A few things about PG (from a person who has worked with it extensively in a lab and another lab):
1. On heating PG, it will vaporize to a thick white smoke like material b/f it or as it appears to boil. It will smoke a lot, it's not true smoke but PG vapor. It is nearly the same as what you see at concerts, Donald Asshole rallies, big events - except in those cases the PG is cool and is vaporized by special machines.
2. Don't heat PG above about about 40C (esp., in a microwave or other enclose oven). 40C is warm enough for all lipophilic benzodiazepines to dissolve within half and hour with stirring. I heat mine on a glass top oven that has a "warmer" burner, not on the regular cooking burners. Water baths work, but I prefer a hot plate on low - medium (at most).
3. Keep PG solutions of research chemicals in amber bottles designed to prevent degradation by light. Bottles from the chemist that held odorless, water or ethanol based tinctures can be used - but be sure they are clean and do not have any, I mean any odor or residue at all. Best to buy a new 15-mL - 30-mL bottle with an airtight seal. I find those very handy, you can get them with droppers too - but they aren't perfectly accurate like lab quality droppers. And they tend to leak, I go with a regular amber bottles with an airtight cap. White taped on labels, protected form dripping PG as it may dissolve your label ink.
I always rinse my newly cleaned bottles and mixing flasks with fresh PG before working.
4. Store your excess PG in the original container, mark the opened date so you know. You can write "thickener for curry sauces", or "swallowing aide" on it for stealth, as these are common food uses of PG.
It will remain good for use for years when stored properly, if with cool, dark kitchen stuff, in any case store cool and dry - not refrigerator or freezer cold - cool (20 C). Allow the container and contents to warm to RT.
5. Seal your containers so that no air can enter / exit the container. Because PG picks up water out of the air and we don't know the degrading effect of water on benzos in PG solution, best to keep it dry. Although some data is online, it could very well vary by compound.
If I recall correctly chlordiazepoxide in EtOH/H2O degraded about 10% to diazepam within 24 - 72 hours when stored at RT and in clear glass. I know it's one of chlordiazepoxide's degradants in EtOH/H20 (likely a 50:50 HPLC mobile phase solution). I imagine there is a slight amount of EtOH in most PG, but it's likely not near a significant amount.
6. Dispense, dilute or otherwise work with these solutions at 20 - 25C for accuracy after they have cooled if heated for dissolution of material.
Cold PG (-18C / 0 F) will cause one to over shoot a measured dose, hot PG will likewise cause under-dosing. I've seen PG expand 10% on high heating (~35C - 40C) and it gets noticeably thicker when cold. You can look up the exact data on this online.
7. PG is an anti-microbial to a decent extent, this is why it's so ideal for many, many purposes - cooking, making smoke that a aerosol of is pushed over a crowd of 1000s. PG won't allow most microbial growth in your solutions unless it has significant water in it. There may be exceptions, but if you see shit floating in your PG you probably either stuck something dirty and possibly moist in the solution, something got in somehow - leaving the bottle open a few minutes in a British summer would do it. Will it affect the solution for certain as above - I cannot say, but it won't help.
I've never had issues with dozens of the solutions I have made but I live where the humidity rarely excedes 50% on the most humid of summer days. Typical is 5 - 15%!
8. It will evaporate but not readily, but it has a low vapor point, and with that may be a close flash point, so I'd not spark a lighter in a room full of warm PG smoke. Could be a bad thing, this vapor is hot, unlike concert / event vapor and may act differently.
9. Always check for crud and undissolved (crashed out) material, likely if some crashed out most of it did and you'll simply need to reheat.
10. Trying to drive off PG to concentrate a solution is not going to work well without some lab condensers, etc... Don't bother to try. You will not enjoy the experience.
I hope this info helps those working with PG solutions. One way to hasten the dissolution is to crush particle that you can with a lab spatula while dissolving it. This makes it much quicker.
Best
