Water is great at dissolving a wide variety of minerals and ionic compounds. Often this is a valuable property, but sometimes it's a nuisance as well. High levels of metals like copper or iron in your pool water can cause the pool water to exhibit strange colours or discolour swimmers' hair. If you encounter these problems, it may be time to add a chelating agent to your pool.
Chemistry of Chelating Agents
Chelating agents are molecules or ions that bind to multiple sites on a metal ion, forming what's called a coordination complex. One common chelating agent is ethylenediaminetetraacetic acid, or EDTA. An EDTA molecule can actually wrap all the way around a metal ion to cloak it in a kind of external shell. Chelating agents like EDTA keep metal atoms in solution and prevent them from reacting with other ions to form insoluble precipitates. Consequently, chelating agents are sometimes called sequestering agents.
Metals in Pools
If the pH of your pool water is too low, the water may dissolve iron in light fixtures and copper in heat exchangers, increasing the concentration of these metal ions in the water. Alternatively, if the water you use to fill your pool contains a high mineral content, metal ions may be present in undesirable quantities. These metal ions can cause all kinds of problems, like stains or discoloured water.
Diagnosing & Solving Metal Problems
Chelating agents are a good solution to problems caused by excess copper or iron in your pool water. If your pool water turns green, test the chlorine level, and if the chlorine is in the ideal range, copper is the likely culprit. If your pool water turns brownish-red, on the other hand, iron is probably at fault. Adding a chelating agent to your pool will get rid of the colour and prevent staining; moreover, once the metal ions have been bound by the chelating agent, they can be removed from the pool water through filtration.
Tips on Using Chelating Agents
A wide variety of different brands are available from pool chemical suppliers; consult your local pool supply store to find out what chemicals they sell. It's very important when using a chelating agent that you also test the pool water pH, since your chelating agent may perform poorly if the pH isn't within the correct range. Moreover, if your pH is too low, the pool water may continue to dissolve metal ions, causing the problem to recur later on.