CH rising?

[DONNIE]

What would cause my CH to rise from 410 to 480 in 19 days? I have added nothing but bleach and muriatic acid in that time.

Temp 41
PH 7.6
FC 6
CC 0
CYA 60
CH 480
TA 70 (down from 80 in same period)
Salt 3600


Donnie

[chem geek]

Assuming that there was nothing wrong with the test for CH itself, the CH will rise if the plaster is curing since that releases calcium hydroxide into the water (it will also make the pH rise). If your plaster is more than one year old, then it is unlikely to still be curing (at least not that fast). If the CH rise was accurate and truly due to the curing of plaster, then you would have added 23 cups of Muriatic Acid to compensate for the very strong pH rise. I doubt very much that you've added anywhere near that amount of acid so I doubt the curing of plaster is the cause.

The other way that CH can rise is if your water chemistry is in a corrosive state with too little dissolved calcium carbonate. Your water is, in fact, slightly corrosive, but only a little bit (-0.3) and not nearly enough to cause etching or dissolving of the plaster at the rate of CH change you are seeing. If the CH rise was accurate and truly due to corrosion of plaster, then you would have added 23 cups of Muriatic Acid to compensate for the very strong pH rise (just as above for curing of plaster), but you would also have had to outgas a whole lot of carbon dioxide as well. Again, a very unlikely scenario.

Your TA is dropping because you are adding acid to maintain pH and the pH may be rising (until you add acid to lower it again) due to outgassing of carbon dioxide, but this does seem strange unless your water is getting aerated. I assume your SWG is off since the water is so cold (and you said you were adding bleach) and I assume you have waterfall or fountain features probably turned off or not running for very long. If your TA truly dropped 10 ppm (it could have been less since the test only resolves to 10 ppm for one drop), then the pH would have first risen from 7.6 to 8.67 due to carbon dioxide outgassing and you would add 2.9 cups of Muriatic Acid to restore the pH (of course, you would acid more frequently and not see the pH rise so much). That sounds a little closer to what is really going on except I suspect you added less acid and had less actual drop in TA. Of course, that only explains the TA and not the CH.

It is very unususal for the CH to rise so quickly. I would suspect that there is something about the test that is off a bit. Maybe it is sensitive to temperature -- the pool water is cold so let it get to room temperature before testing. With my own testing I used to get a "fading endpoint" which made the counts of the last drops difficult, but then read the instructions that said to add a few drops of titrant first before the calcium buffer and indicator solutions and that helped make the endpoint much more obvious and consistent.

So, bottom line, I don't have a good answer for you (but you know how I like to write...)

Richard

[CarlD]

What would cause my CH to rise from 410 to 480 in 19 days? I have added nothing but bleach and muriatic acid in that time.

Temp 41
PH 7.6
FC 6
CC 0
CYA 60
CH 480
TA 70 (down from 80 in same period)
Salt 3600


Donnie

Is your pool gunite/concrete/plaster, etc? That can do it, ESPECIALLY if it's new. If it's a vinyl pool you should be ok, especially with your T/A so low. In a vinyl pool, if your CH reaches 500, and your T/A gets high then you can end up with cloudy water.

Let us know which it is.


And Chem_Geek responded first!

[DONNIE]

#1 my pool is fiberglass. I have been adding 7oz of acid every other day to keep my Ph at 7.5 but in the last week it has been stuck at 7.6 so I just leave it there. I assume thats dut to the lower TA. My SWG is off and I have had to add very little bleach to maintain 5ppm FC. Air temps have been below freezing here for most of the last 6 weeks so I have had a fountain and pool pump running 24/7. Water temps for the last 6 weeks have been between 37 - 45. I did test the CH last night right out of the 41F pool.

Donnie

[chem geek]

Wow, that's still a lot of acid you've added. 7 ounces every other day would be over 8 cups of acid over the 19 days. Since only about 3 cups would be needed to have the TA lowered by 10 ppm, it could be that the TA test is measuring a little low so it's more like a 15 ppm or so drop. That still only accounts for about half of your pool's acid demand. At least your 24/7 use of the waterfall explains why you have to add acid and see a TA drop. What is still not explained is the additional acid demand and the increase in CH. I'll bet it's the water temperature since you did the test without letting the water get to room temperature. Try doing that the next time(s) you test and let us know if it is still rising.

By the way, it is not necessary for you to keep your pH as low when the water temperature is cold. Natural water balance will have the pH rise from 7.5 to 7.7 when the water temperature drops from 80F to 50F so you could keep your pH at 7.7 to 7.8 and probably need to add a lot less acid to keep the pH there. Your water chemistry would be more in balance at the higher pH, though it's not very far out of balance now (so the main reason to do this is to lower your acid demand since you have to keep the water flowing to prevent freezing).

I still wonder where the extra acid demand is coming from. One drop in a TA test isn't very accurate so perhaps over a longer period of time the results will make more sense.

Richard

[DONNIE]

I think I have been trying too hard to keep the Ph at 7.5 so every other day when it hits 7.6 I add the 7oz of acid. Last week I wanted to see how high it would go left unadjusted. To my suprise it has remained stable at 7.6 and I haven't had to add any all week. I'll retest the Ch tonight at room temp. Thanks for your advice.

Donnie

[chem geek]

A change in pH from 7.5 to 7.7 might cut down your rate of CO2 outgassing by almost half and cut the acid demand in half, but a change from 7.5 to 7.6 shouldn't be so dramatic as having virtually no pH rise in a week. I'm sure something else has happened coincidentally (maybe something simple like there is less wind). At any rate, the point is that you can live with a higher pH and this will have you add less acid and that is what you are seeing (albeit more extreme than expected).

Richard

[DONNIE]

Can you actually "smell" out-gassing? thought I could smell my pool the other night for the first time.

[chem geek]

Carbon dioxide is odorless so you can't smell it. If you are smelling chlorine, then that is possible (a "clean" chlorine smell unless you have combined chlorine), but I'm not sure why you'd be smelling that now all of a sudden.

Richard

[DONNIE]

Im not even sure it was my pool. Zero CC's. It's been really wet and cold here so Im sure it was just the weather. That or the Opossum thats been hanging around.

Donnie