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Topic: Can you mix HCl and H2SO4 safely?  (Read 856 times)

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Offline simenmm

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Can you mix HCl and H2SO4 safely?
« on: March 11, 2019, 09:26:12 AM »
Hi, non-chemist nanotech PhD student who needs to work with harsh chemicals here. I'm looking to make an etchant containing both HCl and H2SO4. It needs to be quite acidic, currently we are using 20% HCl in our process. However if we were to mix the two directly we'd make a lot of HCl gas, which is not cool in our multiuser environment. Is there a way to make a solution like this safely? There can be other ions present.

Offline chenbeier

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Re: Can you mix HCl and H2SO4 safely?
« Reply #1 on: March 11, 2019, 04:39:27 PM »
What is the concentration of the sulfuric. Better is to use sodium chloride and sulfuric acid, because the HCl will produced then. Conc. sulfuric adsorbs a lot off water and evaporate HCl gas, not a good idea.

Offline phth

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Re: Can you mix HCl and H2SO4 safely?
« Reply #2 on: March 12, 2019, 01:48:08 AM »
Hi, non-chemist nanotech PhD student who needs to work with harsh chemicals here. I'm looking to make an etchant containing both HCl and H2SO4. It needs to be quite acidic, currently we are using 20% HCl in our process. However if we were to mix the two directly we'd make a lot of HCl gas, which is not cool in our multiuser environment. Is there a way to make a solution like this safely? There can be other ions present.

Mix before use? It seems simpler if you have a problem of a premixed solution fuming while use to switch the acid to a less volatile one.

Offline Borek

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Re: Can you mix HCl and H2SO4 safely?
« Reply #3 on: March 12, 2019, 03:57:02 AM »
Better is to use sodium chloride and sulfuric acid, because the HCl will produced then.

I don't see how it it is better. It is exactly as dangerous, if not worse. As there is less total water, chances that HCl will get airborne are much higher.
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Offline simenmm

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Re: Can you mix HCl and H2SO4 safely?
« Reply #4 on: March 12, 2019, 04:14:45 AM »
What is the concentration of the sulfuric. Better is to use sodium chloride and sulfuric acid, because the HCl will produced then. Conc. sulfuric adsorbs a lot off water and evaporate HCl gas, not a good idea.

The sulfuric acid is 99% and the HCl is 32%.  I don't know what final concentration is necessary because this is a novel process. I've seen youtube videos where people pour H2SO4 onto solid NaCl and it produces a lot of HCl gas, so I'm worried about that.

Mix before use? It seems simpler if you have a problem of a premixed solution fuming while use to switch the acid to a less volatile one.

Sure, I need it to cool to room temperature before use anyway. But I still need to make the solution in a safe way.

Offline AWK

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Re: Can you mix HCl and H2SO4 safely?
« Reply #5 on: March 12, 2019, 06:44:57 AM »
The solubility of HCl in anhydrous H2SO4 is very low. Probably also in aqueous solutions of H2SO4, this solubility will be much lower than in water.
What is the final concentration of H2SO4? When diluting with H2SO4, half of the heat is released at a molar ratio of 1: 1. This temperature causes a huge amount of HCl to be released in gaseous form.
I would advise you to dilute the sulfuric acid so that the mixing will be done in 1: 1 volume proportions. But the solution must be cooled intensively. Mixing the solutions must be done with good ventilation! I am afraid that achieving 20 % of ​HCl will be very difficult, even at 0 C.
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Offline chenbeier

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Re: Can you mix HCl and H2SO4 safely?
« Reply #6 on: March 12, 2019, 07:49:34 AM »
My question is, what material do you want to etch.  Normally it is not necessary to have high concentrated acids. In my expierence in the plating industry the etchants contain sulfuric acid 30 -100 g/l , Chloride mostly only some ppm. And the make up is then to add Chloride first in water and then add the acid.
As already mentioned. HCl will not survive in concentrated acid, because it adsorbs the water and the HCl will evaporate.

Offline simenmm

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Re: Can you mix HCl and H2SO4 safely?
« Reply #7 on: March 13, 2019, 05:04:44 AM »
I don't want to reveal too much about our process because we will publish a paper about it.

Offline hanzdolo

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Re: Can you mix HCl and H2SO4 safely?
« Reply #8 on: April 16, 2019, 08:07:52 AM »
I'm an engineer dabbling in chemistry as well. If you're looking to etch copper, you can make copper chloride or ferric chloride(iron(iii) chloride). I can't really see a reason to combine HCl and H2SO4 because it won't really do much. In fact, in my experience aqueous HCl dilutes the H2SO4 making it weaker.

Peroxymonosulfuric (H2SO4+H2O2) oxidizes diamonds to CO2 under the correct conditions, and will probably do the job, but if that fails Peroxydisulfates are known etchants and can easily be produced electrochemically.

Peroxydisulfuric acid(H2S2O8) can be prepared via anodic oxidation of H2SO4 and can be reacted with aqueous ammonia to produce Ammonium Persulfate which is an excellent etchant.

I'm not sure what you're trying to etch, but I think the Peroxydisulfuric Acid and/or it's conjugate bases should do.

Oh, one more, Aqua Ragia (HCl +HNO3) dissolves gold, so that's another option.

I hope I was able to help.

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