Yes, it does appear that Clorox increased their Regular concentration from 5.25% to 6.0% at some point in time so that 5.25% is now the generic brand concentration.

Additional inconsistencies abound. The chlorine I get from my pool store which comes in gallon plastic jugs labeled Sani-Clor and manufactured by Hasa, says on the bottle that it has 12.5% Sodium Hypochlorite as the active ingredient, but it also says that 3-1/4 quarts added to 10,000 gallons increases chlorine by 10 ppm. This latter calculation only makes sense if 12.5% is a Trade %. The $2.50/gallon price is just for the chlorine. The gallon jugs come 4 to a plastic case and there is a deposit per bottle ($0.50 I think) and case ($1.00 or $2.00 I think), but since I always just exchange one case with 4 bottles for another my effective cost is just the $2.50/gallon plus a tiny amount of interest I'm losing (by not investing) on the $4.00 total deposit.

On the other hand, I found Kem-Tek 10% chlorinating liquid that claims 10% Sodium Hypochlorite and it says that 11 ounces in 10,000 gallons yields 1 ppm which only makes sense if 10% is a Weight % of sodium hypochlorite and even then 12 ounces would be more accurate.

So what's the bottom line? It appears that the manufacturers are not consistent. Combine that with the loss of chlorine over time and this means you can really only know what you are getting by testing it, either through a straight dilution test or by testing your pool before and after a significant addition (waiting an hour). Picky, picky, picky...

Richard