Why Are There Anions In Pool Water? [Facts!]

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You may have heard the term “anion” thrown around in relation to water pollution. While studying for my certification in environmental pool safety, I learned about the various anions that can exist in water, how to identify them, and the effects that they have on the pool environment.

What Are Anions?

An anion is a negatively charged molecule, consisting of one or more atoms (typically oxygen, nitrogen, or sulfur) attached to one or more hydrogen atoms. In chemistry, anions are represented by the Greek letter “-” (minus sign) and they are often bulky and/or colorful.

Most chemical elements on Earth are in an ionic form (i.e., they are in a form that can be held in solution by electrolysis). For example, chlorine is usually found in its ionic form as Cl- due to its solubility in water. Chlorine is a positively charged ion (i.e., Cl- has a net positive charge) and it will react with other ions in the water to form chlorine gas (Cl2) and hypochlorite ions (ClO-). Hypochlorite ions are more stable than chlorine gas and they can act as a stronger oxidizing agent. They can also react with bacteria to form chlorinated byproducts that are toxic to humans. Hence, it is essential to keep hypochlorite and other associated anions (e.g., chlorite and chlorate) at bay throughout a pool’s lifespan.

The Forms Anions Can Take

Ions can take on many different forms. For instance, chlorine gas is usually represented as a yellowish-brownish gas and it solidifies at around minus 40 degrees Fahrenheit. This is contrary to most other gases which become less dense as they get colder.

Depending on the pH of the pool water, chlorine can exist as either a gaseous or a solid form. If the pH of the water is above 7.2, then it will be mostly found in its gaseous form. However, if the pH is below 7.2, then it will be present in either a gaseous or solid form. This happens because solid chlorine (Cl2) dissolves in water to form hypochlorite (ClO-) and acidic materials which lower the pH (e.g., hydrogen ions and carbon dioxide).

The Effects Of Anions On The Pool Environment

The presence of anions in the pool water can have a variety of bad effects, some of which are listed here.

  • Acidic water (pH <7) aggravates the wear on pool pump parts and valves due to corrosion.
  • Anions lower the pH which makes the water more susceptible to algae growth and boosts the development of bacteria. The combination of high algae growth along with high bacterial growth creates an environment where disease-causing organisms can thrive.
  • When chlorine is present in excess, it can replace hydroxide (OH-) as the active oxidizing agent in the water. Thus, it can break down plastics and other organic matter which ends up in the pool.
  • Some anions (e.g., chlorate, chlorite, and thiocyanate) are more effective at removing algae than chlorine gas, due to their ability to do so at much lower concentrations. Thiocyanate in particular has been shown to be a highly effective algae killer at around 5 ppm (i.e., 5 parts per million). Chlorite ions (ClO-), on the other hand, have been found to be more effective than either chlorine or thiocyanate at removing algae, at around 10-15 ppm. Chlorate ions (ClO3-) are also able to kill algae at much lower concentrations (around 1 ppm) than either chlorine or thiocyanate. Hence, it is essential to test for and control algae growth in conjunction with testing for and controlling for the presence of anions in the pool water.

It is also important to realize that most of these effects are exacerbated when the pH of the water is low. The reason is that a) when the pH is low, then the chlorine in its gaseous form tends to remain in the air for a longer period of time and b) most pools are not equipped with a water treatment system which neutralizes the effects of low pH water. If you do live in a household with a pool, then it is a good idea to invest in a pH balance pool controller which can regulate the pH of the water precisely to 7.2 to avoid the abovementioned issues.

How Do You Test For Anions?

There are several ways to test for anions in the pool environment. If it is a gas, then it can be collected and weighed either spontaneously or by using a sensor. If it is a solid, then it can be detected using a sensor or it can be dissolved in water using a chemical agent (e.g., potassium dichromate) and analyzed using liquid chromatography/mass spectrometry (LC/MS).

If you suspect that your pool water may contain excessive levels of anions, then it is a good idea to have it tested by a pool professional. Having your pool tested for anions can help you identify the source of the problem and it can also help you develop a plan of action. If your pool has been around for a while and you are not sure where the source of the problem is, then it may be a good idea to get it checked out by a pool inspector. You should not attempt to test for anions by yourself, as it is not that easy to do and it involves a considerable amount of specialized equipment (e.g., gas analyzers and/or pH meters).

If you would like to find out more about the various anions that exist in water and their effects on the pool environment, then it is a good idea to read the EPA’s Water Quality Fact Sheet on Anions or visit their website at https://waterquality.epa.gov/anion.

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