Thermal Decomposition of Calcium Hypochlorite

raw KClO3 crystals

Based loosely on Wouter's instructions I weighed 120 g of a commercial pool chlorine which claimed to be 65% Ca(ClO)2. 250 ml of water was brought to the boil on a hotplate in a 400 ml beaker with several Alumina polishing media cylinders in it as "boiling chips". I began to add small amounts of the granulated pool chlorine.

Initially the effervescence wasn't too bad, brisk stirring kept it under control. But once about 80 g had gone in whatever binder was used to granulate the product really started holding the foam together and letting it rise. The solution became more and more pink as I added the product, almost the colour of phenolphthalein. Letting it boil hard for a few minutes between additions while stirring like mad kept it all in the beaker. After about 100 g adding small additions seemed to have little effect, so I dumped in the remaining 20 g. My impatience cost me, the seething cauldron overflowed despite my best efforts stirring like a maniac, the foamy mixture flash boiling on the hotplate and forming a chalky deposit the melted and started evolving chlorine dioxide much faster than my ventilation system could deal with!

Quickly turning off the heat, removing the beaker and flooding the hot plate with water controlled the situation. Much foul language was spoken as the hardened deposit was chipped off the hotplate and the surrounding area cleaned up.

boiling down the raw solution

Once that was all over with the solution was filtered while still warm into another beaker, the viscous concentrated solution (largely of calcium chloride) taking almost an hour to gravity filtrate through the triple layers of paper. The powerfully caustic solution bleached and ate three filters before I finally used the best quality paper I had on hand. That got much of the insoluble rubbish out of it, and made the pink colour go away oddly enough.

The cleaner solution went back on the hot plate, diluted a little. A nice rolling boil was established and allowed to continue for over an hour. The solution had turned pink again, strange! The solution was filtered again after a little dilution to deal with its rheology, giving once again a clear but noticeably "yellow" liquid.

It still bleached the paper, suggesting significant hypochlorite left despite over an hour of hard boiling. I'd had enough at this point and prepared the saturated solution of 50 g of KCl. The liquids were mixed and boiled hard for a further hour to reduce the volume to a little over 150 ml. Into the fridge it went to crystallise out.

the dreaded Sodium D-lines

A rather disappointing yield of crystals formed and were filtered out. The crystals were washed quickly with ice water and put back into a beaker with 100 ml of water, boiled down a bit and back into the fridge again to crystallise. Filtering and extensive ice-cold water washing resulted in a 24 g yield that did not show significant Calcium and Sodium colouration in a flame test.

I tried boiling down the filtrate the get extra KClO3 out but little more was obtained. More aggressive boiling down and chilling produced odd-looking crystals that I doubt were KClO3. They were disposed of.


purified KClO3

Was it worth it? Not really IMO.

The calcium scale on the beakers may never come off, yuck!

The difficulty in working with the thick nasty solutions and the lousy yield make this a method for the desperate only.

It appears Calcium Hypochlorite is significantly more stable than the Sodium and Potassium versions, it can sit boiling for almost 2 hours and still have plenty left. Manipulating the pH and using some kind of catalyst might be required.

The process is slow, messy, nasty and inefficient. Why bother?


Parent article: Making Chlorates and Perchlorates.