I think the Milwaukee storage solution is some sort of saline water. When it dries it leaves a thick, salt-like residue.

Water tends to 'go bad' with algae and muck. Not sure the storage solution works any better or worse though. As long as you don't let the electrode dry-out I'm not sure it makes much difference. But Milwaukee does recommend it so I got it. (just before dropping the damn ph pen and cracking my new electrode...)