How CYA handles CL

[dolphin]

Not sure if this question is for the 'Chem Geeks' or not...
I'm just curious on how it all works... from a chemical ( well not a too complicated explanation).

with a CYA of 0 or 100 for example, a test kit will report the 'same' reading as tho CYA is present in the water or not.
Whereas, if the sun where capable to do a test it CAN only see a little bit of CL and i guess the micro organisms can only see a portion of it as well.
My understanding is that CYA does and i dont doubt that it doesn't, protect and hold back the CL from the sun and micro organisms.

my question is: why does a test kit see ALL of the CL in the water and why the sun and micro organisms dont ( hence, why we have to shock proportionally higher as the CYA becomes higher).
The more i understand how to keep a clear trouble free pool ( and thanks to TFP for explanations), the more i want to understand the science on how the sun (UV rays?) affect CL, and the behavior and effects of chlorine

 

[pwrstrk]

Where's chemgeek ? He'll answer that for you.


Sent from my iPhone using Tapatalk.

 

[JasonLion]

CYA binds to a percentage of the chlorine and holds it in reserve. When unbound chlorine gets used up, more of the bound chlorine gets released so that a constant ratio between bound and un-bound chlorine is maintained. The exact ratio depends on the CYA level, with more held in reserve at higher CYA levels.

The test kits measure chlorine by using it up. As they use it up, the CYA releases bound chlorine to maintain the ratio, until eventually all of the chlorine is gone. Because of this, the test kit sees all of the chlorine.

 

[dolphin]

so a test kit uses up the Free CL not held in reserve until the CYA releases more of it which gives the test kit a full reading of CL.
Why is this different for the micro organism to have to see more of the CL that the test kit can see ? is it a time thing ?

 

[JasonLion]

Yes, essentially it is a "time thing". Sanitizing ability, killing algae/bacteria/viruses takes time and the speed that happens at depends on the amount of unbound chlorine in the water, more unbound chlorine means faster kill times for organic contaminates. For example, with algae it is essentially a race between the rate at which algae replicates, and the rate chlorine is killing the algae. When most of the chlorine is bound to CYA, it isn't available to kill algae, so less algae is killed per unit time. A given quantity of chlorine can still kill a given quantity of algae, but with some of the chlorine bound to CYA that process takes longer, and in the meantime your "given quantity" of algae has grown into more algae.

 

[dolphin]

i get it, its who can supply or be there first... the algae cells or the CL atoms ( or free CL the CYA can lend or free up).
is it that a test kit uses so little ( in terms of micro or nano samples) that it is capable of getting the full balance of CL ?
The Test kit consumes the free but i guess so little of it is needed that the time it takes to replenish it is negligible?

 

[chem geek]

Think of soldiers on the front-line fighting a war. The rate of kill is a function of how many front-line soldiers you have. If a front-line soldier gets killed and gets replaced from the reserve, even instantaneously, it doesn't matter how many soldiers you have in reserve. The amount in reserve only tells you how long you can fight, not how quickly the enemy gets killed. The chlorine bound to CYA is for practical purposes inactive in terms of killing pathogens and algae and oxidizing bather waste.

The unbound chlorine reacts very quickly with the DPD dye in the chlorine test. The chlorine bound to CYA is released reasonably quickly where if all the unbound chlorine were to be continuously and instantaneously consumed, half of the chlorine bound to CYA would be released in 1/4th (0.25) second so 93.8% is released in 1 second, 99.6% in 2 seconds. So, in practice, you measure all the chlorine as FC in the time of the test -- you measure the sum total of the original hypochlorous acid, hypochlorite ion, and chlorine bound to CYA in all of its various forms.

In a DPD test, there is enough DPD dye to react with all the FC amount and still have an excess of DPD. That is why the DPD that has reacted with chlorine gets more intense in color. If you did not have enough DPD then you couldn't measure higher amounts of chlorine. The FAS-DPD works a little differently in that it's not critical to have excess DPD because you back-titrate by counting-the-drops of FAS reagent which reacts with chlorine regardless of whether it had colored the dye or is still unreacted with the dye. When the FAS reacts with all the chlorine, then there is none bound to the dye so there is no color and that's the end of the test.

 

[dolphin]

i did not know that how the Test kit behaves with the chlorine, now i know, very interesting and informative.
the 3 criteria rule works. When i left the pool in the hands of inexperienced personnel, the pool turned into a swamp.
When i came back (5 weeks later), the pool was a swamp where you could not see the bottom or the white of the drain lol - and its only a 5 foot deep pool lol.
just like TFP recommended , the 3 rule criteria. the water slowly but surely, completly cleared up ( obviously not overnight but logarithmically clearing up if that make sense).
Since then, 5 months ago, never had any issues with the water.
i do watch the water for signs every day and we do get daily heavy rains now.
sometimes the CL levels go down to 1 @ CYA of 7 , but i quickly top off the levels back up to 3 or 4 but i never get any glow around the drain
( a halo around the drain if the water starts to become cloudy troublesome).
if it does, sometimes i shock according to the Pool Calculator then its gone for good ( the slight halo effect).
Other than that, all is quiet on the war front for my outdoor pool.
Obviously, something is working right lol.
the Pool Calculator and TFP site were instrumental in clearing up and calculating the right amounts of chem for the water
Good Job!
Ps: i feel maintaining a pool is just as much a science as well as an art, you can quote me on that

 

[chem geek]

How are you measuring a CYA level of 7 ppm? Is that from a turbidity meter? It's certainly not from a visual turbidity test kit since those only go down as low as 20 ppm.

