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What's your method for storing air/moisture and temperature sensitive reagents? For example:

  • A solid. Should I just blow some argon, screw the lid, put parafilm and put it in the fridge? Can a dessicator with silica be put in the fridge?

  • A liquid. Do you trust the septa seal that come with some bottles? I guess that after a few punctures, air can get inside the bottle. Moreover, sometimes I get bottles of air sensitive reagents with no septa. What is your procedure with this? Use a rubber septa or schlenk flask, maybe?

  • In general, do you trust your methods or purify your reagent before using it? For example, for something like $\ce{BF3 * OEt2}$.

Thank you in advance.

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  • $\begingroup$ I am not much of a synthetic chemist, but here are my 2 cents. Schlenk + argon line + septa is good for one day's work, not for storage. Other than that, methods vary. Some things may sit in a fridge, some in a glovebox, and some have to be purified or prepared each time before use. $\endgroup$ Nov 4, 2023 at 23:13
  • $\begingroup$ For future reference: for the body of questions, answers, and comments, chemistry.se offers to use mhchem as a comfortable method to add chemical equations. $\endgroup$
    – Buttonwood
    Nov 5, 2023 at 10:25

4 Answers 4

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Three thoughts:

  • As soon as you pierce the septum, it is perforated. Depending on what you aim to store in the bottle, the material of the septum and its age (not age, but what happened to the septum [light, temperature, time of exposure to chemicals so far including the lab dishwasher, etc]), you can't trust the thing as much as you did prior to this. It is noteworthy that lids and septa are available in different materials. The gradual move away from fluorinated materials (PFTE/Teflon) with new seals of different chemical resistance and mechanical properties (more prominently seen on seals on GCMS vials) is a story on its own.
  • You can degrade a chemical in a bottle closed by a septum, but the cause isn't the perforated septum in first place. If the chemical is stored in the fridge to compete decomposition, and your first puncture (esp. across an already pierced septum) is the needle to draw the reagent solution instead of the one with gentle overpressure by the Schlenk line to add dry nitrogen (meh), or argon (better for its density, but more expensive), chances are increased that for pressure release you draw (humid) air back into the bottle. Or you open the still cold container, and humidity condenses on the walls, this water starts to decompose your chemical. The cross on Aldrich's SureSeal septa helps you (like a grid) to minimize the number of punctures, though.
  • What do you understand as storage? If it is long term, infrequent draw, and your chemical is compatible to glass (there a multiple sorts of glass), than sealing it in a glass vial still is the most reliable option in different volumes, white glass/brown glass, under own atmosphere/under inert gas; just a matter of equipment and some training for dexterity.

As for the purification of chemicals, the typical goto in organic chemistry (though not only covering $\ce{C_xH_yO_z}$, but a fair amount of inorganics), is Perrin/Amareggo' Purification of Laboratory Chemicals.

The idea of putting a dessicator into a fridge is foreign to me: typically too heavy to support it on the grid, it occupies too much volume per volume of the sample of interest (don't forget the valve to evacuate the inner), it is too expensive if this falls to the floor and shatters (not only for the chemicals to store + the drying agent).

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    $\begingroup$ Fresh parafilm over septa each time your use it will help preserve the contenrs $\endgroup$
    – Waylander
    Nov 5, 2023 at 17:04
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Your question can not be easily answered. The only valid reply must be: It really depends on the individual case and the reactivity as well as physical properties.

  • Is your compound easy to evaporate? If not, it is good practice to flame dry a Schlenk flask and to flush the flask multiple times with argon. In the best case, you have dried the argon prior with driderite for example. When the substance is transfered to the flask either through the sept or in argon counterflow, it can be put under high vacuum for elongated periods of time to remove residual water and oxygen and than again flushed with argon. Best to to this multiple times. For some hygroscopic compounds, also additional heating might be necessary in combination with high vacuum to get it really dry.

  • If your compound can be easily evaporated, and has a boiling point significantly different from water, destillation in a previously dried apparatus might be an option. I would add activated molecular sieve (see below) to the sample and collection vessel to keep the water isolated.

  • As it was pointed out already, septs are prone to puncture and more often you use them, more leaky they will get. For longer term storage, I put para film around the sept. Personally I found, that silicon septs are better seals, but not as chemically resistant in comparison with rubber. I perform some reactions which destroy the sept after one single use, so keep this in mind that you have to replace them more or less often.

  • When you are using a chemical that is sealed by the manufacturer, use always a balloon with inert gas, ideally argon because it is heavier than air to repressurize. Always use a balloon that was flushed several times beforehand with inert gas, otherwise it will contain residual water and oxygen.

  • Melting and sealing glas valves gives you a very good seal, but you need special equipment (oxygen flame, inert atmosphere). In combination with overpressure this might even be dangerous. Also, you can not easyly remove your compound from such a container.

  • If your substance of choice is a liquid, you may want to put activated, that means very dry molecular sieve to the flask, to keep it dry for a longer period of time. The proper way of activating molecular sieve is to heat it to 400 degrees Celsius under high vacuum until the pressure stabilizes. But be aware that there are substances that will deteriorate the molecular sieve. You can not use it for hydrofluoric acid or a reaction which contains oxalylchloride because they will attack the silicon. I have however used molecular sieve for drying TBAF. If a substance is extremely wet, molecular sieve is also not the perfect choice to dry it, a better way would to start with $\ce{Na2SO4}$ first and than apply molecular sieve. I had to do this once for wet acetonitrile that was delivered in a fresh, sealed bottle. Molecular sieve can be obviously also used in the fridge or the freezer but continued freeze cycles may limit the lifetime.

