Disinfection with Calcium Hypochlorite

This journal entry is a continuation of my series of entries on disinfection of drinking water.   Please read the following linked articles first,  Introduction to Disinfection and Disinfection with Sodium Hypochlorite.  I have covered a lot of background information already, which you will need to fully understand this journal entry.    Go ahead, I’ll wait.

Calcium Hypochlorite, Ca(OCl)₂ is a solid chemical used as a disinfectant and bleaching agent. It is used to store chlorine for long periods of time without having to resort to chlorine gas, which is extremely dangerous if not handled properly.  If stored is a room temperature environment away from moisture, temperature extremes and direct sunlight, calcium hypochlorite can last up to a year without serious degradation.  Beyond a year the chemical will become weaker with time.

Calcium hypochlorite is very similar to sodium hypochlorite.  The biggest difference is in the appearance. Calcium hypo is a white powder and sodium hypo is a yellow liquid. Sodium hypochlorite is a liquid and can be added directly to water for disinfection. Calcium hypochlorite on the other hand, needs to be mixed into solution first. Then added to the water for disinfection.

Calcium Hypochlorite Powder

When the calcium hypochlorite is added to water it will heat up. Wait until it has cooled and any remaining sediment has settled to the bottom. Then only use the supernatant, (the liquid above the precipitate) for disinfection of water.

Types of Liquid Mixtures (Image Source: http://chemwiki.ucdavis.edu)

Note following paragraph is math heavy, but suitably simplified and all units are SI (metric) because concentration calculations work a lot better and are simpler in SI units.  Even American water treatment facilities use SI for concentration measurements and calculations.

How you mix it depends on the initial strength of the dry powder. The chemical formula of calcium hypochlorite is Ca(OCl)₂.   Let’s assume the strength is 100% for two reasons.  One so the math is easier and two that is the quality(%) I use in my water treatment plant.  At 25 degrees Celsius, 21 grams will dissolve in 100 ml of water.  Trust me when I say that this means the maximum percentage you can mix is 21% Calcium Hypochlorite.  Note at this point the not all of the 21 grams are not disinfectant.  That is the total molecule, including calcium.  Only15 grams of calcium hypo become disinfectant, because when dissolved in water, calcium hypochlorite forms the hypochlorite ion (OCl-) and this is the disinfectant we want.

See the chemical reaction equation to see how the hypochlorite ion is formed (or skip it, chemistry can be dry sometimes) Ca(OCl)₂ + 2 H₂O <=> 2 H+OCl- + Ca(OH)₂.

At low pH (acidic conditions) the hypochlorite ion  will bond with hydrogen to form hyopchlorous acid (HOCl) .  This is a weak disinfectant and it should be avoided.  Keep the pH over 7 to favor the formation of the hypochlorite ion.

Calcium hypo powders sold as a disinfectant for swimming pools are 60% to 70%  calcium hypochlorite. If you are using these chemicals take into account the weaker starting point.  Instead of 21 grams you will need 35.7 grams (if you started at 70%) to get the same 15% concentration of hypochlorous acid.

Make sense? If it doesn’t, or I need to explain more I can elaborate in the comments section or expand this article, whichever makes more sense.  Just let me know.

To recap, to mix a 15% solution hypochorite solution, add 21 grams to 100 mL of water.  Here is a list of other common measurements.

  • 210 grams in 1 Liter
  • 7.41 ounces (dry) in 2.11 pints
  • 28 ounces (dry) in 1 gallon (US Liquid)

Remember this has just mixed our disinfectant.  This is now what you will add to your drinking water as a disinfectant.  Now the procedure is exactly the same as with sodium hypochlorite.  I told you, read this Disinfection With Sodium Hypochlorite and when adding your solution; make sure you at least reach the break-point chlorination point (explained in the sodium hypo article).   You will need to measure the residual to know for sure that your water is disinfected.  And as always, try and buy ANSI/NSF 60 grade chemicals. This is the standard for drinking water chemicals in North America.  They are purer and will have less surprises as you use them.


