Hello!  In accordance to page 128 of the following Fact Sheet from the World Health Organization (WHO), "all forms of calcium hypochlorite contain some inert material which is insoluble".  Therefore, your Calcium hypochlorite [Solubility = 21.4% in 76 degrees F, or 25 degrees C  (Toxic Substances Portal -ATSDR)] is in fact dissolved. You will need to separate out the insoluble inert material from your solution.  A method of separation may be found in Figure 1, page 129 below:

Fact Sheet 2.19 - Calcium hypochlorite (WHO)

http://www.who.int/water_sanitation_health/hygiene/emergencies/fs2_19.pdf

Hi Hamilton,

Thanks for the advice! However, in sometimes after I dissolved Calcium Hypo with acid buffer, the solution caked after it was stored at room temperature for few days

Hello Again!  Calcium hypochlorite solutions will begin to degrade after four (4) hours at room temperature. Although you are using the acid buffer to stabilize your solution for storage, perhaps the formation of calcium hydroxide, or a random insoluble chemical "by-product" is forming nonetheless.  This might be due to the reversible chemical reaction occurring between calcium hypochlorite  and water, or the presence of some minute residual contaminant (e. g., detergent residue on glassware). Therefore, these could be possible explanations as to why your calcium hypochlorite solution "cakes" only some of the time.

Another WHO publication that may be of interest is as follows:

Chemistry of Disinfectants and Disinfectant By-Products -- Part 2 (WHO)

http://www.who.int/ipcs/publications/ehc/216_disinfectants_part_2.pdf

Hi Hamilton:

Thanks for your comment, I wonder that, will the calcium hypochlorite solution "cakes" more easily in neutral / basic solution than acidic solution?

Thanks!

Hello Again!  Perhaps the following article might be found useful regarding your most recent pH question:

Hypochlorite Solution Expiration and Stability in Household Water Treatment in Developping Countries (LANTAGNE et al., 2011)

http://www.antenna.ch/en/medias/lantagne-expiry.pdf

Commercial Ca(OCl)2 contains up to 30 % lime.  You can filter it as suggested above. However, it is known that Ca(OCl)2 is susceptible for metal metathesis reactions. Thus for example, it can react with sulfate or even with carbonate contents giving you precipitate (if it is not dissolved in distilled water or the solution is exposed to air), because HClO is a very weak acid PKa~7.4. Thus,  Ca(OCl)2 + MSO4= CaSO4 + M(OCl)2 and analogs can take place. Ca(OH)2  content is probably neutralized by your by acid buffer to avoid formation of CaCO3? By the way what else is in your acid buffer? You are probably aware that you can add cyanuric acid and anti-scaling agents to your Ca(OCl)2 solution in order to stabilize it. Another certain way is to increase the pH above 11.

Hi Hamilton,

Thanks for the article, it helps a lot!

Hi Imre,

I am using citric acid buffer. I also suspect that the caking of my hypochlorite solution is due to the by-product of the hypochlorite. I found that after I lower the amount of citric acid in buffer, caking prone to happen. I am trying to stabilize the hypochlorite with sulfamic acid.

Besides, increase the pH higher than 7, the reaction will favor formation of chlorate, instead of hypochlorous acid? What I understand that hypochlorous acid has better oxidation ability that chlorate.

Thanks! 

You are quite welcome.  I'm pleased to have been of some assistance.

Dear Gan Jen,

Then your cake is probably due to CaCO3 formation (either because Ca(OH)2 is not neutralized completely and reacts with CO2 from air and/or H2CO3 is competitive with citric acid (their pKa's are similar and precipitate formation can always shift the equilibrium). Indeed, very high pH-s and especially heating would favor chlorate formation (and run away reactions) using concentrated hypochlorites. I think you can increase the stability of OCl- anion until pH 10-11 and may be in the presence of Cl- , because other than LiOCl, house-hold bleach (mixture of NaOH, NaOCl, NaCl) is the most stable hypochlorite solution. Some extent of dilution also helps to stabilize HOCl.

Regards,

Hi Imre,

stabilizing the hypochlorite in acidic condition, will it better than stabilize hypochlorite in basic condition? Thanks!

Hi Gan,

Apparently, HOCl is the most effective (has the best anti-microbiological activity, etc.) under pH 5.5 and above 3.5 (best is around pH 5). (Ca(OCl)2 or NaOCl is less efficient than HOCl). The addition of 0.5-1.0 % saline (in order to provide Cl-) is enough for the stabilization. If you haven't done so, please read the nice internet article below.

https://aquaox.wordpress.com/category/hypochlorous-acid-sodium-hypochlorite-and-chlorine/