I thought it best to post this as I've seen two posts in the last couple of days from folks with dodgy nicotine.
The UK is about to drop the TPD so if you're buying nicotine that's on sale in order to stock up then be aware that you could receive old stock. Before purchasing I'd suggest contacting the company you're buying from and asking when the nicotine was made/batched/bottled etc.
how could one tell that nicotine is old if he/she had already purchased some? any specific smell or color to look out for?
Cleanest highest quality nicotine should be clear and smell like nothing (unless you can smell pure Pg then that's how nicotine solution should smell). In reality your nicotine will be either slightly yellow (oxidised nicotine, fine to use) through very bright yellowish-red (oxidised nicotine, fine to use but might feel harsh. Either use 1-3mg less than normally or just put up with it) going through black nicotine (trash it) or even pink (pink nicotine have been treated with something I don't remember name of, it's fine to use and it was treated with spray that remove air from the bottle, I think it's argon spray but dot take my word for it)
Nitrogen will give it the pinkish hue.
Ideally it should look and smell very much like the base it's in. I generally first taste new nic by dropping a single drop on my tongue. I just got some 100mg from Carolina, and a drop of it tastes pretty much of pepper and nothing else. I'd like to do a small blind comparison with other nic soon, though I'd need to dilute it if tasting more than a couple of samples.