What is it
Sodium Hypochlorite is also commonly called liquid chlorine or bleach. When it is labeled as bleach, it comes in concentrations of 3 or 6%. The bleach labeled 3% is often sold in deep discount and dollar stores and is not a cost effective source of chlorine for the pool. Bleach labeled 6% is the most common strength of bleach sold in stores. Chlorinating liquid labeled for pools is sold in 10 or 12% strength, and the cost varies from region to region.
When buying bleach, you want only plain, unscented bleach. Additives such as fragrances, thickeners (splashless), or surfactants (outdoor bleach) may cause foaming. Bleach with fabric protectors (chloromax technology) may also cause foaming.
In some regions grocery store bleach is a better value than pool store liquid chlorine. You can use the PoolMath app to compare how much 6% bleach will cost vs 10-12% liquid chlorine cost to raise the free chlorine in your pool to the target level, and then buy whichever is the better value. Do a google search on the Trouble Free Pool homepage for the current year's bleach price thread. For example, "2019 Bleach Prices" pulls up this thread
If you buy liquid chlorine at a big box store, avoid bottles that have been stored outside in the heat. Heat accelerates the breakdown of bleach, lowering its potency. Also check the Julian date code on all bottles of chlorine. The date code will show the date the chlorine was bottled. You want to buy the freshest chlorine available.
Store your liquid chlorine in a cool, dry place out of direct sun if possible. If you must store it outside, consider a deckbox to provide some protection from the sun, and only buy enough chlorine for a week or 2 at a time. If you can store your liquid chlorine inside you can buy larger quantities to take advantage of any good sale prices during the season.
What else does it add
There is extra salt in the liquid chlorine due to the production process. 1 ppm chlorine added by sodium hypochlorite adds 1.64 ppm salt.
How to add
Bleach should be added to the pool by pouring slowly in front of a return jet with the pump running. You should pour slowly enough that pouring an entire jug takes more than one minute, preferably two minutes. Leave the pump running for at least 30 minutes after adding bleach.