Why would clear pool water turn yellow when a chlorine tablet was added?

Pool water turning yellow after adding a chlorine tablet is usually caused by the presence of high levels of nitrogen in the water. When chlorine comes in contact with these nitrogen compounds, it can form a chemical reaction that results in the formation of chloramines. These chloramines are often responsible for giving the pool water a yellow or amber tint.

Here's a more detailed explanation of the chemical process involved:

When chlorine is added to pool water, it reacts with water molecules to form hypochlorous acid (HOCl). HOCl is a powerful oxidizing agent that helps to kill bacteria and other microorganisms in the pool water. However, when HOCl comes into contact with nitrogen compounds, such as urea and ammonia, it can react to form chloramines. Some of the common chloramines formed are monochloramine (NH2Cl), dichloramine (NHCl2), and trichloramine (NCl3).

These chloramines are less effective than HOCl at killing bacteria, and they can also cause eye irritation and other health problems. In addition, they can contribute to the formation of scale and other deposits on pool surfaces.

If you notice that your pool water turns yellow after adding a chlorine tablet, it's important to test the water for nitrogen levels and to take steps to reduce the levels of nitrogen in the pool. You can do this by using a pool chemical specifically designed to remove nitrogen, or by using a flocculant to remove suspended particles from the water. Once the nitrogen levels have been reduced, the pool water should return to its normal color.

To prevent the formation of chloramines in the future, it's important to keep the nitrogen levels in your pool water low and to use a chlorine stabilizer to help extend the life of the chlorine. You can also minimize the amount of nitrogen entering the pool by keeping the pool covered when not in use, and by preventing swimmers from using the pool when they have recently used sunscreen or other personal care products that contain nitrogen.