Calcium Hypochlorite (Cal Hypo) in Pools: All You Need to Know

Calcium Hypochlorite, an unstabilized, inorganic compound, is available in granular form, tabs, or briquettes. It offers 47-78% available chlorine and the same percentage of active strength. In a 1% solution, it raises the pH to a range of 8.5 to 11.  Calcium Hypochlorite, also known as Cal Hypo, is a chemical compound with the formula Ca(OCl)2. It is a dry form of chlorine that is available in granular, tablet, or briquette form. It is a white powder, granule, or tablet with a high pH and provides 48 to 75 percent available chlorine. It does not contain any stabilizer but if cyanuric acid is present in the water, it will protect Cal Hypo. It is often used to superchlorinate water due to its high available chlorine content and moderate solubility.

How is Cal Hypo produced?

Cal Hypo is typically made by mixing chlorine gas with caustic soda and calcium hydroxide. This reaction produces calcium hypochlorite, sodium chloride, and water.

How Stable is Calcium Hypochlorite?

Calcium hypochlorite is unstable. It is a chemical compound with a stability rating of 1 and does not contain any stabilizer.

What are the Properties of Calcium Hypochlorite?

Calcium hypochlorite is unstable, slow dissolving, and leaves substantial sediment. It contains 65% available chlorine and has a pH of 11.5. It’s often used as a shocking agent due to its potency.

How does calcium hypochlorite interact with chlorine compounds?

Chlorine compounds like calcium hypochlorite form HOCl by releasing calcium and picking up a hydrogen ion. The solubility of calcium hypochlorite in water is 180 grams per liter at 77° F (25° C) and 1 atm.

How Much Chlorine Does Calcium Hypochlorite Deliver?

One pound of calcium hypochlorite with an ACC of 47% to 78%, when dissolved in 10,000 gallons of water, delivers 5.6 ppm (mg/L) to 9.4 ppm (mg/L) of chlorine, respectively.

How much Chlorine is in Calcium Hypochlorite?

Calcium hypochlorite provides 43-73% available chlorine. This percentage can vary depending on how it is manufactured, with some sources stating it can provide between 48 to 75 percent available chlorine. For example, a granular form of chlorine, often referred to as calcium hypochlorite, is generally produced in a compound of 70 percent chlorine and 30 percent inert materials.

Is Calcium Hypochlorite and Cal Hypo the same?

Yes, Calcium Hypochlorite and Cal Hypo are the same.  Calcium Hypochlorite, also known as Cal Hypo, is a chemical compound with the formula Ca(OCl)2. It is a white powder, granule, or tablet with a high pH (10.0 to 11.8).

Where is Calcium Hypochlorite used?

Calcium Hypochlorite is often used to superchlorinate water, particularly in pools and spas.  It is a chemical compound that is commonly used to treat water, making it safe for use in swimming pools and spas. It is available in granular, tablet, or briquette form and can be added directly to the water or used in certain erosion feeders. It is also used for regular chlorination. The granules can be broadcast into the pool or pre-dissolved and added as a liquid. The tablets are used in a special chlorinator, some of which are for skimmer use.

How is Calcium Hypochlorite Used?

Calcium hypochlorite is often used to superchlorinate water due to its high available chlorine content (ACC) of 47-78% and moderate solubility. It can be added directly to a pool or spa, and maintaining good circulation and brushing granules off the bottom can prevent vinyl liner bleaching. The tablet form can be used in certain erosion feeders. It is also used for regular chlorination. The granules can be broadcast into the pool or pre-dissolved and added as a liquid. The tablets are used in a special chlorinator, some of which are for skimmer use.

How to increase Chlorine in the pool with Calcium Hypochlorite?

To increase Chlorine in the pool with Calcium Hypochlorite, add 2 oz. of Cal Hypo (65%) to 10,000 gallons of water. This will increase the chlorine residual by about 1 ppm. However, care should be taken when adding Cal Hypo directly to a vinyl-liner, fiberglass, painted, or colored-plaster pool as it may bleach areas where the granules lay. Any settled granules should be brushed up until fully dissolved.

How to Superchlorinate a Pool with Calcium Hypochlorite?

Superchlorination can be achieved by adding 1 pound of Cal Hypo (65% available chlorine) to 10,000 gallons of water. However, care should be taken when adding Cal Hypo directly to a vinyl-liner, fiberglass, painted, or colored-plaster pool as it may bleach areas where the granules lay. Any settled granules should be brushed up until fully dissolved.

What Happens When Calcium Hypochlorite is Added to Water?

When calcium hypochlorite is added to water, it reacts with the water to yield hypochlorous acid and calcium hydroxide. This reaction can be represented as: Ca(OCl) + 2H O → 2HOCl + Ca+ + OH–. The calcium hydroxide then ionizes into a calcium ion and a hydroxyl ion. The by-product of calcium hypochlorite, calcium hydroxide (Ca[OH]»), increases the water’s pH. The pH of calcium hypochlorite ranges from 8.5 to 11.

What Happens When Calcium Hypochlorite is Added to Pool Water?

When calcium hypochlorite is added to pool water, it can form calcium carbonate with the alkalinity in the water, potentially causing the water to become cloudy. However, if the water is kept at recommended targets, the cloudiness will dissipate. Depending on the region, adding more calcium to hard water may not be advisable due to potential scale formation. Additionally, calcium hypochlorite reacts with water to yield hypochlorous acid and calcium hydroxide. It also increases the chlorine residual in the water and adds alkalinity to the water but does not increase the pH.

What is the Effect of Adding Calcium Hypochlorite to Pool Water?

Adding 2 oz. of Cal Hypo (65%) to 10,000 gallons of water will increase the chlorine residual by about 1 ppm. However, mixing different types of chlorine, such as calcium hypochlorite tablets and trichlor tablets, can cause an explosion or fire and release toxic chlorine gas.

How Much Calcium Hypochlorite is Needed for 1 ppm?

To find out how much calcium hypochlorite is needed for 1 ppm, divide the weight needed for 1 ppm by the available chlorine content. For example, 0.4289 lbs ÷ 0.65 = 0.6598 lbs of calcium hypochlorite.

What are the Benefits of Calcium Hypochlorite?

The benefits of calcium hypochlorite include its high available chlorine content (ACC) of 47-78% and moderate solubility, making it effective for superchlorinating water. It can be used directly in pools or spas, and in tablet form in certain erosion feeders. It also provides 48 to 75 percent available chlorine and does not contain stabilizer. However, if cyanuric acid is present in the water, it will protect calcium hypochlorite. It is also a potent shocking agent due to its potency.  Calcium hypochlorite increases the chlorine residual in pool water, and one pound of it can deliver 5.6 ppm (mg/L) to 9.4 ppm (mg/L) of chlorine when dissolved in 10,000 gallons of water. It can also be used to superchlorinate a pool by adding 1 pound of it to 10,000 gallons of water. In soft-water regions, calcium hypochlorite can beneficially add calcium hardness.

How to store Calcium Hypochlorite?

Calcium Hypochlorite should be stored in a cool, dry, well-ventilated area away from incompatible substances. It is stable and can be stored for long periods, typically 12 years, depending on storage conditions.

How is Calcium Hypochlorite Sold?

Calcium hypochlorite is sold in granular, tablet, or briquette form. It is often referred to as cal-hypo and is available with an active chlorine content (ACC) that can vary from 47% to 78%. The form in which it is sold can depend on how it is manufactured.