Misconceptions about chlorine demand

By Alicia Stephens

In an era of technological advancements and equipment upgrades, pool and spa chemistry is often viewed as being an unchanging element of water care.

However, over the past several years, chemistry has actually undergone many changes, including an increase in saltwater pool care, a focus on ‘low sanitizer residual’ systems, and an overall increase in problem pools/spas due to an ever-changing environment. Throughout all of these changes, one problem that remains prevalent and continues to frustrate pool/spa owners and professionals alike is chlorine (Cl) demand.

This is defined as the inability to maintain a chlorine residual in a pool or spa even after repeated application of a chlorinating product. There are an infinite number of substances that can contribute to chlorine demand. These include (but are not limited to) bacteria, algae, ammonia, urine, sweat, health and beauty products, and bather and animal waste. These contaminants can enter the water in a number of different ways.

Determining the cause of chlorine demand in a particular pool may seem like an insurmountable task. In many cases, the root cause of the demand will be impossible to uncover; however, it is usually not truly relevant to the treatment needed. There are many misconceptions about what actually causes chlorine demand, as well as when a demand is present and how it should
be treated.

Is the chlorine working?

One of the most common misconceptions about chlorine demand is the thought that the chlorine is not working when it is applied to the pool/spa. Consumers often complain the chlorine they are using is weak or ineffective because they keep adding it to the water, but nothing seems to happen. By nothing, they are referring to constant addition of product but not establishing a free chlorine residual.

In reality, the lack of residual is caused by an overload of contamination in the pool/spa that depletes the amount of chlorine available to sanitize the water. It often appears as if chlorine is not working, while in reality it is working overtime to try and overcome the impurities in the water.

Water contamination is reduced as more chlorine is added; however, the inability to maintain a residual will continue until all chlorine reactive pollutants are removed from the pool water. If the contamination is substantial, it often takes a large amount of chlorine not only to eliminate the problem, but also to re-establish a chlorine residual in the water.

Do phosphates and nitrates consume chlorine residuals?

The second common misconception is phosphates and nitrates in the pool eat up chlorine residuals and, as a result, contributes to chlorine demand. Hypochlorous acid (HOCl), or free available chlorine (FAC), reacts easily with many different types of materials. By looking at the chemical structure of some contaminants, one can predict whether or not there will be an interaction with chlorine.

All atoms have what is referred to as a preferred ‘oxidation state’ or ‘oxidation number.’ This is simply a number assigned to a particular atom based on its chemical properties. For example, the preferred oxidation number for chlorine is -1. Atoms in an oxidation state that are not preferred are very reactive, while atoms in their preferred oxidation state are stable and are much less reactive.

It is not important for one to know how the oxidation numbers are determined, but knowing what they are is very helpful. It may sound complicated at first, but it is an extremely useful way for scientists to predict which chemical reactions are likely to occur.

In hypochlorous acid (or FAC), chlorine actually has an oxidation number of +1, which is not preferred. Because chlorine is constantly trying to reach its preferred state of -1, it is very reactive. This is why hypochlorous acid is such a great oxidizer. When it oxidizes other material, the chlorine atom ends up where it wants to be at -1.

Because of oxidation numbers, there are compounds that do not tend to react with hypochlorous acid. For example, the nitrogen in nitrate (NO₃-) is already where it wants to be at +5. The same is true for phosphate (PO₄³⁻). In the orthophosphate molecule, the phosphorous atom is also where it wants to be at +5. This makes these compounds quite stable and unlikely to react with hypochlorous acid. If the material does not react with hypochlorous acid, then it does not contribute to chlorine demand. If orthophosphate or nitrate reacted with chlorine and caused a chlorine demand, then these compounds would be removed when shocking the pool—this does not occur.