I shocked with chlorine, and the water changed color. Why?

Chlorine can oxidize various things in water, and sometimes the reactions produce colors in water. Here's why.

If your water changes color immediately after chlorinating (or using a non-chlorine shock oxidizer), it could be for a number of reasons. This help article will offer some possibilities for each color. Nobody wants their water to be a tint of brown, green, yellow, purple, or anything else.  We all strive for safe, clean, and clear blue water.

Why does clear water look blue?

According to the NOAA website, the ocean is blue because water absorbs colors in the red part of the light spectrum:

"The ocean is blue because water absorbs colors in the red part of the light spectrum. Like a filter, this leaves behind colors in the blue part of the light spectrum for us to see.

The ocean may also take on green, red, or other hues as light bounces off of floating sediments and particles in the water." - NOAA

According to the Scientific American journal, the sky is blue for the same reason:

"The ocean looks blue because red, orange and yellow (long wavelength light) are absorbed more strongly by water than is blue (short wavelength light). So when white light from the sun enters the ocean, it is mostly the blue that gets returned. Same reason the sky is blue."

In other words, the color of the ocean and the color of the sky are related but occur independently of each other: in both cases, the preferential absorption of long-wavelength (reddish) light gives rise to the blue. Note that this effect only works if the water is very pure; if the water is full of mud, algae or other impurities, the light scattered off these impurities will overwhelm the water's natural blueness.

That same Scientific American article goes on to share a comment from a physics professor named Michael Kruger from the University of Missouri:

"The sky is blue not because the atmosphere absorbs the other colors, but because the atmosphere tends to scatter shorter wavelength (blue) light to a greater extent than longer wavelength (red) light. Blue light from the sun is scattered every which way, much more so than the other colors, so when you look up at the daytime sky you see blue no matter where you look." - Michael Kruger

So water looks blue because of how water refracts light and absorbs red light from the spectrum.

Now let's talk about other colors of water.

Green pool water

If water is discolored at all, it will most often be green.  Pool water can turn green for a number of reasons, and you can easily diagnose a green pool using the White Bucket Test.

• Green algae
• Oxidized copper
  • copper from tap water
  • copper from algaecide (copper sulfate)
  • copper from corroding pool heater (heat exchanger)
  • copper from a mineral system or ionizer
• Too much alkalinity relative to calcium hardness
• Bromine

Green algae is probably the most common, and it's also the easiest to remedy. You can follow our Green Pool Cleanup Procedure. But diagnose first! Because if the pool is green from copper, shocking the pool (super-chlorinating) will oxidize the copper and make it worse.

If it's copper, SC-1000 can help chelate the copper, but consider using a metal removing filter to get the copper out, and address the source of the copper. If it's in the tap water, install a metal filter on the fill line to take metals out before they get into the pool.

If the White Bucket Test shows no change in the water sample, test total alkalinity (TA) and calcium hardness.  If the TA is more than 1/3 of the calcium hardness level, that could be part of the issue. We don't know why, but sometimes reducing TA and adding calcium is enough to flip a green pool to clear blue in under an hour. We've seen it happen many times.

When there is too much alkalinity and not enough calcium, water can turn green too. We honestly don't know why, and nobody has been able to explain it to us. It's a mystery, but yet we know it happens. We have seen it many times.

In this case, reducing TA with diluted acid and increasing calcium hardness may be all that's necessary to flip the pool back to blue within minutes. It's fun to watch. If you know why this happens, please contact us! We want to learn why this happens.

Finally, bromine tends to turn water a yellowish green.  We will cover more on that in a moment.

Yellow pool water

We know of three ways water can turn yellow, but there may be more that we are not yet thinking of.

• Mustard algae (yellow algae)
• Too much sodium bromide (bromine)
• PR-10,000 in a hydrogen peroxide/biguanide pool

Mustard algae tends to grow on walls and surfaces in the shade. It does not often change the color of the water, unlike certain types of green algae.

The more common cause is when chlorine (or potassium monopersulfate) oxidizes sodium bromide.

Sodium bromide is often marketed (ironically enough) as an algaecide that helps kill yellow algae. Many of these sodium bromide products even have "yellow" in their name. What most people do not realize is that bromide ions don't go away. They continue to be oxidized and recharged by chlorine to create more hypobromous acid (HOBr), which is a decent sanitizer and oxidizer itself. But HOBr cannot be stabilized in sunlight, so it breaks down quickly into a known carcinogen called bromate ion.

As mentioned earlier, bromine can also turn water green. It depends on light refraction and how much there is. A bromine pool tends to look green, while a chlorine shock that interacts with sodium bromide tends to look more yellow.  Bromine itself is brown, and in blue water it changes the color.

We cover bromine in more depth here.

Another cause of yellow-ish green water is if PR-10,000 is used in a biguanide pool with hydrogen peroxide. It's a chemical conflict, and we advise customers with biguanide pools to avoid using any of the Orenda products, because they directly conflict with the biguanide/hydrogen peroxide chemistry.

Brown pool water

Brown (or reddish brown) is usually caused by iron. It can also be called by mud or other soils/sediment that gets into the pool, or other organics like tannins. For instance, if the pool floods during a rainstorm, or has rain flowing in from a muddy hill, that can turn the water brown too.

Leaves sitting on top of a mesh safety cover for months can also cause brown water under the cover.

Generally, if a weather event is not involved, and the pool turns brown or red after chlorinating, the chlorine is oxidizing iron.  Sometimes iron can appear green too, but that's more of a question of the pool surface color, brightness of the sky (or natatorium), and how the light refracts in the water. Either way, the White Bucket Test will diagnose if heavy metals are the issue, and the solution is still the same, regardless if it's iron or copper.

Purple pool water

Water can turn purple (or stain purple) for two reasons that we know of.

• Copper cyanurate
• Manganese

Copper cyanurate is pictured above. When cyanuric acid (CYA) levels are high enough, and the copper levels are high enough, these two can combine into copper cyanurate.  There may also be oxidation involved, but we have not yet done enough research on this to know whether or not oxidation is needed for this reaction to take place.

The other way water can turn purple is from manganese being oxidized. Manganese is a metal that behaves just like iron or copper. Thankfully, it is more rare.  Manganese can stain dark purple, blue, or even black.

White pool water

White water is usually from cloudiness, or some other chemical conflict. More often than not, if the water is pure white or gray, it's carbonate clouding. PR-10,000 phosphate remover will cloud the water too (if phosphates are present).  Neither of these are related to chlorine oxidation, but we felt they were worth mentioning.

We often get asked about pools becoming cloudy and white, especially after shocking with calcium hypochlorite.  Yes, it's a powerful chlorine, but usually the white color comes from calcium coming out of solution...or in the case of cal hypo, not going into solution in the first place.  If the pH and LSI are too high when the cal hypo is added, it will cloud the water.

Closing

If chlorine changes the color of the water, it's most likely a chemical conflict (usually oxidation).  If the water is discolored and chlorine helps clear it up, it was either organic (tannins, etc.) or some type of algae. Once you know what chlorine is interacting with, you can come up with a strategy to remove it from the water and prevent more of it from getting in. If you need help, ask us.