you are doing it, it would be very hard to give you answers to your questions.

There is no need to make fuming nitric acid, 70% nitric is as strong as you need to make aqua regia, and for silver or base metals dilute acid works much better, if the acid is strong some metals passivate, other like silver will not hold as much silver in the solution, 35 to 40% HNO3 works just fine, actually with heat even more dilute nitric acid can be used.

Sulfuric acid and NaCl (sodium chloride) (salt) makes HCl (hydrogen chloride gas) which if dissolved in water, or in the liquid of your lungs forms HCl acid (hydrochloric acid). Sulfuric acid and NaCl will not make nitric acid, although there are ways you could make nitric using HCl in refining silver we would not want a chloride contaminate.

From what I can gather from your question it sounds as if you are strongly heating the dry, or relatively dry sodium bisulfate (NaHSO4) and the potassium nitrate (KNO3) to produce NO2/NOx gases, then dissolving these gases in water or a solution of water and hydrogen peroxide to make the nitric acid (some guessing here on my part), using a oil bath between your heat source and your vessel, you can do this with water in these salts and distill the nitric from the reaction vessel, and still get the nitric acid plenty strong enough to recover and refine precious metals with.

In my opinion the oil bath is more dangerous than a sand bath.

I also do not know what your heat source is and how much control you have over it.

Nitric acid has different boiling points for different concentrations, knowing the concentration you can determine boiling point, and vise versa, knowing the boiling point you can pretty much determine concentration (although specific gravity or titration would be better methods of determining concentration).

I can think of much better ways to make nitric acid for refining, (than what it sounds like you are doing from what I can gather from your question), one of them you can find in the guided tour (found in the general chat section) (using the recipe found there and distilling it).

There are several other methods or ways to make nitric, we can discuss if you wish; I can also help with data and facts about nitric acid.

But you will also have to give more detail in your questions; it is hard to give answers to a question when you do not know what the question is asking or have all of the details.