Cyanuric Acid Location: Top, Bottom, or Everywhere?

[TraylorMan]

Has anyone collected and tested simultaneous samples of a still pool, one from the top and one from the bottom? If so, we could answer the question of whether cyanuric acid is evenly distributed or settles near the top or bottom of the pool.

If we knew where CYA collected, we could more efficiently drain our pools to remove excess quantities.

Here are some relevant molecular molar masses:
18 g/mol Water (H2O)
58 g/mol Salt (NaCl)
111 g/mol CalciumChloride (CaCl2)
129 g/mol Cyanuric Acid (C3H3N3O3)
232 g/mol Trichloroisocyanuric acid (C3ClN3O3)

So if cyanuric acid is more than 7 times heavier than water, it should exist in higher concentrations at the bottom of the pool provided there is sufficiently low turbulence. But even absent turbulence, diffusion competes with gravity, so the question is whether gravity is able to make a meaningful difference.

I am considering setting a sample collector at the bottom of the pool right-side up and one at the top of the pool upside down, leaving it over night, then stealthily capping the lids first thing in the morning and testing both. Has anyone already done this?

 

[chem geek]

Welcome to TFP!

When water-soluble chemicals are mixed in water, they stay mixed even when there is no circulation. See the NASA paper Fugacity and Concentration Gradients in a Gravity Field. Basically, the concentration effect in normal gravity is less than 0.1% in 1 meter so would still be negligible at pool depths. At ocean depths, the concentration ratio of bottom depths divided by that at surface depths for sodium chloride salt would be around 2.4. For something like Cyanuric Acid, then it might be somewhat higher, but even substances that have concentration ratios of 243 at ocean depth (e.g. for maltose), still have less than 0.1% concentration ratio in 1 meter depth.

We really need to squash this idea of denser soluble substances somehow measurably concentrating at depths in pools. That simply does not happen, period. If you want to waste your time doing some experiments on this, go ahead, but the science is very clear on this. One should not think that an effect that is only seen (with normal gravity) at ocean depths is going to show up at pool depths. We are not talking about settling of insoluble materials, but soluble chemicals which by definition means diffusion effects far outweigh that of gravity unless the depths are large or the substances have huge buoyancy differences.

Now this should not be confused with the opposite scenario which is starting out with a denser chemical that is unmixed with water above it. This is the principle in The Liquidator where denser chlorine is below water that is above it. If there were no water flow, it would take diffusion to mix the two (if there were no thermal gradients) and that is very slow taking 18 hours (at spa temperatures, so even slower at pool temperatures) to get a 50% concentration 1 centimeter into the water above the chlorine. What happens in practice is that the slow water flow also participates in pulling some of the chlorine from the bottom layer (i.e. there is some modest circulation in the water above the chlorine).

People sometimes report gradations of CYA, CH, or salt when opening pools, but that is because winter rains sit on top and have not been mixed with the water below. There was virtually no separation of soluble chemicals in the water itself. Instead, water without such chemicals was added to the pool without sufficient mixing/circulation.

 

[TraylorMan]

Thank you, chem geek, for your insightful response. I studied the NASA report you provided, and I agree with your conclusion: cyanuric acid will exist in equal concentrations everywhere in the pool.

 

[Ale_Brewer]

You can't argue with that response! Makes sense though. Funny story... When I first bought my house with a pool, I knew nothing about taking care of a pool. So I hired a "pool professional" to "maintain" the pool for 3 months while we finished unpacking and getting settled in, with the idea that I will watch her closely and learn what I need to do. Yes, my pool guy was a woman. And she was pretty easy on the eyes, to boot. Anyway, she's the reason my pool was jacked before I found this site. I know that as a responsible adult I should have done the research but I trusted the "professional". When she came by the first time, she tested the water. This was the first and last time I saw this test kit used. She said the CYA was too high. So high that if my pool were a public pool, they would have to shut it down for safety reasons. I had no idea what cyanuric acid was at the time. She said to turn the pump off for 8 hours and the CYA will rise to the top like cream in milk. Then turn on the water hose and put it at the bottom of the pool and leave it on for 8 hours. The theory was that the CYA will drain out of the pool from the overflow. So basically her method worked because I replaced some water, but I have no idea what my level was, what it went down to, and I wasted a lot of time when I could have just pumped some water down the drain. This is why I tell people not to use "pool guys."