The National Fire Protection Association (NFPA) has ratings known as Standard 704 for flammable and hazardous substances. A detailed description of the ratings may be found here.

In addition, the NFPA Code 430 has classifications for oxidizers mostly relating to their storage requirements (see this link and this link).

The following are the ratings for chlorine products used in pools (see this link for oxidizer ratings).

CHLORINE TYPE ..... HEALTH ..... FLAMMABILITY ..... REACTIVITY ..... OXIDIZER
..... Trichlor ................ 2 .....................0 ....................... 2.................Class 1
Dichlor (dihydrate) ...... 2 .................... 0 ...................... 1 ............... Class 1
Dichlor (anhydrous) .... 2 .................... 0 ....................... 2 ............... Class 3
Calcium Hypochlorite . 3 .................... 0 ....................... 1 ............... Class 2 (<=50% concentration), Class 3 (>50% concentration)
Lithium Hypochlorite .. 3 .................... 0 ....................... 1 ............... Class 1 (<=39% Avail. Chlorine), Class 2 (>39% Avail. Chlorine)
Sodium Hypochlorite .. 2 .................... 0 ....................... 1 ........... not an oxidizer (<=12.5% concentration; see this link)

Remember that the above is for the concentrated form of these chemicals, not for their very diluted use in pools and spas. The only real controversy over the years has been with fires associated with storage of high concentrations of Cal-Hypo. None of the chlorine products are flammable in the sense that temperature alone does not ignite the products, but the higher classes of oxidizer can self-sustain their burning once they get started by having the oxidizer come in contact with other substances such as organics (oils).

Richard

Thanks for posting that, Richard. I read through most of the links you cited, and I now feel a lot better about the carboys of chlorine I have in a Rubbermaid box outside by the shed. But I'm a bit worried by the 2 rating for Reactivity for Trichlor. Before I found you guys Leslie's sold me a big bucket (25 lbs?) full of 3" pucks and I've been saving them thinking that eventually I'll need to add CYA. Why waste it, right?

These are sitting up on the top shelf of the closet in the den. Should I be worried about water getting to it? It's not too likely, but if the downside risk is high, then maybe I should just get rid of it?

Very interesting data. Intuition would've made me guess that the solid form was less worrisome, but that doesn't seem to be the case. Oh well, as I tell my students, intuition is mainly a way to arrive at the wrong answer very quickly.

Thanks again, Gary

P.S. Same goes for the cal-hypo I used to use thinking it was "safer to store" than the liquid chlorine. Class 3 oxidizer!?

The "reactivity" of Trichlor has to do with it's very concentrated chlorine content and high acidity, both of which make this potent when combined with certain substances such as inorganic nitrogen compounds (e.g. ammonia) or organics (e.g. oils). Though having it stored in a sealed bucket is not normally a problem, I wouldn't keep that inside the house but instead in an outside well-ventilated shed. Even so, if the container is not air-tight, you can find any metal nearby becoming rusted.

Cal-Hypo is more troublesome only because once it gets started reacting with a chemical, it can be more self-sustaining in its breakdown reaction so can accelerate a fire. You can usually get Cal-Hypo in 48% strength which is less reactive than 65% or especially 73%.

The main risk with the liquid forms of chlorine (e.g. chlorinating liquid and bleach) is if the bottle leaks since the liquid will spread while chlorine in solid form won't (except for gas). Most bottles of chlorinating liquid are usually vented which means that they can emit chlorine gas (or hypochlorous acid gas) so should be stored in a well-ventilated area (and if tipped over, they can leak liquid).

Richard

Interesting link, thanks for posting!

You mentioned that the tri-chlor will cause nearby metal to rust. Is this also the case with vented bottles of 12% liquid chlorine? I'm wondering because I store mine in a shed with snowblowers and lawn equip. and I don't want it causing any problems. There's usually anywhere between 4 and 8 1 gal. jugs in there at any one time.

Thanks,
Adam

Stored chlorine runs the risk of corroding metal when it is not in an airtight container. Liquid chlorine/bleach is almost always sold in airtight containers, and so does not normally pose a risk. Trichlor containers are not normally closed tightly enough to become airtight, either the container isn't airtight to begin with or the lid is not put on tightly enough to achieve an airtight seal, and so can often corrode nearby metals.

How much corrosion you actually see also depends on how well ventilated the storage area is. A well ventilated storage area will limit both the range and severity of any corrosion.

The caps on chlorinating liquid may or may not be air tight depending on where you purchase them. The ones I get from my pool store have vented caps so are not air tight if pressure builds up. This is intentional since impurities in the chlorine can lead to breakdown of chlorine to produce oxygen gas, but if the pressure builds up then the vented cap releases some gas and this will include some chlorine gas. If this did not happen, then the container could bulge and burst sending liquid spraying which would be worse. Such containers must be stored upright and have warnings about not tipping them over.

The quality of the chlorinating liquid product matters as shown in this link where you can see that lower quality 12% chlorinating liquid can build up oxygen gas of over 50% of the volume of the liquid over one month and clearly that is a product that should have a vented cap.

Technically, if pressure isn't building up, then gas won't be escaping from a vented cap container (depending on cap design). Bleach usually doesn't have a vented cap since it's concentration is lower so is less likely to build up pressures from contaminants (mostly metal ions).

I keep my chlorinating liquid in the pool shed and that has equipment in it, but it's also well ventilated. I've been monitoring for any potential damage and don't see any, but pool people have told me it would be best to have the chlorine stored separately away from equipment. In any event, you should make sure that the cap is on tightly since a loose cap of any kind will allow more gasses to escape.

Technically, Muriatic Acid is probably much worse in terms of storage and venting issues as far as metal corrosion is concerned. There are several people who have reported storing Muriatic Acid in their garage and found that nearby metal (bicycles, etc.) rusted. A post about rusting from Cal-Hypo in a shed is here, and from Muriatic Acid in a trunk here and in a store here.

Richard

Following Richard's advice, I moved the big bucket of Trichlor outside to the storage box by my shed. It's well enough ventilated, I think, but temperatures are 85+ and relative humidity is 90%+ for much of the year here.

Is that a problem? I initially put it inside because of the "store in a cool dry place" sort of instruction on the label. Will the heat and humidity cause a safety issue? Or will it make the pucks degrade in some fashion to being useless?

These blasted pucks were what put my pool over the edge in the first place, and were also my first victimization by the pool store. And now this storage issue. But given the cost I hate to just dispose of them. Maybe I could just frisbee them through the window at the pool store some night. At least I'd get my money back in entertainment ...

If the container is well sealed, then the humidity will not be a problem. The pucks should do OK even at the higher temps. It's chlorinating liquid that degrades noticeably faster at higher temps.

Thanks for the reassurance. I'll reinforce the seal tomorrow if that's the best solution.

Do you think the frisbee idea is a good one? I can make video to share ...

As a related aside, the Costco 4-in-1 dichlor dihydrate 'package' (a mix of 4 components) is actually classified as a non oxidizer according to the patent application.

Read what they say! They're not saying that the 4 in 1 product isn't an oxidizer. They're saying that a CLARIFIER based on the application wouldn't be.

Hi Bama Rambler,
I did read the patent, quite a few times. It's a hard read, so I very well could have gotten it all wrong - but - it seemed pretty clear to me that they were saying one of the benefits of mixing the four things together was that it lowered the DOT requirements.

Anyway, if it matters, I think JasonLion summed it up nicely when he said: