[ignoic]

Why is anhydrous sodium hypochlorite less stable than its calcium counterpart? Thanks much in advanced!

[Corribus]

When I saw this question, a sketch of an explanation came to mind. But even if I'm absolutely sure what an answer is, I rarely answer a question without doing at least a quick scan of the internet to make sure I'm not way off. I did that in this case as well and the fact that I couldn't find any obvious mechanistic explanation (even in lay terms) after about 10 minutes told me the answer is probably complicated. Ok, next stop is usually the primary literature, but no, not a whole lot there either. Forget about comparisons, then, let's start with what is known about decomposition mechanisms for anhydrous sodium hypochlorite. Even here, there's not a whole lot, and what there is is complicated.

Here is one study on anhydrous sodium hypochlorite: Lister et al. DECOMPOSITION OF SODIUM HYPOCHLORITE:THE UNCATALYZED REACTION, CANADIAN JOURNAL OF CHEMISTRY. VOL. 34. 1956, 466 https://cdnsciencepub.com/doi/pdf/10.1139/v56-068

And one on calcium hypochlorite: Bibby and Milestone, J Chem Technol Biotechnol, 1984, 34, 8, 423-430, https://onlinelibrary.wiley.com/doi/epdf/10.1002/jctb.5040340804

Focusing on sodium hypochlorite for a moment, the cited reference indicates multiple pathways for decomposition, with different products and rates that are sensitive to different conditions. Carbon dioxide, atmospheric water, and the presence of trace metals also influence the kinetics. Similarly, calcium hydroxide also has several decomposition routes available and relative humidity plays a large role.

Given the complexity of these decomposition chemistries, there is probably no simple explanation why anhydrous calcium hypochlorite is more stable than sodium hypochlorite. We don't even have a lot of good structural information to help us out. With a deep dive into the literature maybe you could come up with a hand-wavy explanation but not sure how easy it would be to verify or how useful it is.

If you limit yourself to the anhydrous hypochlorites going up and down a single group (lithium, sodium, potassium), here you may have some more luck and I'd probably start by trying to draw an analogy to the stability of the carbonate or nitrate series. Likewise doing up and down the hypohalite series (chlorite, bromite, iodite, etc.) you could probably come up a decent hypothesis. In these cases, the crystal symmetries and stoichiometry at least stay reasonably similar.

[marquis]

Corribus knows the theoretical.  Haven't been to that part.  However, when and if you deal with these chemicals, extreme caution is needed.  We used 12.5% bleach for chlorination of polymers ( usually natural rubber).  It was easy to generate this strange green gas ( chlorine for those not in on the joke) accidentally.  This happened under a hood, so no problem.  But....without knowing the chemistry, problems could easily have developed.  Please use caution!!!

[marquis]

A further note bleach is sodium hypochlorite in aqueous solution.