I have a large, single crystal, very pure piece of MgF2 that I want to etch a bit with Nitric Acid (HNO3). I believe I know what will happen but I was hoping that somebody with some chemistry skills here on the dope could validate my expectations.
As I will be using a dilute nitric acid (68%), I believe the products will by HF (actually dilute hydroflouric acid) and Magnesium Nitrate (which I believe will be soluble in my dilute HNO3, HF solution). Does this sound right? After etching, I need to make sure I do not have any contamination of the MgF2 when I am finished, so I will rinse with DI water and then ethanol (Mg(NO3)2 is also soluble in ethanol I believe). I might even even scrub it a bit with ethanol to ensure I don’t have any magnesium nitrate on the surface. Everything will be done with full protection in a hood, I know HF can be some really nasty stuff. Anyway, does this sound right? Can I effectively etch with the HNO3 and end up with an uncontaminated MgF2 crystal* at the end? I need to etch away quite a bit of material, is HNO3 the best solvent for this?
I will be using this piece of material as a UV window for Hydrogen Lyman Alpha (121.6 nm) in a vacuum chamber when finished, so if there is any contamination I will fail in my task.,
I don’t really know what will happen, but I’m a bit concerned that you think commercially available concentrated nitric acid at 68% is “dilute”. Thats some serious nasty stuff in the highest concentration normal-ly (heh!) available.
THis tells me you aren’t exactly knowledgeable and comfortable with using this sort of material. Good job with the fume hood, you might want to dilute the acid by 100 fold at first to be a little safer and have some control over the reaction.
It will also turn your skin yellow/brown
I am lapping it; I am lapping it before I etch it. What I am trying to do is create an optical diffuser for vacuum ultraviolet (VUV) light. Basically, this thing is just a piece of shower glass that needs to transmit all the way down to 120 nm. MgF2 is one of the few materials that still transmits at this wavelength. The amount that light scatters with a roughened piece of glass is inversely related to the feature size of the roughening. Or, better yet, the scattering angles are proportional to the spatial frequency distribution of the roughened material. A ground piece of glass has a huge range of spatial frequency and will scatter light a bit too much for my application. If I can grind it and then soften the rough surface by etching away all the sharp corners, I will end up with a piece of glass that diffuses light in a smaller cone.
You can buy this kind of stuff for visible light from companies like Newport or Thorlabs, but nobody makes this stuff for VUV.
I have higher concentrations than 68%, and I am fully aware how nasty and dangerous it is. That said, I am a physicist and not a chemist even though I regularly use a lot of dangerous materials. I know the proper procedures to stay safe with this acid and I am aware of the dangers of gaseous HF which is probably the most dangerous part of what I am planning to do. What I don’t know know is whether I will contaminate my glass with something I can’t remove or if there is a better chemical out there to knock down the high spatial frequencies on my VUV diffuser.
MgF2 is slightly soluble in water, and its too slow for you.
When you used HNO3, you got HF, as the H and F are stronger and stick together like procreating flies. and you get MgNO3 which is highly soluble. So the speed of reaction is good, and it doesn’t produce a poluting solid.
So how about proceed and make amounts of HF ???
sounds good ? the bloody fume hood will get acid attack ! It will get pitted and start falling apart.
HF is a dangerous gas. So how about simply avoid making HF ?
You want something to dissolve the MgF2. and you do not want to make dangerous gases, so you do care that its not a low pH … high in H+…you want high pH.
So NaOH… that could make NaF , and MgOH. MgOH is not soluble in water, so you make a film of MgOH over the crystal. However that easily cleans off.
The critical factor is the scale of reaction - how big is the piece of MgF2 you want to etch?
If it is small, like <1cm, then I would do what any good chemist does - just try it Properly like - In a hood, protective clothing, correct H&S form for your juristicion, someone else in the lab - wouldn’t have any qualms about giving that a go. Ideally, you would trial the process on the smallest possible crystal / quantity of liquid you could to see if the etching process works.
Where these things go wrong - (hospitals, burns, poisoning, that sort of wrong), is a lack of appreciation of scale. Like if you were talking about a plate of MgF2 that needed several 100 mL of HNO3 to immerse - you would be crazy to try something like that straight off, with no experience of the process. Could get an unexpected exotherm and the reagents are extremely harsh.
A corrolary to that is you need to scale up incrementally - if you etch a tiny 100 mg crystal successfully, you DO NOT then say - great, let’s do it on 20 g [a scale up factor of 200]. It needs to be far more gradual. Some of the worst accidents I have heard of as a chemist have occurred like this - someone successfully does a reaction on mg scale and mentally files it away as works no problem. Scales it up to 50 g - different vessel, solvent quantity, surface to volume ratio etc etc and has an explosion. Idiotic in hindsight, but the initial microscale reaction working smoothly can be very misleading.
Properly like - In a hood, protective clothing, correct H&S form for your juristicion, someone else in the lab - wouldn’t have any qualms about giving that a go. Ideally, you would trial the process on the smallest possible crystal / quantity of liquid you could to see if the etching process works.