The Specifics of Cyanuric Acid

[kbrendan]

Hello,

I am starting my first season as a pool owner. I have had a fair amount of chemistry training in college and grad school and I am trying to understand cyanuric acid.

The owners of the pool the previous three seasons were using trichlor tablets, algecide, and some liquid chlorine. I tested the CYA levels at the beginning of the season and they are high, around 60 to 70. So, my plan is to continue using liquid chlorine and to increase the pH (it seems to be very low, like not even on the chart), but NO more trichlor. I should mention, our water is perfect right now. It was crystal clear last season when the pool was closed, and has been the same the past 10 days since opening.

When the pool was first opened 10 days ago the FC levels were testing at 0 and I put in about 10 gallons of bleach. My pool is 28,000 gallons, and I wanted to get the FC level up to about 28 to shock based on our CYA levels (about 60). I tested a day or so after the shock and the FC levels were very high. When I tested again about 4 days later (too long, I know), the FC levels were again zero. So, I added another 2 gallons of 10% liquid chlorine and I am going to test tomorrow.

When you use the daily chlorine/pH test in the "TF FAS-DPD Test Kit," the chlorine test is for FC, right? Meaning if it is testing zero, (i.e. still clear water after adding the drops), then you might have chlorine in reserve with CYA but no active free chlorine? Is that the correct way to look at it?

This is what I don't understand about CYA.... (again, our levels from the previous owners are pretty high, about 60). Why do you need to maintain higher levels of FC at baseline for higher CYA levels if, once you saturate the CYA reservoir, then the FC should be able to do its job? Does CYA really keep a "reservoir" of chlorine? Because if so, then even when the FC levels are testing at zero, I wouldn't necessarily have to put as much FC in to get the levels back up, because there is already a fair amount of chlorine in reserve just waiting to tip out of the CYA reservoir and into the FC "pool" (no pun intended). Is that an accurate way to think about it?

I have not seen a speck of algae in our pool at any point last year or this year (knock on wood) and I really want to keep it that way, but I have no idea how much chrloine I should be adding on a daily to weekly basis in a 28,000 gallon pool with a CYA of 60 (and NOT adding more right now) with minimal use, cool temperatures so far, and moderate sunlight. I honestly don't even know what would be a reasonable range for maintenance chlorine.

Sorry for the longer post. Thank you so much to anyone who takes the time to reply!

 

[JohnT]

Welcome to TFP!

The chlorine protected by the CYA is just a percentage of the FC in the pool. When it tests as 0ppm, there is none in there.

The daily or OTO test gives you total chlorine, not free chlorine. In the absence of issues, the OTO test is fine.

 

[Bama Rambler]

Welcome to TFP.

I don't really like the "Chlorine Reservoir" analogy, but it's an easy way to explain kind of what happens. CYA really protects the FC from being consumed by UV rays from the sun and it also moderates the strength of the FC in the water. The more CYA you have the more FC you need to provide the same killing power (for lack of a better description).

When testing using the OTO as JT said, it's Total Chlorine and if it says zero then it is. Also keep in mind that it's only designed to measure to 5ppm TC.
The Fas-DPD test is designed to test up to 50ppm FC.

Also 60 ppm CYA isn't really that high. I regularly run 50 to 60 ppm CYA in my pool due to the amount of direct sunlight it gets.

In a good situation you should have to add about 2 to 3 ppm FC each day. You can use PoolMath to figure how much liquid that is for you.
Regardless to what you need to add. You simply run the chlorine test, add the suggested amount from PoolMath, then retest in an hour to see if it hit target.

 

[samt]

I have also found out like Bama that my pool likes a little more cya than the recommended. Keeping a log of my pools chemistry has helped me tweaked what it needs.

 

[ewkearns]

When the pool was first opened 10 days ago the FC levels were testing at 0 and I put in about 10 gallons of bleach. My pool is 28,000 gallons, and I wanted to get the FC level up to about 28 to shock based on our CYA levels (about 60). I tested a day or so after the shock and the FC levels were very high. When I tested again about 4 days later (too long, I know), the FC levels were again zero. So, I added another 2 gallons of 10% liquid chlorine and I am going to test tomorrow.

Shocking the pool is not a one-and-done process. Shocking is a process carried out for a period of time until the pool meets certain standards. 10 gallons at one time is overkill and letting the FC drop back to zero wasted those 10 gallons. You need to return to Pool School and read the specifics of SLAM.

