The two most common chemicals used to purify water during an emergency are bleach and iodine. There are others like Potassium Permanganate, Sodium, Chlorite and Chlor-floc tablets or crystals, but they are not usually found or kept on–hand unless someone has purchased emergency supplies or is an avid outdoors person.
Bleach is almost always readily available and should always be the first choice for water treatment. If iodine is used, it is recommended that it contain at least 5.25% sodium hypochlorite to be the most effective, though that is not always possible. If possible, try to collect water that is odor-free and free of large pieces of floating material.
If it is an emergency situation, the water may be cloudy with dirt and other contaminants. The following procedures assume you will need to first remove as much dirt and other materials first.