When distilling, we are heating solutions to gases, these gases need to be condensed back to liquid, some types of gases condense back to liquid and some do not convert easily back to a liquid, depending on the gas that is formed.
When dissolving metals we can form gases from the reaction of the acid on metals.
Lets look at nitric acid, as we know when we dissolve a metal with it we create a brown gas, this is not just one gas, and depending on water involved in our nitric solution can determine what form the gas takes, we can say it is ,nitrogen dioxide (NO2), nitrogen oxide (NO), nitrous oxide (laughing gas)(N20), or many of the other gases of nitrogen and oxygen, we call NOx gas to refer to the lot of them, in nitric acid the two we are mainly concerned with are NO2, and NO, the nitrogen oxide gas (NO) needs oxygen to convert to nitrous dioxide gas (NO2):
2NO + O2 --> NO2 (gas)
The oxygen is very important here, as nitrogen monoxide will not make nitric acid (when in contact with water), NO gas would just pass through the solution.
But NO2, nitrogen dioxide when in contact with water does form nitric acid, and does so better with some oxygen present:
4NO2 (g) + 2H2O (L) --> 4HNO3 (aq) .
When distilling Nitric acid (HNO3) we see we need the water.
this water can be part of our boiling solution (and condense) (cool back to liquid)in the condenser, or it can be in our receiver in which we are bubbling our NOx gases into, either way we are distilling and reacting these gases with water to form nitric acid.
So we see we can convert our NOx gas to nitric acid by bubbling them into water, some hydrogen peroxide in our receiver flask can also give the No gas some oxygen converting it to NO2 and in water forming HNO3 in solution.
The longer the gas is in contact with the water the better the conversion, the amount of water in your boiling vessel can be the major factor in determining how strong the nitric will be, if bubbling your gas into water in a receiver (condensing with water there) or (condensing in a condenser with water from reaction vessel), either is distilling nitric acid, you do not need much water if bubbling NOx gases into the receiver, as the boiling vessel is giving water from the reaction also, if you are just dissolving metals in nitric acid collecting the fumes and bubbling into water and some H2O2, you can start with minimum water in receiver, and when gas no longer converts to nitric acid (concentration of acid so high the gas does not react and passes out of solution, then just add a little more water or dilute peroxide.
NO + H2O2 --> HNO3 (plus water)
NO + H2O --> HNO3
How high of percentage of nitric acid we can make depends much on our method used to make the nitric, and since we only need azeotropic nitric for refining. (So that is the strongest I will discuss, we can make this 68% nitric from figuring the mole ratio of our chemicals we add to our boiling distillation vessel (reactor vessel), or we can actually make a weaker nitric acid (such as we need for silver), which is much easier to deal with this solution when distilling, then take this dilute nitric acid solution and then concentrate this up to 68% azeotropic nitric acid solution, as strong as we need for aqua regia.
We can also distill a 35% to 40% nitric solution for use in dissolving silver.
The more dilute solution in our boiling reactor vessel the less trouble we will have with the salts that will be left from our fertilizer and our acid sodium or potassium sulfate's left behind. And we will have less trouble with this "bumping and burning salts or hardening salts inside our reaction vessel.
