How Much Will CYA Lower pH?

[crokett]

Has this ever been measured? Finished filling the pool yesterday. Tuesday I started adding CYA and I added Metal Free. Before the CYA I tested out the pH at .2 points above nominal - based on readings from testing my well water last season this is accurate. When I tested yesterday it was reading .8 points below nominal. I added Borax accordingly and the pump has been running overnight. This morning the reading appears unchanged so either

a) pH was actually lower than that (My water drop test kit has me comparing colors) and more Borax is needed
b) the CYA is still dissolving and therefore still keeping pH down

On a side note, I added enough metal free to treat the 3400 gallons still having another 800 or so gallons to go. Did I waste some of it? Or if there's too much will the unused part sit there waiting for water to treat?

 

[JasonLion]

CYA is acidic and it keeps on lowering PH the entire time it is dissolving. Metal Free is also somewhat acidic.

Sequestrant slowly decays in the pool. That is why regular maintenance doses are required. If you add extra it will sit around waiting for some metals to sequester but you will also slowly lose it to decay.

 

[chem geek]

Though the actual pH drop depends on the TA level, the initial CYA level and the initial pH, for typical starting numbers (TA 100, CYA 30, pH 7.5) an addition of pure CYA to increase the CYA level by 10 ppm would result in a pH drop of 0.16 units with no change in TA. Another way to look at it is that one pound of pure CYA has the acidic equivalent of about 9 fluid ounces of Muriatic Acid.

 

[crokett]

Hrrm.. well TA is 130 , CYA was 0 (unless there is some already present in ground water) and pH was 7.8 (or thereabouts, the water drop test kit is a bit subjective)

When I tested yesterday evening after adding enough CYA Tuesday and wednesday to get to 40 on the full 3400 gallons and enough sequestrant to treat the full 3400 gallons that is there now, ph was 6.8 (or below) - the test won't register if below that. I still had a few hundred gallons of water to add - finished that up last night after the test. I added enough Borax to get it back to 7.6 and tested pH this morning it was still 6.8 or below. So using chem geek's numbers the CYA by itself would lower pH ~ .64 units leaving me at 7.16. I am guessing the Metal Free dropped it further. So now should I keep trying to boost pH or wait for the CYA to dissolve? I am concerned if I keep trying to boost pH I will get a monster swing the other way in a week or so and have to adjust down.

 

[JasonLion]

If your PH test reagent is more than one year old there is some chance the reagent has gone bad. Other than that possibility you should go ahead and raise the PH to at least 7.0 as soon as possible.

 

[crokett]

Dang. sounds like I need to take a sample in to verify my pH testing is accurate.

Thanks.