Pool shock is a common term for a concentrated chemical treatment designed to restore water quality by rapidly increasing the chlorine level or oxidizing power in a pool. This process, often called superchlorination, involves dosing the pool with a quantity of chemicals much higher than the normal daily amount. The primary purpose of these potent oxidizers is to overcome sanitation challenges and address water issues that regular chlorination cannot quickly resolve. They act quickly to neutralize contaminants and refresh the water chemistry.
The Primary Goal of Pool Shocking
The main objective of pool shocking is to achieve “breakpoint chlorination,” the chemical point where all combined chlorine compounds are broken apart and destroyed. These combined chlorine molecules, known as chloramines, form when free chlorine reacts with nitrogen-containing contaminants like sweat, urine, and cosmetics. Chloramines are less effective at sanitizing the water and are the source of the unpleasant “chlorine” odor and eye irritation.
Introducing a large dose of a powerful oxidizer eliminates these chloramines, allowing the remaining free chlorine to work efficiently as a sanitizer. Superchlorination also serves to oxidize non-living organic contaminants that accumulate from bathers and the environment. This oxidation process helps restore water clarity by breaking down microscopic particles and aids in killing resistant strains of algae and bacteria.
Calcium Hypochlorite (Cal-Hypo)
Calcium hypochlorite (Cal-Hypo) is one of the oldest and most widely used chemicals for pool shock, typically sold as a granular solid. It is composed of calcium and the hypochlorite ion, which is the active disinfecting agent. Commercial Cal-Hypo products are potent, usually containing between 65% and 75% available chlorine by weight. This high concentration makes it a fast-acting option for quickly resolving water quality problems.
Cal-Hypo is an unstabilized form of chlorine, meaning it lacks cyanuric acid (CYA) to protect it from the sun’s ultraviolet rays. Consequently, the chlorine residual dissipates quickly in outdoor pools. The chemical is highly alkaline, possessing a high pH that will raise the pool water’s pH level. Furthermore, Cal-Hypo introduces calcium, which can lead to an increase in the pool’s calcium hardness over time.
Stabilized Chlorine Shocks (Dichlor)
Another major category of chlorine shock is Dichloroisocyanuric Acid, commonly referred to as Dichlor. Dichlor is a stabilized compound because it is chemically bonded with Cyanuric Acid (CYA), the pool’s built-in “sunscreen.” This pre-mixed formulation means the water receives a dose of CYA along with the chlorine, shielding it from rapid degradation by sunlight. This stabilization makes Dichlor a popular choice for routine maintenance shocking, especially in sunny climates.
Unlike Cal-Hypo, Dichlor is considered nearly pH-neutral upon dissolution, so it does not drastically alter the water’s pH balance. However, the continuous addition of CYA requires careful monitoring, as excessive levels can inhibit the effectiveness of the chlorine sanitizer. Dichlor shock is typically sold as a granular product that dissolves quickly in water.
Non-Chlorine Shock Options
Non-chlorine shock is a distinct alternative that achieves oxidation without adding active chlorine to the pool water. The primary chemical used is Potassium Monopersulfate (MPS), an oxygen-based oxidizer. This compound works by chemically breaking down organic waste and chloramines through oxidation, similar to chlorine shock. It is an excellent choice for routine oxidation and eliminating the irritating chlorine odor.
A major advantage of MPS is that it allows swimmers to re-enter the water much sooner, often within 15 to 30 minutes of application. Because it is an oxidizer and not a sanitizer, MPS does not effectively kill bacteria or algae on its own and must be used with a regular sanitizer. MPS also does not add calcium or cyanuric acid to the water, which helps maintain overall water balance.