While I agree with Patrick_B, it should be noted that chlorine has a really short half-life and the more concentrated, the quicker it begins to degrade to a lesser percentage. I've often wondered if fairly frsh 8.25% from the grocery store isn't about the same strength as 12.5% from the local pool store, given the time of stock rotation......
Considering price per unit volume, pool store chlorine is MUCH more expensive than the grocery store variety.