High CYA, High TA, Reasonable FC & confused.

[daisy]

OK.

Some cousins moved to our neck of the woods. We agreed to help them with their pool, which was filled with algae. Initially we added pure (unbuffered/unconditioned) shock. They got some other shock which obviously had CYA in it.

Here's where things are:

CYA 100
FC 7
TC 18
TA off the scale
pH about 8.

Pool is approx. 15,000 gal.
In ground/Gunnite
Outdoor direct sunlight
Cartridge Filter
Pump FR etc. unknown.

Not really worried about the pool maintainance/issues per se, realize we will have to drain about 10 - 20% H20 to realize a lower CYA, also know that the FC maybe should be a little higher, but I'm puzzled.

If the CYA "binds" the Cl, then how can CYA test +? Is the CYA test testing for a part of the CYA rather than CYA per se? (Doesn't make sense to me, as I vaguely remember from HS Chemistry that acid + alkali = salt + H20 ----so why isn't the CYA being used up?). Unless the CYA is acting as a catalyst, but then, why wouldn't the pool test more acidic......???

Hoping some chem nerd out there can give me a formula or two and set me straight.....

Thanks!

Daisy

 

[Ohm_Boy]

First - that ph needs to come down asap. You should get some muriatic acid in the pool ASAP.

CYA buffers the chlorine, but the CYA itself doesn't get used up - the chlorine does. CYA effectively holds chlorine in reserve, if you will, and releases it as the available chlorine goes away. It tends to reduce the effectiveness of the chlorine in the process, which is why one must maintain a higher FC for higher CYA levels.

Note that most CYA tests have a high limit of 100, and anything above that will read 100. So you may need to replace more water than you think to get the CYA levels where they should be.

And, by now, the liquid chlorine you added will most likely be gone. It goes fast when fighting organics in the water.

I strongly suggest that you read through the topics in Pool School - see the big button at the top right of the page.

 

[JasonLion]

Welcome to TFP!

CYA doesn't get used up. It binds to chlorine and then unbinds and binds again and unbinds, remaining unchanged throughout. The CYA test measures the total CYA level, all of the chlorine unbinds during the test. CYA has nothing much to do with the PH. CYA is only a weak acid and will not react with any of the basic chemicals in the pool.

One thing to watch out for is that the CYA test will read 100 even if the CYA level is way over 100, so your CYA level may or may not be much higher. That means that 20% water replacement is the minimum you might need. The actual amount of what replacement you might need to do could be much higher.

You should lower the PH to around 7.2 while the FC level is below 10.

"Shock" is a process, not a product. When you say "shock" you appear to be referring to a product. While there are some products that have "shock" in their name, that is really very misleading and doesn't tell us much of anything about which chemical they actually contained.

With CYA around 100, shock level is FC up around 30. Having FC at 7 isn't going to do much of anything against algae as long as CYA is that high. Also, if TC is really 18, you have CC at 11, which is suggestive of some fairly serious problems. If those test results are correct, it is going to take a good amount of water replacement and some dramatic amounts of chlorine to clean things up.

If you are interested in the chemistry, take a look at the Pool Water Chemistry topic.

 

[chem geek]

This post also goes into the chemistry of chlorine and CYA and the back-and-forth that Jason talked about.