I hate mentioning brands, but in South Africa, "HTH" - dry granular
chlorine does the job. A quick Google for "HTH chlorine seems to indicate
that 68 or 70% is the norm in the US - maybe your laws are different to
ours. Anyhoo, that's what you need - Calcium Hypochlorite.
HTH is a trademark for calcium hypochlorite, made by chlorinating lime,
which leaves a portion of inert material, thus the varieties of
concentration which are left in for economic reasons.
HTH was originally an Olin brand, but now I understand owned by Arch,
and the sole US manufacturer I hear. Lately the buckets have been
showing up in even lower available chlorine, something like 50 percent.
So to my knowledge no product in the US is un-"stabilized" [sic] and 80
percent available chlorine.
By the way, "available chlorine" does not mean chlorine by weight. It
means the weight of Cl2 gas which would give the equal free chlorine in
use. For example, 1 kg of 80 percent HTH would give you as much
chlorine in the pool as 800 grams of pure chlorine gas, but only half of
chlorine gas actually goes to work. In the solid products all the
chlorine goes to work, and that's how you get such high proportions
despite the low proportion of chlorine in the molecular mass.
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