In an ideal world, the pool season would end just as you use-up the last chlorine tablet, pour-in your last bags of pool shock, and perfectly adjust your water balance with the last scoops from the bucket.
However, one look inside of any pool owner’s chemical closet will tell you that leftover pool chemicals are the norm. So then, do pool chemicals go bad, and will they still work next season?
Shelf Life of Pool Chemicals
Sodium Hypochlorite, or chlorine bleach is a comparatively unstable form of chlorine, in that it can lose up to 50% potency within the first 6 months, and 90% within one year. If subjected to hot or cold temperatures or direct sunlight, degradation occurs more quickly.
Trichlor tablets have the longest chlorine shelf life. Individually wrapped tablets have been lab tested for potency over time. At 36 months, the tablets still have 98% of their original available chlorine, when stored in constant 50-70° F temps with a tightly closed bucket lid. In the right conditions, trichlor tablets can remain viable for over 5 years.
Chlorine granules sold in buckets Calcium Hypochlorite, aka pool shock or granular chlorine also has a long shelf life, if kept in an air tight container, in a cool and dry (indoor) location. Pool shock in 1 lb. bags absorb moisture from humid air, and bags can dissolve in 12-24 months. For longer term storage, re-pack into zip-loc freezer bags or buy Chlorine Granules, sold in buckets. Properly stored, pool shock has a shelf life of over 5 years.
Cyanuric acid, or chlorine stabilizer will maintain potency indefinitely, even if it absorbs moisture, as long as it is stored in a tightly closed container. Be sure to label cyanuric acid as Conditioner or Stabilizer, so the white powder won’t be confused with another chemical, if the container label fades or becomes unreadable.
When properly stored (cool and dry indoor location), pool algaecides are quite stable and most will maintain potency for at up to 5 years. Temperatures over 75°F and direct sunlight will reduce algaecide shelf life, especially lower concentration algaecides (10% and 30% formulations), but freezing temperatures can ruin pool algaecide.
Similar to algaecides, pool clarifiers have a shelf life of up to 5 years, stored tightly closed in consistently cool temperatures, in a dark cabinet or container. Outdoor storage in a garage or shed with high temperatures can reduce effectiveness, and freezing can damage the polyamide structure, rendering clarifiers useless.
Stain & Scale
Most Stain & Scale chemicals, have an indefinite shelf life, with the usual caveat – when closed tightly and stored in a cool, dry location (indoors). As with other liquid pool chemicals sold in bottles, the formulations are very stable, but high and low temperatures should be avoided to maintain product strength.
Alkalinity & Calcium Increaser
Sodium Bicarbonate and Calcium Carbonate are both dry white powder or flake chemicals, and both can remain viable and potent for over 5 years. As with other dry pool chemicals, shelf life can be extended with tightly closed lids to keep out moisture, which can cause clumping and hardening of your water balance chemicals.
Sodium carbonate or soda ash (pH Increaser) can remain viable for over 5 years, when stored in an air-tight container, to lock out moisture. If stored outdoors, humid conditions can pull moisture into the bucket, clumping and hardening, but also converting sodium carbonate to sodium bicarbonate over time, slightly reducing effect on pH.
Sodium Bisulfate and muriatic acid could have a 5 year shelf life, however pH decreasers are acids, and a larger shelf life concern about pH decreasers is the strength of the container. Over time, thin plastic bottles or packaging can break down from contact with acids. So, although pH-Down chemicals last for years, you should probably use them up.