ldanielrosa - 19-9-2010 at 17:44
I'm a bit puzzled by this- maybe it's a marketing gimmick. Any time I look at the "available chlorine" of a chlorinating product, it's more than the
actual chlorine present.
One I'm looking at now is "100% Calcium Hypochlorite with 65% available chlorine", and I can only find a shy 50% (70.906 out of 142.985). Never mind
that the product title is "Zappit Pool Shock (68% Cal. Hypo.)". What am I missing?
Magpie - 19-9-2010 at 19:47
This might be of interest:
http://www.sciencemadness.org/talk/viewthread.php?tid=2230#p...
[Edited on 20-9-2010 by Magpie]
woelen - 19-9-2010 at 22:31
Available chlorine is a theoretical quantity. It measures the amount of elemental Cl2 when excess HCl is added.
E.g. if available chlorine is 68%, then adding excess HCl to 100 grams of the solid gives you 68 grams of Cl2.
I myself have TCCA and the label on the bottle says it has 90...92% available chlorine. This means that when I add excess HCl to 100 grams of this
solid, then 90 to 92 grams of Cl2 is produced. The compound itself, however, has much less than 92 grams of Cl per 100 gram of solid (its formula is
C3N3O3Cl3), it only has 45.7 grams of Cl per 100 gram solid if it is 100% pure. The remaining part of chlorine is coming from the HCl.
There is even a compound with more than 100% available chlorine. Solid LiOCl, which sometimes is used as pool chlorinator, has an available chlorine
content of 121%.
ScienceSquirrel - 20-9-2010 at 03:18
The term available chlorine has a historical basis in early assay procedures for hypochlorite.
The hypochlorite would be treated with excess acid potassium iodide solution and this would release iodine according to the equation;
OCl- + 2I- + 2H+ ---> I2 + H2O + Cl-
( Sorry about the formatting 
)
This would then be titrated with sodium thiosulphate using starch as a indicator to give an iodine concentration twice the concentration of the
original hypochlorite.
woelen - 20-9-2010 at 04:44
Sciencesquirrel, for hypochlorites your definition gives exactly the same results as the method I outlined. The above method with excess HCl, however,
is more general and also applies for e.g. TCCA or BCDMH, non-hypochlorite pool chlorinators or even brominators.
ScienceSquirrel - 20-9-2010 at 05:20
Yes it does come to the same thing.
I think iodine was used because it is comparatively non volatile and iritating, easily soluble in excess potassium iodide solution and titrates well
against sodium thiosulphate to give a good end point with starch solution.
The figure obtained for iodine would be translated in to available chlorine for the assay on the bottle.
I think the assay would work with any other chlorinator or brominator.
[Edited on 20-9-2010 by ScienceSquirrel]