  • For storing water sensitive solids I would use an excicator. The best way probably would the use proper high vacuum grease and use Schlenk flasks that are put under vacuum and than placed in the excicator. Subsequently the excicator itself would be put under vacuum. For the drying agent I use anhydrous $\ce{CaCl2} that was heated to 250 °C under vacuum for a couple of hours.

  • Regarding temperature dependence of compound stability, one has to keep in mind, that if the flask and its internal atmosphere are cooled down in the freezer, this creates a negative pressure and might suck humidity into the flask. You could counteract this be cooling a flask to freezer temperature under protective atmosphere first. If I have a compound that is very prone to hydrolysis and is kept in a freezer I put it immediately to the Schlenk line before it warms up, so the moisture is evaporated before it can react.

  • For substances that need to be cooled and still kept dry, I would use a Schlenk flask under argon and put these in a plastic container either with silica gel or $\ce{CaCl2}$ to avoid condensation water upon warming up to room temperature.

  • There are substances that are so reactive, that they are delivered to you in sealed glas valves. Example are TMSI or N-(trimethylsilyl)dimethylamine. In these cases it is best to order just quantities that can be readly consumed in a small timeframe, because they will often attack most sealing materials (TMSI especially). On the other hand, due to their reactivity some substances like this might be used to dry your reaction mixture very efficiently.

  • Other ways to dry liquids, especially solvents, are the usage of elemental sodium or destillation over benzophenon. Here it is always necessary to check the compatibility. For example one can not use sodium with DCM but with THF.

Useful info on drying solvents:

https://chem.libretexts.org/Ancillary_Materials/Demos_Techniques_and_Experiments/General_Lab_Techniques/Drying_Solvents

http://books.google.com/books?id=SYzm1tx2z3QC

  • When you are not sure if a sample contains water, it is always good practice to do a quick proton NMR to check for a residual water peak. This will obviously just work when the water is not reacting itself with your compound or only very slowly. In this case you can test the fresh compound and also deliberately add a little bit of water to see how the spectrum changes to have a reference chemical shift for hydrolysis deterioation product. For many compounds, this works especially well with heteroatoms like 31p. (Here you can give very easily give an estimate in percent for deterioation just by comparing the integrals.

  • A special case might be the drying of gases: As mentioned above one can use an indicator drying agent like driderite that changes it's color when it absorbed specific quantities of water. Alternatives would be the stream though concentrated sulfuric acid or through granulate of $\ce{NaOh}$, $\ce{CaCl2}$...

  • In some cases it is not sufficient to keep a compound or reaction dry but it is necessary to remove oxygen beforehand. For liquids and solvents I flushed solvents with argon. Helium might also work in some cases. Otherwise sodium benzophenone ketyl not only reacts with water but with oxygen and creates a blue color, when no oxygen, water or peroxides are present.

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  • $\begingroup$ Thanks for the comprehensive answer. However, I have so quick doubts that I hope you could help me with. - If the reagent has to be stored under inert atmosphere, as is sensitive to oxygen and moisture, would you store it anyways in a vacuum desiccator or simply in a Schlenk flask under argon/nitrogen? - Does your institution pose a problem if you store reagents in recipients without a proper sticker with pictograms and information? Or you place the sticker (or a new one) on the new recipient? $\endgroup$
    – user78790
    Nov 7, 2023 at 9:29
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To store air-sensitive and temperature-sensitive reagents:

Air-Sensitive Reagents:

  • Use air-tight containers.
  • Inert gas can help displace air.
  • Store in a cool, dry, dark place.

Temperature-Sensitive Reagents:

  • Follow recommended storage temperatures.

  • Use appropriate storage equipment.

  • Prevent temperature fluctuations.

    Label containers clearly, monitor conditions, and handle with care, following safety guidelines.

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Regarding the third bullet, mentioning BF3·OEt2, two answers.

1, As has been noted by at least one other, in the safe practice of preparative chemistry, especially involving anhydrous/air-sensitive reagents, there are no good, sufficient answers to overly general questions. Purification, handling, and storage of each reagent, product, solvent/solution, is an individual problem to be addressed.

2, With regard to the specific reagent mentioned as an example, BF3·OEt2 [CAS No. 109-63-7]—taking this not as a general example, but as a specific question of how that reagent should be handled, purified, stored:

Veronica Cornel addresses the BF3.Et2O reagent in her entry in the Encyclopedia of Reagents for Organic Synthesis (EROS), here: https://onlinelibrary.wiley.com/doi/10.1002/0471264180.or013.01

Cornel addresses the observation that advantitious air oxidizes (and so darkens) commercial preparations, making redistillation prior to use the standard course for careful work (because the if reagent titre varies with time since distillation, a portion used in chemistry that is not redistilled is being used without knowing actual concentration). She cites Zweifel and Brown's 1963 Organic Reactions chapter with regard to use of Ca(OH)2 for capturing impurities and addressing problematic bumping. [https://doi.org/10.1002/0471264180.or013.01]

She also generally addresses this reagent's precautions, and handling and storage, but in this regard we refer to our longer post here, that such reading is not sufficient to teach these things.

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  • $\begingroup$ Feel free to flag questions or answers that seem to be a safety issue. The community can close questions they feel are off topic. The moderators can do so as well. $\endgroup$
    – Karsten
    Mar 24 at 2:39