Cyanuric Acid Solubility

[chem geek]

I was asked in a PM about Cyanuric Acid solubility, but thought I'd put this into a Deep End thread so others can see the answer.

The chemical structure of Cyanuric Acid is seen here, but that is when in water where it can go back and forth in two forms, though in practice the acid form on the left releases one of its hydrogen atoms into the water so that the dominant form at pool pH is the cyanurate ion (singly charged).

In solid form, Cyanuric Acid has the form with the ketone (with double-bonded oxygens; known as isocyanuric acid) and as shown here it forms strong hydrogen bonds between the hydrogen attached to the nitrogen and the oxygen (i.e. NH:::O hydrogen bonding). This is what makes it relatively slow to dissolve since the hydrogen bonds are reasonably strong (and there are many of them) so take more energy to break apart. Putting the cyanuric acid into a sock or stocking and hanging it over a return flow or in the skimmer helps it to dissolve more quickly.

If one adds a strong base to Cyanuric Acid, then one can form a sodium cyanurate and this is what is done in the Natural Chemistry® Instant Pool Water Conditioner product which is the sodium salt of cyanuric acid (i.e. sodium cyanurate) in a slurry so dissolves much more quickly. It's about twice as expensive as pure CYA for the same CYA level, however.

 

[simland]

Thanks for the insight.

I see the sock method prescribed a lot. What is the reason for the sock in the skimmer and why should one not pour directly into the skimmer?

 

[jblizzle]

Since it may take awhile to dissolve, putting it into the filter mean you can not clean the filter for a week. And you can never be sure when it is dissolved.

There was one thread when it was believed that the CYA formed a blockage in the pipe on the way to the pump ... not sure if that was confirmed and what the resolution was.

 

[techguy]

I placed my CYA (1.75 pounds) in one of those bra washing bags and placed the bag on my weir, blocking it open in the right side of my wide mouth skimmer, and all of the CYA pebbles were dissolved/gone by the next day.

 

[JasonLion]

There are a number of ways that CYA poured directly in the skimmer can cause problems, though all are rather rare. For example the CYA can accumulate in the pump strainer basket and potentially break the strainer basket. The CYA can also block the pipe and cause the pump to lose prime or possibly even overheat to the point where it damages it's self. As I said, both of these are rare, but we strive to give instructions that are completely trouble free, thus the sock approach.

 

[anonapersona]

Wehn we were working on a friend's pool we bought two different CYA products. The first was HTH brand and it dissolved really quickly when hung in a stocking over a return. The entire container was dissolved in a few hours. Since Walmart was so far away we bought the next CYA from a local pool store.

The new CYA was much denser than the HTH brand. Not sure of the brand from the pool store, instructions said to adjust pool pH (lower I assume) and then add product to the skimmer. As we were already in shock mode we could not reliably test pH so we put it into a stocking and waited, and waited. It took most of the day to dissolve and a lot ended up on the floor and was picked up by the Polaris.

It seemed to me that the HTH brand Stabilizer and Conditioner was processed differently than the second brand, HTH was softer, less dense, and so it dissolved faster.

 

[rbarba]

Thanks for making my question public, chem geek! Very helpful stuff. I suspected that an easy-dissolve formula would be much more expensive.

I'll start recommending the sock approach I think. That makes the most sense. I too have heard of a clog caused by adding CYA to the skimmer, though that was only one case, so that goes with the rarity.

Is there any harm in letting the CYA sit at the bottom of the pool to dissolve?

 

[JasonLion]

CYA is acidic, so it could potentially damage the pool surface if it stays in one place too long. If CYA gets into the pool you should vacuum it up.