PH/ALK Results -- Should Not Be Possible

[johnsimion]

I now have both the Taylor and ColorQ test kits. ChemGeek sent me a standard 100 sample of ALK and the Taylor tested exactly 100 whereas the ColorQ showed 91. I mention this only to show that I believe I am doing the tests right. However, I am still getting bizarre results. I have been testing with both kits just to see what differences result, and not only are there differences in the kits, but the pool is simply not responsive to acid. I give the Taylor results first with the ColorQ results in parentheses.

10/10 (test 1): PH 8.2+ (7.8), ALK 110 (89). I wanted to bring the ALK down to the recommended level by reducing the PH to 7.0. PoolMath said to add three quarts (96 oz) of acid so I did so. This was labeled as 29% variety and is only a couple of weeks old. Waited 1-1/2 hours and retested.
10/10 (test 2): PH 7.5-7.8 (7.5), ALK 110 (81). PoolMath said to add 74 oz more to get to PH 7.0, but I only had 60 oz on hand, so I added that. By this time there was not enough time to circulate a full 1-1/2 hours so I waited until morning. Tested at about 10:00 after the pump ran a couple more hours in the morning.
10/11 (test 1): PH 7.2-7.5 (7.3), ALK 110 (77). PoolMath said to add 80 oz more acid to reach PH 7.0, so I added that. This was fresh acid labeled 31.45% that I got from Leslie's. It smells strong. Waited until 2:30 to retest.
10/11 (test 2): PH 7.2 (7.1), ALK 100 (65). PoolMath said to add 47 oz more of the 31.45% acid to reach PH 7.0, but I actually added 49 oz to finish out another jug. Pool ran about an hour but I waited until it ran a couple of hours this morning (i.e., 10:00) before retesting.
10/12 (today): PH 7.2 or more in that the color is still pinkish, not orange (7.3), ALK 100 (61).

According to the pool builder, the pool has 19,000 gallons. According to the construction diagram, it has 610 sf of area. I have measured it at 39.5' long and 17' at one end and 14' at the other. It slopes gradually from 3.5' down to 5.5', with about 700 gal in the spa and there is also a sun area that is circular and around 7' diameter with about 1.5' depth. These give me volume results greater than the pool store, but not a huge amount greater.

At this point I have put 2 gallons and a quart of acid into the pool and never yet achieved a PH of 7.0. Granted, I have a hard time reading the Taylor PH test but even the ColorQ has never reached 7.0. Moreover, the ALK has dropped a less-than-whopping 10 from the starting point according to the Taylor test. Even with the ColorQ, it isn't much of a drop in ALK considering how much acid has been added. It is incredible that the PH actually rose last night even though I put 49 more oz of acid into it. Nothing else has been added to the water other than a little from the feed line -- which Taylor shows has a PH of 7.5 and ALK of 160. Really there shouldn't have been very much feed line water going into the pool in the last two days anyway, considering it's cool here now.

These results should be chemically impossible, right? How the heck can anyone add over 2 gallons of acid to a pool of this size and still have PH over 7.2 and ALK barely changed. For that matter, how could PH actually RISE overnight after adding 49 oz of acid??? The only thing I can think of is the spillover from the spa due to the in-floor system, but it really only spills over for about 5 minutes at a time for a couple of times an hour. For a long time it has sat at PH 7.8 and ALK in the 90s (according to the ColorQ, anyway). I never saw the PH go over 7.8 with the ColorQ whether I added acid or not.

Any ideas what is going on here? I am beginning to think it is impossible to reduce the PH and ALK in this pool.

FYI, the last full test of the pool water was also on 10/10 and showed this:
FCL: 5+ or 7.5 using the "better" Taylor test (5.17 on ColorQ)
CH: 400 (232)
CYA: 37 (39)
Salt: 4200

 

[JasonLion]

There are several different possibilities. Most likely more than one of them is involved.

The acid you are using might not be a strong as you think it is. 31.45% muriatic used to be very common, but more and more stores are now carrying 15.7% instead.

You may have a significant borate level. Borates buffer PH changes separately from TA.

You may be adding more high TA fill water than you think.

How recently was the PebbleTek applied? If the PebbleTek is less than a year old, it will be raising the PH as the plaster cures. If it is less than a month old it will be raising the PH rapidly.

There may be a significant amount of PH drift going on due to aeration, which constantly raises the PH. This is actually certain to be happening to some extent, but from your description it is difficult to tell if it is the dominant effect or not. High TA plus aeration results in CO2 outgassing, which raises the PH. TA at 100 is certainly high enough to cause the effect, and you certainly have some aeration. Unfortunately, it is difficult/impossible to quantify just how much aeration you have.

By the by, why are you trying to get the PH down to 7.0? Lowering PH that low just speeds up the rate of PH drift.

 

[johnsimion]

Thanks for helping. I can give a little more information.

You said, "The acid you are using might not be a strong as you think it is. 31.45% muriatic used to be very common, but more and more stores are now carrying 15.7% instead." This is a valid comment but all I can go by is what's on the label, and the first acid said 29% and the new one says 31.45%. Maybe they lie? But they came from different pool stores, maybe they are both lying?!?

You asked if I had a significant borate level. I have never added borates to the pool, unless they are getting in from somewhere else (?).

