I have had a bottle of resublimed iodine crystals for years -> I use them for water purification as described in this 1975 Western Journal of Medicine item (see the two attached pages).
I take the small one ounce bottle with a few crystals in it in my pocket on trips – it can be used over and over again.
I use my bandana to filter out large particles when filling my 1 quart water bottle with contaminated water. I simply cover the bottle opening with the bandana and place the bottle in the river or under a running water source to fill it. Supersaturated iodine solution from the little bottle gets added to the water in the 1 quart bottle, with 40 minute wait until it is ready to drink.
I always loosen the cap on the one quart bottle and invert it after adding the iodine. The treated water gets into the threads of the cap that way, preventing illness from retained unpurified water retained there.
Larger amounts of water can be prepared / treated in the same manner as the 1 quart bottle. Just increase the number of milliliters of supersaturated iodine solution accordingly for however many quarts of water you have in your storage container. The forty minute wait period still applies, and the water stays clean so long as it remains in the closed container.
Many water purification systems purchased by campers or international travelers are bulky, expensive, prone to failure or require batteries. This method has no such disadvantages and is light / easy to carry. My bottle at home has a lifetime supply of crystals, too. Even though my bandana is not a 2 micrometer filter (usually required to stop all bacteria), iodine takes care of any bacteria passing through. Salmonella, Shigella, Campylobacter, Yersinia, Vibrios, protozoa, viruses and Cryptosporidia are all killed by the halogen.