Chlorine tablets are solid forms of chlorine. Chlorine tablets have different shapes. Some are shaped like a hockey puck; others are shaped like a stick; others are granular.
For this discussion we will use the term chlorine tablet to generically describe solid chlorine in its various shapes and forms.
Some chlorine tablets have only an active ingredient for sanitizing the swimming pool. These chlorine tablets are referred to as unstabilized chlorine tablets. Other chlorine tablets contain, in addition to chlorine, an acid (usually isocyanuric acid or cyanuric acid, commonly referred to as CYA) to reduce the pH of the swimming pool as the chlorine tablets dissolve. These chlorine tablet products are referred to as stabilized chlorine tablets. A popular stabilized chlorine tablet is called Trichlor. Trichlor and other chlorine tablets can be found at your local pool supply store.
Chlorine tablets contain calcium hypochlorite as the active chlorine sanitizer. Calcium hypochlorite reacts differently with swimming pool water than liquid chlorine bleach, as calcium hypochlorite will increase the hardness or alkalinity and raise the pH of the pool water. If the pool water is considered hard water, and has a high alkalinity (high pH), the effectiveness of the chlorine is greatly reduced.
Additionally, as the pH rises as a result of increased alkalinity, calcium begins to precipitate from the pool water as a solid form of calcium carbonate, sometimes referred to as calcium scale. The calcium will typically cause buildups on pool side walls, ladders, pool decks and pool bottoms.
A common example of a precipitation reaction is putting sugar in iced tea. As more sugar is added to the tea, the sugar concentrates until sugar crystals appear on the bottom of the tea glass. The tea reaches the point of being “supersaturated” with sugar and a solid form of sugar will precipitate from the liquid.
It is important when using chlorine tablets to frequently monitor the pH of the pool water. As the pH increases, the addition of an acid will bring the pH back to acceptable ranges. If the pH of the pool is not corrected, calcium carbonate will build up on pool side walls, ladders, pool decks and the pool bottom. Additionally, when the pool walls are brushed, the displaced calcium carbonate will build up on the pool filters, thus increasing pool filter maintenance requirements.
Chlorine tablets have a higher chlorine availability than liquid chlorine. Liquid chlorine has typically 10% available chlorine while chlorine tablets will have an effective chlorine availability of 35% - 90% depending on the type of chlorine tablet selected. (10)
Stabilized chlorine tablets which contain trichlor as the active sanitizer ingredient usually contain cyanuric acid to prevent sunlight from reducing the effectiveness of chlorine.
While unstabilized tablets will typically increase the pool pH, stabilized tablets which contain acid will lower the pH of pool water. Understanding the chemistry of stabilized vs. unstabilized tablet products is essential.
Chlorine tablets using calcium hypochlorite are very reactive due to their higher percentage of available chlorine. Accordingly, great care must be exercised in storing and handling these tablet products and users should thoroughly familiarize themselves with, and observe, the supplier’s directions for use and precautionary warnings on the product label. These tablet products should not come in contact with organic materials such as leaves, twigs, oily rags or gasoline residues, as a fire or explosion may result. Never mix different types of chlorine products as harmful effects may occur. (10) (11)
Chlorine Tablets: Other Pool Chemicals Needed Maintenance Requirements Long Term Costs