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I'm following a protocol from some old literature (1962), and I want to make sure I'm being safe. It asks you to dissolve some salts in 150 mL water + 25 mL concentrated HCl, then evaporate to dryness at 100 °C. So the question is whether the acid is dilute enough that this is safe to do in a benchtop drying oven (i.e., not inside a fume hood). I imagine back in 1962, you might have done this drying on steam bath inside a fume hood, but I haven't seen a steam bath around for ages. If this is not safe, please suggest how it could be done safely (I do have a fume hood, but the oven won't fit inside).

Telling me to check with my PI or safety officer isn't going to be helpful in this case.

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    $\begingroup$ It depends on the nature of the salt.There are a huge amount of different salts. Some support a temperature much higher than 100°C. Some are decomposed already at 100°C or a little bit higher. Without knowing the nature of your salt, nothing can be proposed. $\endgroup$
    – Maurice
    Sep 5, 2023 at 19:12
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    $\begingroup$ @Maurice, I believe it's the HCl fumes that are of concern, not the solid. $\endgroup$ Sep 5, 2023 at 23:15
  • $\begingroup$ What is the salt to isolate? $\endgroup$
    – Buttonwood
    Sep 7, 2023 at 9:20
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    $\begingroup$ Personally, I'd go for it - back then, you'd have a scotch in one hand and a cigarette in the other. Just do it outside or in your garage. $\endgroup$ Sep 8, 2023 at 13:48

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The $\ce{HCl}$ vapor given off certainly could be an issue. It's not only a health hazard, but would likely corrode the oven. Many stainless steels and nichrome are attacked at elevated temperatures by moist $\ce{HCl}$ vapor.

Since you've got less than 200 mL, you might dry the container, e.g., a beaker, inside a desiccator that can take that mild heat. Add a safe alkali (e.g., $\ce{CaCO3}$ or or $\ce{MgOH}$) to the desiccant mix, enough to neutralize 25 mL of concentrated $\ce{HCl}$. If there is an exhaust from the oven, you might pipe that through some marble or limestone chips.

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    $\begingroup$ The application of vacuum of a membrane pump (down to $\approx \pu{20 mbar}$, they should have a mark stating the permitted pressure next to the valve) on desiccators is easier done, than gentle (and evenly distributed) heating of them. To place their lower part in a water bath is clumsy and impractical. $\endgroup$
    – Buttonwood
    Sep 7, 2023 at 9:18
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25 mL of 32% HCl will weigh 28.975 g at 20ºC. This is about as much as you need to clean a toilet (which then gets flushed down the drain, to no great environmental damage). Ref 1.

If you do the preparation outdoors with a moderate wind, you could assume safety. Otherwise, you could trap the HCl or divert it. Trapping the HCl would involve a lot of glassware and preparation.

Diverting it could be done simply with an aspirator (Ref 2)(purchased for less than $5).

enter image description here

Attach the aspirator to a water source and lead a tube from the aspirator to the vessel to be heated.

enter image description here

Ref 3 . Place this tube at the opening, as in the image, or loosely, slightly inside. If placed loosely, the aspirator will draw air from around the vessel opening - and from within the heated vessel - along with the HCl.

For greater safety, you could consider reducing the size of your experiment. In addition, depending on the salts involved, you might be able to attain complete solubility with less water - especially since the mixture will be heated to 100ºC.

Ref 1. https://www.handymath.com/cgi-bin/hcltble3.cgi?submit=Entry

Ref 2. https://www.nascoeducation.com/chapman-aspirator-filter-pump-sb53131.html?gad=1&gclid=EAIaIQobChMItIPa8baXgQMVIQizAB2iMQE2EAQYByABEgK-d_D_BwE

Ref 3. Setting up a water aspirator pump for vacuum distillation

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On the back of an envelope: you start with $\pu{25 mL}$ of concentrated $\ce{HCl}$. Often seen among the suppliers, this is about $\pu{37.5 mass\%}$, or $\pu{12 mol/L}$ (reference), clearly corrosive. A dilution in $\pu{150 mL}$ of water yields a total volume of about $\pu{175 mL}$. With a dilution by a factor of 7 (you recall $n = c \cdot V$), the concentration now equates to $\pu{\approx 1.7 mol/L}$.

Your question does not detail out what salt you are going to isolate. An example could be light weight (organic) amines eventually isolated (thanks to differences of p$K_a$ values) as ammonium hydrochlorides. Here, the addition of the acid is just the necessary quantity to obtain the corresponding protonation, with practical no remaining corrosive acidity of $\ce{HCl}$ left. (Check with a droplet of the diluted solution on a stripe of pH paper.)

What scale we talk about?

$$n = c \cdot V$$

$$n = \pu{12 mol/L} \cdot \pu{25E-3 L} = \pu{0.3 mol}$$

For anilinium chloride obtained from aniline, for example, this corresponds to about $\pu{39 g}$ of product (with a reported density of $\pu{1.68 g/cm^3}$, less than $\pu{25 mL}$ occupied volume if it were a compact powder). So pick your heating plate from the shelf, a wide beaker, and a magnetic stirring bar to concentrate the diluted solution with gentle heating while stirring. This is a small setup to fit into a hood. Add the diluted solution in portions; when nearly dry, remove the magnetic stirring bar and stir with glass rod instead. As for OSHA, keep the sash down, protect your eyes, skin, clothing. (It doesn't leave a good impression about the chemistry and work ahead if you can't ask a TA / colleague [of your, or of an other group of the department], nor supervising PI for a safety check...)

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  • $\begingroup$ I'm the PI, and the safety officer doesn't have enough practical knowledge to be of any help. I appreciate you for taking the time to give this some thought $\endgroup$
    – ike9898
    Sep 9, 2023 at 14:22
  • $\begingroup$ @ike9898 Multiple days after addressing the question, you still do not indicate the chemistry intended which would narrow, and possibly yield a fitting answer. That is to say: HCl used to purify an inorganic salt, to digest a compound (salt metathesis), to precipitate an amine (the example above) as a couple of options. This is, not only because chemistry.se has such a category to vote for closure of a question, but more general, an instance «lack of detail, too little information shared». $\endgroup$
    – Buttonwood
    Sep 10, 2023 at 17:28
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Some steam baths require a steam source, either a connection to piped stream ("house steam") or a local steam generator. There are, however, items that look very much like the old steam baths (concentric rings which you remove to match the size of your vessel), but instead of being connected to a steam line, they just sit on an ordinary hot plate (inside a fume hood). Presumably this is slower than using house steam but that's acceptable for infrequent use.

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  • $\begingroup$ As it’s currently written, your answer is unclear. Please edit to add additional details that will help others understand how this addresses the question asked. You can find more information on how to write good answers in the help center. $\endgroup$
    – Community Bot
    Sep 6, 2023 at 21:41
  • $\begingroup$ And it does not answer, "please suggest how it could be done safely." Your proposal vents 25 mL HCl to lab! $\endgroup$ Sep 7, 2023 at 0:39
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    $\begingroup$ What is the purpose of the HCl. If it doesn't react you are evaporating 25 mL of concentrated HCl. A steambath is a temperature control at 100C aka boiling water. Direct heating of such a solution will concentrate HCl to the high boiling azeotrope about 20% HCl Why not simply distil off the water and acid and collect the distillate? $\endgroup$
    – jimchmst
    Sep 7, 2023 at 7:37
  • $\begingroup$ I'm new here. I've rewritten my answer to be more generally useful. I'd appreciate it if you could remove the down votes $\endgroup$
    – ike9898
    Sep 9, 2023 at 14:35

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