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It was brought to Bio-Lab's attention that a New Zealand-based supplier of calcium hypochlorite is claiming that its product is "safe" simply because it has a higher moisture level than competing brands. The product in question has a 12.5 % moisture content, which is roughly 5 % higher than the level usually found in commercial calcium hypochlorite.


The product may be marginally "safer" than some other brands, but it is certainly not "safe"!


According to the internationally applicable United Nations classification system, calcium hypochlorite may fall into one of four categories depending on chlorine and moisture content:



On this basis, a product containing 12.5 % water would be classified as UN 1479. The appropriate entry in the Dangerous Goods Code defines such compounds as "… Substances which will produce highly flammable and, in some cases, explosive mixtures with combustible material."


The supplier made the claim that a mixture of its "hydrous" calcium hypochlorite with oil is "non-flammable". According to the Dangerous Goods Code and independent evaluation, this is clearly not the case. It is true that the spontaneity of combustion will be lessened because of the increased water content, but the exothermic reaction between strong oxidiser (the calcium hypochlorite) and fuel source (the oil) will proceed nonetheless. Given time, the heat generated in the reaction will dehydrate the mixture completely, resulting in combustion.


Another questionable feature of the "fact sheet" issued by the supplier was that comparisons were made between anhydrous calcium hypochlorite (i.e. no water) and the supplier’s hydrated product. It is true that calcium hypochlorite is at its most dangerous in the anhydrous form, as testified to by the restrictive classification in the Dangerous Goods Code (UN 1748). However, due to the hygroscopic nature of the compound, it is most unlikely that any manufacturer’s calcium hypochlorite would fall into that category. A 6 – 8 % moisture level (UN 2880) appears to be the norm throughout the pool chemicals industry.


In terms of potential hazard and emergency response, there isn’t a lot to differentiate between calcium hypochlorite classified as UN 1479 and that classified as UN 2880. They are both Class 5.1 Oxidisers, they are both Packaging Group II items and, most importantly, they both pose a fire and explosion hazard if stored or handled incorrectly.


Almost all commercially available calcium hypochlorite products will have a minimum of 65 % available chlorine. Inert constituents are added to reduce the fire hazards posed by the hypochlorite; these tend to be water-soluble calcium and sodium salts and, of course, water itself. The main differences between "good" and "bad" quality calcium hypochlorite lie in the overall solubility of the product, the amount of residue left after dissolution, and the level of potential contaminants in the blend. For instance, poor quality brownish-coloured material will be slow to dissolve, leave large amounts of undissolved sediment and, due to the presence of iron compounds, may lead to the staining of pool surfaces.


Given that all calcium hypochlorite is potentially hazardous, it is this quality aspect that should be considered when purchasing calcium hypochlorite. Remember that, as with any chemical, proper storage and handling is the key to minimising health, fire and explosion risks.


The above information is supplied by Bio-Lab and represents its best interpretation of available technical information at the time of preparation. The sole purpose is to supply factual information to Bio-Lab customers. It is not to be taken out of context nor used as support for any other claim not made herein.