| Product | Sealed/Unopened | After Opening | Signs of Degradation |
|---|---|---|---|
| Calcium hypochlorite granules (65%) | 3–5 years | 1 year | Clumping, yellow-brown color, weak chlorine smell |
| Dichlor granules (56%) | 2–3 years | 1 year | Clumping, faded white color |
| Liquid sodium hypochlorite (10–12.5%) | 6 months (from mfg date) | 1–3 months | Yellow color, watery, weak smell |
| Potassium monopersulfate (MPS) | 3–5 years | 2 years | Clumping, discoloration |
| Trichlor tablets | 3–5 years | 2 years | Crumbling, unusual odor |
Calcium hypochlorite is chemically unstable and reacts with moisture, heat, and light over time. The active chlorine percentage (65% when new) drops as the product ages. After 5 years, a "1 lb bag" may only deliver 30–40% of the active chlorine it would have when fresh.
Liquid shock is the most unstable. Sodium hypochlorite in water actively decomposes, releasing oxygen. Higher concentrations degrade faster. Pool supply store liquid shock (10–12.5%) loses about 1% active chlorine per month at 70°F — and faster in warm storage.
When buying liquid chlorine, check the manufacturing date on the container (or ask the store). If it was bottled more than 3 months ago, its effective concentration may already be 10–20% lower than labeled.
Once added to the pool, free chlorine from shock treatment dissipates within 24–72 hours depending on sunlight, temperature, and organic load. In direct summer sun, expect chlorine to drop 3–5 ppm per 8 hours of sunlight exposure. This is why shocking at night is always recommended — it gives the chlorine maximum contact time before UV starts degrading it.
Record every chemical addition with a date in PoolLens. Know exactly when you last shocked and compare expected vs actual chlorine rise to gauge product effectiveness.
Open PoolLens Free →Yes, but it may be less effective. Old shock has lower active chlorine. You may need to use more to achieve the same result. Test after shocking — if chlorine didn't rise as expected, the shock has degraded. Clumped or yellowed granules confirm degradation.
Store granular shock sealed in a cool, dry, well-ventilated area away from sunlight, heat, and moisture. Never store near acids, flammables, or other pool chemicals. A temperature-controlled storage area dramatically extends shelf life.
Pool shock loses active chlorine content with age — it becomes less effective but not hazardous. Granular shock exposed to moisture may clump or turn yellow-brown. Liquid shock turns yellow and loses potency rapidly. Both should still be discarded if significantly degraded.
Once added to the pool, free chlorine from shock dissipates in 24–72 hours depending on sunlight and organic load. In full summer sun, chlorine can drop 3–5 ppm in 8 hours — which is why shocking at night maximizes effectiveness.
Granular shock and tablets are worth buying in bulk (use within the season). Liquid shock should be bought in smaller quantities since it degrades quickly. Never buy more liquid chlorine than you will use in one season, and check the manufacturing date before purchasing.