Chlorine is one of the most common disinfectants used for swimming pools and spas. It helps kill bacteria and algae, keeping the water clean. However, chlorine can become less effective over time as contaminants in the water use up the available chlorine. This is why pool owners need to periodically shock their pools – to reactivate the chlorine.
What is Pool Shock?
Pool shock, also known as pool oxidizer, contains chemicals that help reactivate and boost the chlorine in pool water. The active ingredient in most pool shock products is either calcium hypochlorite or sodium dichloro-s-triazinetrione, commonly known as dichlor or sodium dichlor.
These chemicals work by raising the chlorine level quickly to attack contaminants and reactivate the chlorine in the water. After shocking, the chlorine level will drop back down to the normal range within 24 hours.
How Does Shock Reactivate Chlorine?
Shock helps reactivate chlorine through a chemical process known as oxidation reduction, or redox. Here’s how it works:
- Active chlorine has the chemical formula HOCl. It’s a strong oxidizer that kills bacteria and algae.
- Over time, the active HOCl gets used up and converts to the chloride ion Cl-. This chloride ion is inactive and does not sanitize the pool.
- When pool shock is added, the chemicals react with the chloride ions and convert them back to active HOCl chlorine through oxidation.
In essence, shock brings the inactive chlorine back to life. The boost in chlorine quickly destroys contaminants and re-establishes the proper sanitizer levels.
Does Every Type of Shock Work the Same?
There are a few different types of pool shock on the market:
- Calcium hypochlorite – This is sold as a powder or tablets and provides 65-75% available chlorine. It fully dissolves and works well in all water conditions.
- Sodium dichlor – Also known as dichlor, this fast-dissolving shock has about 62% available chlorine.
- Lithium hypochlorite – With 35% available chlorine, this is a popular granular shock for soft water pools.
While the active ingredients vary, all these shock products work to oxidize chlorine and boost sanitation levels. Calcium hypochlorite and lithium hypochlorite may raise calcium and lithium levels in the water over time.
When Should You Shock a Pool?
Here are some guidelines on when to shock your pool:
- At the start of each swim season
- After heavy bather loads
- If the water looks hazy
- When free chlorine drops below the ideal range
- After heavy rains or wind storms
- If there is algae growth
Shocking 1-2 times per week during peak swim season is recommended. More shocking may be needed if chlorine demand is high due to lots of swimmers, high temperatures, or contamination issues.
How is Shock Added to Pools?
Shock treatment should be added in the evening or when the pool is not in use, as the high chlorine levels can irritate skin and eyes. Here are some tips for adding shock:
- Follow dosage instructions on the product label based on gallonage.
- Powder or granular shock should be broadcast around the pool perimeter. It will quickly dissolve and distribute.
- Pre-dissolve tablets in a bucket of water before adding to the pool.
- Run the filtration system to circulate the shock treatment.
- Maintain proper water balance – shocking is less effective if pH and alkalinity are off.
- Consider using a shock pot or feeder for a controlled release of shock over time.
Test the chlorine level the next day and repeat shocking if still below desired range. Remember to backwash filters after shocking to rinse out debris freed up by the oxidization process.
Can You Swim After Shocking?
It’s best to wait 24-48 hours before swimming after adding shock to allow chlorine levels to stabilize. The high initial chlorine concentration right after shocking would be irritating and uncomfortable for swimmers. Fortunately, the chlorine dissipates quickly back to the normal range within a day or so.
Are There Alternatives to Pool Shock?
A few options to consider besides traditional shocking:
- Maintain constant low-level chlorine – Using a chlorine feeder or generator provides a steady low dose of sanitizer instead of shocking.
- Ozone generator – Ozone powerfully oxidizes contaminants without irritating chloramines.
- UV system – Ultraviolet light destroys chloramines and does not alter water chemistry.
- Mineral sticks/cartridges – Release small amounts of oxidizers over time, reducing the need for shocking.
However, most pools still require periodic shocking since nothing works as quickly and powerfully as high chlorine to restore sanitation. Alternatives may reduce the need for shocking but are not complete substitutes.
Risks of Improper Pool Shocking
Adding too much shock or shocking too frequently can cause issues like:
- Skin and eye irritation
- Surface corrosion from high chemical levels
- Degradation of metals in the pool equipment
- Etching or discoloration of the pool surface
- Unpleasant chemical odors
Additionally, shock will be less effective if the water balance is off. Make sure to monitor and adjust pH, total alkalinity, calcium hardness, and cyanuric acid as needed.
Only use shock products specifically formulated for pools. Never mix types of shock together or mix with other chemicals directly. Always follow the dosage instructions carefully.
Conclusion
Shocking a pool with chlorine oxidizers boosts sanitizer levels to destroy contaminants and reactivate inactive chlorine. All types of pool shock work by the same redox chemical process to sanitize pool water. Shocking 1-2 times per week during summer is usually sufficient, with additional shocks as needed based on bather load, weather, and visual water quality. While alternatives exist, periodic shocking remains the quickest, most potent way to maintain the cleanliness of pool water.
| Shock Type | Form | Active Ingredient | Available Chlorine |
|---|---|---|---|
| Calcium hypochlorite | Powder or tablet | Calcium hypochlorite | 65-75% |
| Sodium dichlor | Powder | Sodium dichlor | 62% |
| Lithium hypochlorite | Granules | Lithium hypochlorite | 35% |
Ideal Chlorine Levels
| Pool Type | Ideal Free Chlorine Range (ppm) |
|---|---|
| Residential | 1-3 |
| Public | 2-4 |
| Spa | 3-5 |
When to Shock Your Pool
- Start of swim season
- After heavy bather load
- Hazy water
- Free chlorine below ideal range
- After rainstorms
- Visible algae growth
Tips for Adding Shock
- Follow label dosage for pool gallonage
- Broadcast powder/granular around perimeter
- Pre-dissolve tablets before adding
- Run filtration system to circulate
- Maintain proper water balance
- Consider a shock pot for controlled release