I have not seen a speck of algae in our pool at any point last year or this year (knock on wood) and I really want to keep it that way, but I have no idea how much chrloine I should be adding on a daily to weekly basis in a 28,000 gallon pool with a CYA of 60 (and NOT adding more right now) with minimal use, cool temperatures so far, and moderate sunlight. I honestly don't even know what would be a reasonable range for maintenance chlorine.

If that much chlorine disappeared in that short a period of time, my guess is you do have algae.... or something.

Check here to modify your signature.

Here is a MUST USE calculator.

Latest scoop on SLAMming.

 

[Patrick_B]

Welcome to TFP kbrendan!

Your off to a good start with those questions. It won't take you long to master this at all.

 

[kbrendan]

Thank you all very much for your help! I have obviously been monitoring the clarity of the pool water and also the other typical levels including the daily test for total chlorine, but I forgot about also keeping in mind the CCs and the loss of FC overnight.

Currently the water is clear (which it has been consistently thus far, thankfully), has 2 to 3 ppm of FC, and has no detectable CC's. I am not sure yet about the loss of FC overnight. So, is this a reasonable state for my pool even with 60 of CYA? I have been getting really caught up in the higher target level for FC with my CYA level (the chart says minimum FC of 5 and target of 7).

Thanks again!

 

[jblizzle]

No it is not reasonable. Your FC should never be below 5ppm or you are in danger of algae developing.

- - - Updated - - -

FYI, it is normal to have to add 2-4ppm of FC every day. PoolMath will calculate how much bleach that is for your pool.

 

[kbrendan]

Ok that makes sense, thanks.

 

[chem geek]

I have had a fair amount of chemistry training in college and grad school and I am trying to understand cyanuric acid.

When you use the daily chlorine/pH test in the "TF FAS-DPD Test Kit," the chlorine test is for FC, right? Meaning if it is testing zero, (i.e. still clear water after adding the drops), then you might have chlorine in reserve with CYA but no active free chlorine? Is that the correct way to look at it?

This is what I don't understand about CYA.... (again, our levels from the previous owners are pretty high, about 60). Why do you need to maintain higher levels of FC at baseline for higher CYA levels if, once you saturate the CYA reservoir, then the FC should be able to do its job? Does CYA really keep a "reservoir" of chlorine? Because if so, then even when the FC levels are testing at zero, I wouldn't necessarily have to put as much FC in to get the levels back up, because there is already a fair amount of chlorine in reserve just waiting to tip out of the CYA reservoir and into the FC "pool" (no pun intended). Is that an accurate way to think about it?

If you know chemistry then you can look at the technical details regarding the chlorine/cyanuric acid relationship in the thread Pool Water Chemistry and in particular the derivation of the FC/CYA ratio as being proportional to the active chlorine (hypochlorous acid) level as described in this post. There is a chemical equilibrium between chlorine bound to CYA (a series of compounds called chlorinated isocyanurates) vs. chlorine that is unbound (hypochlorous acid and hypochlorite ion). The chlorine that is bound to CYA is for practical purposes not a disinfectant nor an oxidizer (< 1/150th the oxidation rate of HOCl). You can see more details in the "Chlorine/CYA Relationship" section in the first post in the thread Certified Pool Operator (CPO) training -- What is not taught.

Chlorine is released from CYA fairly fast (if HOCl is depleted) with half released (through one of several pathways) every half a second. So the analytical chlorine tests that are based on wet chemistry (e.g. OTO, DPD, FAS-DPD) measure the sum of hypochlorous acid, hypochlorite ion, and the chlorine bound to CYA because as hypochlorous acid is used up more is created from the other forms to maintain equilibrium and this happens well within the timeframe of the chlorine test. However, the rate of disinfection or oxidation depends on the instantaneous concentration of the active chlorine species hypochlorous acid (some oxidation reactions are done with hypochlorite ion, but most with hypochlorous acid, and disinfection rates with hypochlorite ion are from 20-50 times slower than with hypochlorous acid). So direct and indirect measurements (e.g. ORP, HOCl sensors, pathogen kill rate experiments) that depend on hypochlorous acid give results proportional to the FC/CYA ratio rather than to the FC level by itself.

As for the "reservoir" concept of chlorine bound to CYA, one analogy is to think of soldiers fighting a war where the front-line soldiers are in battle with the enemy trying to kill them. The fact that you have a large reserve of soldiers able to replace those of yours that have been killed has nothing to do with the rate of killing the enemy. Such rate is dependent on the number of front-line soldiers and not on the amount in reserve. The amount in reserve just tells you how long you can continue to fight.