pH Problem?

[Jaywalker]

On 10 May 2010, due to high CYA (~150) and CH (~550), I drained our pool of approximately 80%-90% of the water and began to rebalance. Unfortunately, I've been unable to keep the pH balanced and have had to continuously add acid. Here's a recap, summarized from our chemical log.

11 May - pH=7.8, TA=170, FAS FC=1, FAS CC=0, CYA=0 (or less than 20, anyway), CH=240.
I added Bleach and Baking Soda to balance and won't comment further on them except as necessary.

16 May - pH=7.6, TA=110, DPD FC>2,
By this time I have added 166 oz. of Muriatic Acid and believed I was nearing balance. This was not to be.

29 May - pH= 8, TA=90
By this time I have added an additional 62 oz. of Muriatic Acid. Since I use Trichlor pucks when gone for a few days, I decided to bring the TA up to about 100. I used 1 pound Dichlor to bring up CYA.

17 June - pH=8, DPD FC~3
By this time I have added an additional 125 oz. of Muriatic Acid. Also, I've become tired of breathing MA fumes and have bought Dry Acid.

24 June - pH=8, DPD FC>2, CYA =30
I've added an additional 63 oz. of Dry Acid in the past week. I've also noted an increase of pH immediately after the addition of liquid bleach, which I first attributed to the local H.E.B brand, but it appears present even with regular Chlorox. I have now shifted to Trichlor tabs exclusively, which seems to slow the need for Acid, adds CYA, and is simpler, given my constant need to add acid.

2 July - pH=8
Have added an additional 63 oz. of Dry Acid. Have noted that rainwater and tap water have pH=7.4.

9 July - ph=8, TA=110
Have added an additional 38 oz. of Dry Acid.

17 July - pH=7.9, TA-130, CYA=34, FAS FC=3.5, FAS CC=0
Have added an additional 40 oz. of Dry Acid.
Edited after questions: Also, to level the TA, since 29 May I have added 27 pounds of Baking Soda.

So there you have it - over the time period, I've used 9.8 oz. of Muriatic Acid per day , then changed to Dry Acid and am using 7.0 oz per day.

Do I have a problem?

Thanks.

 

[Guest]

Are your tests really that far apart?

I add about 24 ounces of acid a day to my pool. I have lots of aeration, and it rises pretty quickly. My TA is at 80, and I am considering Borates, but for now I just keep adding acid. If you are adding about 10 ounces a day (I don't see if you have any aeration), I don't think that is unreasonable.

 

[JasonLion]

It sounds like your fill water has a fairly high TA level. Other than that, everything sounds normal.

High TA tends to cause the PH to drift up. If you aren't already, try lowering the PH down to 7.2 each time it gets to 7.8. That will give you a little more time between acid additions (compared to lowering PH to 7.5 as we normally recommend).

 

[chem geek]

I assume the pool is not new or newly re-plastered, otherwise the curing of plaster will require a lot of acid addition, especially in the first months.

 

[Jaywalker]

chemgeek, The pool was new 2006 and is not replastered. Some granular, non-dissolved, acid grains may have opened dots of white in the blue plaster. That began after the pH issue which started with the pool drain/refill in May. I did not have this pH issue prior to drain/refill. However, this is my first season with this pool, and I can't say if the previous owners had the same issue last year.

simicrintz, No, I test the pool daily for FC and pH, with CC tests weekly. I didn't believe anyone would read my complete log, so I summarized pH and acid additions.

Jason, I haven't tested the tap water TA, so there certainly could be something there. We have lots of evaporation here in Texas, so there's lots of corresponding refill. This wouldn't have shown up as much in the Spring. I'll check it and report back. However water stocks here vary their sources from groundwater normally to river water at times, so I won't be sure at any point what I'm testing. Still, I'm having to add occasional Baking Soda to raise it as I depress TA with my acid additions, and I haven't noted variances except what appear to be related to acid and Baking Soda additions.

I'm gaining the impression that this isn't an issue.

 

[chem geek]

9.8 fluid ounces of full-strength Muriatic Acid (31.45% Hydrochloric Acid) would be equivalent to 13.2 ounces weight or around 8.8 ounces volume of dry acid (93.2% Sodium Bisulfate). The 9.8 ounces of Muriatic Acid per day in 12,000 gallons would lower the TA by 3.2 ppm per day while 7 ounces volume of dry acid would lower the TA by 2.5 ppm per day.

Even if you were using bleach that was high in lye with a pH of 13 and had a chlorine demand of 3 ppm FC per day, the TA from this lye would rise by around 0.3 ppm per day.

As shown in this map, evaporation rates in Texas can be as high as 120" per year. If the pool water is heated, the rate would be even higher. 120" per year is 1/3" per day. Let's say it's 1/2" per day for the purposes of calculation and let's say that the fill water is high in TA at 200 ppm and that your pool has an average depth of 4.5 feet. Evaporation and refill would add (0.5/54)*200 = 1.9 ppm TA every day.

So evaporation and refill could certainly be a significant contributing factor, but does not seem to explain the full extent of what you are seeing unless 1) the TA level in the fill water is higher than I assumed or 2) the rate of evaporation is higher, say due to more wind and higher temps than usual (remember that I used an annual average so perhaps the evaporation rate in the summer is double).

