I recently started trying out DIY eLiquid and I bought 500ml or 48 mg/ml nicotine. It came in an amber glass bottle so I have been storing it in my fridge. When I plan on making some eLiquid, I take it out and let it warm a little, shake, and then put back in the fridge when done. Is this likely to shorten the life of my nicotine or make the nicotine more likely to oxidize or otherwise go bad?
Yes, you are doing it wrong.
Long term storage should be in a freezer, not a refrigerator. It's not cold enough, and there are too many fluctuations in temperature which brings up the second issue. You aren't supposed to be using that bottle every time. You should have a separate, smaller working bottle out to use. The nicotine cannot be subject to temperature changes that often. You also don't want to be opening the bottle that frequently.
The temperature changes shouldn't matter much (beyond keeping it cold), it's more the introduction of oxygen that causes problems. I agree a smaller working bottle is a better way to dispense the nicotine, since that reduces how much you need to uncap the bottle, therefore introducing less oxygen.
Maybe not since it's in the refrigerator, but if it were in the freezer, the change would matter
I buy 500mls at a time of either 100mg or 250mg so it'll last awhile. I have 100ml amber glass bottles that I divide the 500mls into and they all go in the freezer. When I mix I take only 1 able bottle out to "thaw" that way I'm not taking it all out every time
Best thing to do is separate it into smaller bottles and put it in a freezer. Take one out at a time to use.
You can get 32ml glass bottles in packs of 12 off amazon for pretty cheap. Or any other size like 16ml and 64ml. Fill them with the nicotine, put them back in the box, put the box in the freezer. Neither PG or VG will freeze at normal household freezer temps. If you use VG based nicotine it will become thicker so you may have to let it warm up before use. Don't microwave it, just let it sit out of the freezer for a few hours.
For the sake of safety, the bottles will of course be below freezing temps when taken out of the freezer and frost will form on them. Any water on your hands can freeze to them. Your hand can warm them enough to melt the frost and a wet glass bottle can slip out of your hand so careful not to drop a bottle.
I only buy enough to last 6 months, sometimes more. Always fresh and the savings lost are worth the convenience of always having fresh base to work with. Unless your concerned about legislation changing I don't see much value in long term storage, but it has its place. If you sell a little ejuice to break even I consider it a courtesy to others who vape your juice.
How long would 72mg nicotine liquid stay ‘fresh’ eg useable when stored in a freezer? Months? Years?
Mine seems fine after months (gets discolored but that's all I notice) with the worst possible storage conditions short of leaving the bottle open all the time in the heat. I would say proper storage and you should get years of life out of a bottle (and you will probably use it before then).
I've talked to people using nic thats been stored in the freezer for 2+years and they claim it's still like new.
EDIT: I hope they are right, because I recently re-distributed 2 liters of 100mg into manageable, amber glass, Argon gassed, and stored in the chest freezer. lol