Pool (or spa) chemicals that are labeled as stain preventatives or metal removers help prevent staining caused by metals and can contain 2 very similar ingredients known as a sequestering agent or a chelating agent. And to make it more confusing, chelating agents are sequestrants but not all sequestrants are chelating agents. Although these agents do the same thing, they do it in a slightly different way and can affect water chemistry in different ways. They both act as magnets to attract and bond with the electrons of metal ions and therefore prevent metals from being oxidized. When oxidized, Iron causes brown stains, copper causes green stains, and calcium will combine with carbonate to deposit calcium carbonate scale.
Comparison of Sequestrants and Chelators
How Do They Work?
- Sequestering agents attract and attach multiple ions together and form clumps that can sometimes be big enough to filter out, therefore it is said that sequestering agents hold metals and minerals in suspension.
- Chelating agents divide and conquer by spreading out and attacking individual ions, which means they are much smaller and pass through a typical pool (or spa) filtration system. Therefore, it is said that chelating agents hold metals and minerals in solution as they basically prevent them from being oxidized.
- Both will need to be added periodically to be effective.
Chemical Composition
- Most sequestrants are phosphate-based (polyphosphates). Under certain conditions, they can break down into orthophosphates (the ones algae like to eat), but this shouldn’t be a concern in typical applications as they will be used up before the reaction can take place. And you will keep your water balanced and sanitizer level at an acceptable level so the algae can’t survive and therefore can’t eat – right?
- Chelating agents are most typically created from ethylenediaminetetraacetic acid (EDTA), and are different from phosphate-based in that they are not affected by phosphate removers so there isn’t a risk of being converted to orthophosphates. EDTA is also oxidized by chlorine so you may see your chlorine drop in the presence of EDTA.
Should You Use a Product With Sequestering or Chelating Agents?
Generally speaking, either can be used as long as you follow the manufacturer’s instructions, however, phosphate-based agents seem to be more effective. Some brands are not compatible with other chemicals such as biguanides and/or you may have to wait a certain period of time after shocking as chlorine can deactivate the agent. You should also consider which metal you are trying to remove and choose a product specific to that type of metal.
Don't know what chemicals to add or when? Sutro will tell you!