Sciencemadness Discussion Board

Nitric Acid

Chemgineer - 15-6-2021 at 08:21

If I have a 10% concentration of Nitric Acid, can I boil it to increase the concentration? I note Nitric Acid boils at 83 deg C but a 68% concentration boils at 121 deg C.

Will boiling my 10% just boil out my Nitric Acid lowering the concentration?

Antigua - 15-6-2021 at 08:23

Yes - you can concentrate HNO3 up to 68%. At this concentration it's going to form what's called an azeotrope - when boiling it, you'll evaporate the same ratio of water and acid, thus distilling the same thing you heated up.

woelen - 17-6-2021 at 00:31

In practice the above is limited. When you boil nitric acid, then the vapor, leaving the liquid will contain a percentage of HNO3, which is a function of concentration of the acid. For concentrations, less than the azeotropic concentration, the concentration of HNO3, leaving the boiling liquid, is lower than the concentration of the acid in the liquid, but it is not zero. The closer you get to the azeotropic concentration, the closer the concentration of HNO3-vapor comes to the concentration of HNO3 in the liquid.

E.g. with 10% HNO3, near boiling temperature, the vapor will be almost 100% H2O.
With 40% HNO3 in the liquid, near boiling temperature, the vapor will contain around 25% HNO3.
With 50% HNO3 in the liquid, near boiling temperature, the vapor will contain around 40% HNO3.

So, concentrating the acid by boiling is possible, but the losses increase with increasing concentration of HNO3 in the liquid. In practice I think that you can go to 25% or so without excessive losses. Of course, you can collect the vapors (especially from concentrations above 20% or so) and keep that liquid as an additional dilute, but high purity, stock of nitric acid.

Amos - 17-6-2021 at 06:45

woelen is correct; when I've done this in the past even the first few drops of distillate turn blue in contact with copper. When you distill nitric acid from a mixture that has a considerable amount of dissolved solid, like the leftover material from a nitric acid preparation that has been re-dissolved, it's actually possible to pass up the azeotrope; the concentration of nitric acid keeps increasing well into the 80-90% range. So when I add water to that big solid puck of sodium bisfulfate/nitrate/sulfate etc. and continue my distillation, I usually collect a large, somewhat dilute fraction, and then a fair bit of very concentrated acid as well.

Chemgineer - 17-6-2021 at 08:12

Thanks for the detailed explanations, i'm just getting the concept straight in my head of how azeotropes work and which vapour will come off first. I'm going to open boil my 10% to half the volume first and then probably distil it to give two concentrations... then bottle it.

arkoma - 17-6-2021 at 11:29

bee careful. no2 fumes are ugly. invest in a disty kit if u dont have one kb got wet

[Edited on 6-17-2021 by arkoma]