The TA of the (soft) fill water has been tested with both the Taylor and ColorQ several times. Also tested the hard water and soft water from the inside softener and I always get about 150-160 on Taylor and 110-120 on the ColorQ.

You may be onto something with the Pebbletec. The pool was brand new in February this year. This is the first time I've heard that Pebbletec has that kind of effect, but that could be true. And the spa spillover is almost a constant aeration. Dunno what I can do about that other than changing the way is spills over so it makes one thick stream rather than the present wide sheet of water that falls a couple of feet and then splatters against the side of the spa when the wind blows (and loves to make calcium there, too).

You asked why I'm trying to get the PH down to 7.0. Well, I was trying to lower the alkalinity -- like you said, "High TA plus aeration results in CO2 outgassing, which raises the PH." The PoolMath tells me my goal for TA should be 60-80, soooo ... that's what I was trying to achieve. And, according to the Pool School (and as stated directly on PoolMath), to lower the TA I was supposed to lower the PH to 7.0 or 7.2. Did I misunderstand this?

 

[JasonLion]

Thanks for the clarifications.

If I am following correctly, the fill water TA is higher than the pool water TA. If so, it is going to be impossible to lower TA permanently. Adding water to the pool will just raise the TA back up. Fill water TA at 100+ is what I call high. With high TA fill water keeping the PH under control is often a constant battle (Depending on how often you add water to the pool, but in Nevada I'm assuming that is going to be often.) An automated acid feed system is often a good choice in this kind of situation. In any case, one way or another you will need to add acid regularly to keep the PH under control.

Fresh PebbleTec will constantly raise the PH for roughly a year. The effect isn't as large as it would be with regular plaster, but there is still plaster between the pebbles and you still see PH increases as that plaster cures. My guess is that this isn't the main issue, TA+aeration sounds like the main issue, but it is still enough to be noticeable even without the TA+aeration issue, so things will improve at least a little next year.

 

[chem geek]

If there was any curing of plaster where calcium hydroxide is produced, then you should find the CH level rising as well as the pH and TA. If you didn't add acid and calcium hydroxide was coming from the plaster between the pebbles, then you'd see 10 ppm CH and 10 ppm TA rise and the pH would rise substantially from 7.5 to 8.5 in this example (with a starting TA of 80 ppm).

Certainly, the unusually high CH readings and the inability to lower pH and TA with acid could be due to this effect though I wouldn't think the effect to be so strong and so fast with the plaster/pebbletec being done in February. After adding acid, that would lower the pH and TA back down to where they started, but the CH would remain high. This does sound like what you are seeing -- I'm just surprised if that much calcium hydroxide is getting into your pool.

Basically, the curing of plaster (concrete) is the following equation:

2Ca₃SiO₅ + 7H₂O ---> 3CaO•2SiO₂•4H₂O + 3Ca²⁺ + 6OH^-
Tricalcium Silicate + Water ---> Calcium Silicate Hydrate + Calcium Ions + Hydroxyl Ions

 

[johnsimion]

I think Chem Geek and Jason Lion are correct. I think the combination of curing Pebbletec, spa spillover, and high TA content in the fill water are simply combining to raise the TA, and I'm inclined to agree that there isn't much I can do about it, unless I can shut down the spa spillover that goes on all day long. I still don't understand why the acid isn't having the expected effect, though. Man, it seems like I've dumped a lot of acid into the pool recently and didn't achieve much of anything. Weird. Say -- what would happen if I just ignored the PH and TA and let them do what they want? At what point would my pool become an unpleasant place?

FYI, my neighbor just around the corner from me put in his pool at practically the same time as I did, and with the same builder. The only difference is that he has a plaster pool, hard water fill line, and he still uses the inline feeder. He wanted me to help him clean his filter today and my "price" was to test his water to see if he was where I was before I drained and refilled. The Taylor test says his six month old pool has CH of 600, TA of 110, CYA of 110+, and FCL of 10.5. Those are very nearly the same as I had before I drained and refilled, except his PH was only 7.5. When we bought our pools, the builder told us we could use the inline feeders for five years before needing to drain and refill. BTW, he has a pool guy and also tests his water at Leslie's. They both told him his water is perfect.

Thanks to everyone for helping, I just don't think there is much else I can do ... it's the water and the climate here. At least I am no longer accumulating CYA and can run a lot less chlorine since I put in the SWG. And the soft water fill at least slows down the calcium accumulation. But the TA and PH seem like they are going to be a continuing battle.

 

[JasonLion]

I assume that the acid is having the expected effect, but the constant PH increase has enough time between PH measurements to partly/mostly cancel out the PH reduction from the acid.

If you stop adding acid both the PH and TA will go up. Eventually the PH/TA will get to levels high enough to cause calcium scaling, which is rough, unsightly, and annoying to remove.

 

[chem geek]

I think Chem Geek and Jason Lion are correct. I think the combination of curing Pebbletec, spa spillover, and high TA content in the fill water are simply combining to raise the TA...

Technically, the release of calcium hydroxide from curing would raise both the TA and the pH, but the aeration from the spa spillover would not change the TA and would only have the pH rise. Evaporation and refill with high TA fill water (or any TA in the fill water, for that matter) would have the TA rise.

Your neighbor's pH is likely staying in check from the "inline feeder" if he's using Trichlor pucks since they are very acidic. In fact, he might even be adding pH Up to compensate because otherwise his TA might even drop though that depends on the evaporation/fill amount.