15 thoughts on “Disinfection with Calcium Hypochlorite

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  4. Thanks for your post. I was searching for some precision in the recipe for mixing Calcium Hypochlorite with water to make the stock solution and then the stock solution with water to disinfect for drinking. Yours is the most precise that I have found. I am still looking for a bit more precision and wondering if you can help. First we should be able to put together a precise formula that takes into account the percentage of available chlorine, be it 52% or 78% or whatever. Second, we should be taking into account the nature of the water being used. Is it clear, cloudy, or what? Second, is it cold or warm? The dirtier and colder the water the more stock solution we should be using, e.g. Thoughts?

    • There is obviously room for more precision if you are prepared to measure temperature, turbidity and other water quality indicators. The problem is different contaminants will change the chlorine equation. It may become too complicated to be used practically.

      • I understand especially about the second part. How about the first? E.g. am I correct to assume that if my Ca(ClO)2 is made with 78% available chlorine that I would use 27 grams instead of 21 grams — 21 / 0.78 — to mix with 100 mL water to make the bleach like solution in this recipe?

        • Without being able to check the math. I think your assumption is correct if you had a solution with 78% available CL2. However the numbers I used in my article assumed the purity of the calcium hypo was 100% which has a fixed amount of CL2 which is less than 78%.

  5. A few point that need elaboration: 1st is that, according to Clorox, laundry bleach has a shelf life of about 6 months; if stored below 70 degrees you may stretch to 1 year, if mixing calcium hypochlorite in the place of bleach, add 1 heaping Tbls to 3 cups water. If you want a water purifier, add 6 drops of the solution to 1 gallon of water, shake, let it sit for 30 minutes; smell. If it smells like chlorine the water is sufficiently purified, if not repeat process. See, I didn’t get all lost in scientific calculations. The info has come rather from or confirmed by chemist and biologist. 2nd point is that According to spec data sheets, the powder form does not have a shelf life, but best to discard after 5 years. Just keep it sealed, cool and in the dark

    • If you don’t want calculations then you should read a different post. As I posted in other articles I am trying to help people go beyond just using thier nose which leads to over dosing and adding chlorine taste.

  6. I love the math; however, I am afraid there is a miss-understanding of terms.

    It is Hypochlorous Acid that is at least 60 times the more potent, powerful oxidizer and deproteinizer – disinfectant – and not its companion the Hypochlorous Ion. As Hypochlorous Acid is consumed during its stronger Oxidative disinfection, Hydrogen atoms are stolen and added to the Ion to maintain the Acid-Ion equilibrium determined by the pH point.

    Hypochlorous is a weak, acid, yes; but the most powerful of biocides by 60 time far, than its pH dependant reserve form the Hypochlorous ion. Optimum disinfection occurs when the pH equilibrium balances between ph 5.7 and 6.3, where the Hypochlorous Acid form of Cl dominates nearing 100% in that pH range. By the time you get to pH 7and above, over 50% of the chlorine is in the Hypochlorite Ion form (the weaker biocide).

    The following passage of writing below is incorrect:

    “At low pH (acidic conditions) the hypochlorite ion will bond with hydrogen to form hyopchlorous acid (HOCl) [ CORRECT ! ]. This is a weak disinfectant [ NOT ! CORRECT ] and it should be avoided. Keep the pH over 7 to favour the formation of the hypochlorite ion.”

    In conclusion Hypochlorous acid is 60 to 100 times more effective than the Hypochlorite Ion at killing or oxidizing micro-organisms. One may like the the more favourable higher pH equilibrium for swimming….but it is an equilibrium of weaker disinfection, and becomes increasingly weaker as the pH climbs.

  7. I am too dumb to understand this. I need to calculate amount of calcium hypochlorite needed for disinfecting1000 m3/hr feed (sea water) for a desalination plant – the dosing is at the inlet of MMF filters. if the BOD COD values in water will consume say 2 ppm chlorine how much calcium hypochlorite do I need to dose to have 1 ppm residual chlorine? in kGs? can you please help me with calculation steps for the same so that I will learn.. thanks in advance

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