 

[dolphin]

that is correct. it is a turbidity test kit with with graduating markings as low as 20 ppm on the rod. Its an L shaped rod with the black dot facing upwards.
Letting the rod sit where it can be its lowest : 0 ppm, the dot is 1/2 or 3/4 still visible, so i'm kinda interpolating between 0 and the 20 ppm mark
i'm slowly dissolving 95% trichlor tab pieces but very slowly so as to just keep the CL ppm above the happy point - 1.5 or 2 .
if it needs help, say on a sunny day, i'll pour Sodium Hypo now 11% strength ( i keep it in a spare fridge @ 55 F).
most times its cloudy, so the CL ppm doesn't usually fall between 2 or 3 but i'm careful to be sure the CYA doesn't creep too high either.
So a balance of slow releasing triclor (monitored closely) with a boost of Sodium Hypo to touch it up or fine tune it - all that and no sings of
algae or the start of cloudiness.
If i see signs of trouble usually a little more chlorine.
a good question i can ask is :
if the water looks great - no noticeable changes to the clarity or quality of the water with an appropriate amount of CL to keep things in check, is it still possible for something in the water that is consuming (CL Demand) the daily CL ppm readings or is it just the sun depleting the CL?
i am very slowly releasing Triclor to shore up the CL during a direct above sunlight?
The water usually behaves and becomes nice and clear when i add or boost the CL levels a little higher than 3 or 4 ppm.
BTW, i get alot of comments on the quality of the water presently and do think so to

 

[chem geek]

if the water looks great - no noticeable changes to the clarity or quality of the water with an appropriate amount of CL to keep things in check, is it still possible for something in the water that is consuming (CL Demand) the daily CL ppm readings or is it just the sun depleting the CL?
i am very slowly releasing Triclor to shore up the CL during a direct above sunlight?

You will have chlorine demand during the day from the loss of chlorine from the UV in sunlight. At your low CYA level, a sunny day will deplete a lot of chlorine -- as much as 80% of your starting FC level. That's why we recommend higher levels. Even in an area with mixed clouds and sun a level of 30 ppm would be more appropriate. You've apparently been lucky and probably not had a full sunny day which I find surprising for Puerto Vallarta, but with daily rains that implies clouds to shield the sun.

I wouldn't go below 20 ppm because you can't test that reliably and you want some CYA in the water to moderate chlorine's strength. Your active chlorine level is higher than it needs to be, oxidizing your swimsuits, skin and hair more than need be. Most users on this forum don't have a problem with water clarity maintaining an FC that is at least 7.5% of the CYA level whereas you are much higher than that is from 14% to 57% so on the high end it's higher than our shock level recommendation which is 40%.

 

[dolphin]

i will keep those figures in mind. I had not known that..
As a result, I will increase the Triclor ( so to bring up the CYA) but at the same maintain a 2 or 3 ppm ( ideally 1.5 ppm) CL.
I wont artificially boost the CL with Sodium Hypo if it doesn't fall below the above levels.

I'm just worried the CYA levels could get too high but i dont think its going to happen anytime soon, since the dissolving of the Trichlor is slooowww.

 

[chem geek]

For every 10 ppm FC added by Trichlor, it also increases CYA by 6 ppm. With 2 ppm FC daily chlorine usage, if fully provided by Trichlor, would increase CYA by 36 ppm per month. If you only have 1 ppm FC daily usage due to the cloudy weather, then that's 18 ppm CYA per month increase. So it's faster than you think unless you have water dilution, which you might if you get rain overflow.

 

[dolphin]

as a matter of fact, yes we have torrential rains (now into hurricane/wet season).
every other day i do have to remove an inch or 2 of water, so yes, some dilution occurs.

absolutely, i do check CYA concentration every 10 days, to see if its not getting out of control.
I do keep very in touch or in tune with the pool calculator for i know that as the CYA increase, so to is the need for FC or shock or both.

the CYA tests are expensive! i get 10 CYA test pills for $20.00 , eechh! so i'm careful how i dispense the test ( every 10 days more or less).
if i get close to 20 or 30 ppm CYA i'll have to perform the test a little more often so as to not exceed 50 ?
if thats the case, then i'll have to switch to Cal Hypo or Sodium Hypo.
Replacing some of the water is an option but the price of water is pricey lol.
For now the rains do dilute the CYA to some extent by draining the excess.
One supplier said having CYA of 100 is the norm for Puerto Vallarta but i find that a little too extreme !
it would be like a heroin addict having to want heavy doses of CL if the CYA gets to that point.
I dont think i'll get i past what the Pool Calculator suggest's ( from what i can recall it's between 50 ~ 80 ppm, 80 being MAX).
We had very heavy rains last night, turned on the pool light (excellent way for turbidity test), it got notably cloudy but by this morning
the cloudiness wasn't present .