If you turned off your fill water for 24 hours and carefully measured the drop in pool level (make sure this doesn't bring you down below the skimmer so that you don't suck in air into the pump) and if you measured the TA of the fill water, then we can see if this explains your acid demand. Alternately, if you have a pool cover and can put it on the pool then you can see if this eliminates most of the acid demand (to prevent the pool from getting too hot, you could use a white or reflective cover during the day that you could remove at night).

I assume that except for the initial raising of TA you mentioned after a refill and the raising to 100 when you used Trichlor, that you did not use any baking soda to raise TA at any other time.

Richard

 

[anonapersona]

My TA in The Woodlands is 340 in tap water. When it stops raining and I start adding water for evaporation, the TA will rise maybe 10 pts even with a top off of just an inch and a half.

The pH of the tap water initially is not important, since CO2 is outgassing and the pH lowering in the first 24 hours or less. After sitting for a day, the pH will be much higher, or you could shake it a lot to force the outgassing to find pH.

 

[Jaywalker]

In my edited post I now note that I have added 27 pounds of Baking Soda since 29 May. TA of tap water is 250. Shaken for 45 seconds and vented three times, my tap water's pH reads 7.6.

 

[chem geek]

27 pounds of baking soda in 12,000 gallons would raise the TA by 160 ppm! So from May 29 to July 17th that's 8 ppm TA increase per day and doesn't sound right given the amount of acid you have been adding (unless you've been using a lot of Trichlor). Why are you adding so much baking soda? If you are trying to keep your TA up higher, stop doing that. Your pH rise is likely due to the TA being too high and your keeping the TA up just makes the pH rise faster having you add more acid -- a vicious cycle.

If you were to add acid and let your TA drop, even to 60 ppm, then you may find some balance between the rise in TA from your evaporation and fill vs. your pH rise. You should be able to find a "sweet spot" TA level where it stays fairly constant with some acid addition and that the pH is stable there as well.

 

[Jaywalker]

Richard,

Letting the TA sink is easy enough to do, but I understood the use of Trichlor required a TA of 100 - 120, per the "Recommended Levels" page, true? On 29 May, TA was then at 90, so I began raising it. (My need to add acid was greater before I began adding Baking Soda on 29 May; it was one of the things I tried in order to get to a balance.) I'm certainly not trying to be argumentative, but I would like to understand. If I can let TA sink, that would be fine with me.

As for the Trichlor pucks, I've used 4 - 8 of the 3-inch pucks, and a medium setting ("3.5" on the regulator). Using them was easier than buying water conditioner in order to get the CYA reading to 40, and we're getting there slowly.

One thing I mentioned in my first post was my observation that pH spiked immediately in relation to my addition of chlorine bleach; at first I attributed it to the local brand of bleach, but I don't think that was it. That was another reason to try Trichlor.

Thanks.

 

[chem geek]

The pH rises when you add any hypochlorite source of chlorine (i.e. bleach, chlorinating liquid, Cal-Hypo, lithium hypochlorite), but it normally drops back down when the chlorine level drops since chlorine consumption/usage is acidic. In your case, the pH stays high because you've got other things going on raising the pH -- most likely outgassing of carbon dioxide due to the higher TA level.

As far as Trichlor is concerned, it is acidic so normally you keep the TA level higher to compensate, but the REAL rule for TA is to adjust it downward if the pH is tending to rise. So if your pH is still tending to rise even with Trichlor, then let the TA drop lower.

Do you have any unusual water aeration features that might explain the tendency towards higher pH? Any waterfalls, spillovers, fountains, etc.?

 

[Jaywalker]

We have two water "features." There are three "sheer" waterfalls that I use only (1) sufficiently to ensure the water in the pipes is fresh and in proper chemical balance (perhaps 10 minutes twice a week, unless I need to raise the pH - rarely, lately), and (2) the primary water filter pump roils the water surface significantly. I run the latter six hours a day.

Okay, I'll let the TA drop naturally as a result of adding acid initially, and I'll take an hour/day off the filter pump and evaluate the need for the longer run. I assumed I needed TA higher to provide pH buffering, and wrongly assumed that lower TA would have created a need for even more acid. Is there any TA level below which I should be concerned?

Thanks.

 

[chem geek]

I wouldn't go below 50 ppm, but if for some reason you needed to be at a rather low TA you can still get a proper saturation index by having a higher pH target (say, 7.7) and/or a higher Calcium Hardness (CH) level. I'd see how things go with the lower TA first and then you can adjust the CH later if the pH becomes more stable. However, if you are still going to be using Trichlor, I'd be surprised if you needed to drop the TA that low, but with chlorinating liquid or bleach you might need to go well below 80 ppm. If you wanted more pH buffering, 50 ppm Borates would give you that, but first you want to get things stable before making other adjustments.

TA is not only a pH buffer but a SOURCE of rising pH in its own right due to carbon dioxide outgassing. Perhaps it is the 6 hours/day of significant roiling of the water surface that is exacerbating the problem. Is it possible to have the returns not pointed up as much or is that necessary to get proper surface flow to move things to the skimmer?

 

[Jaywalker]

I don't know if the current roiling is necessary. The nozzle looks adjustable, but not by hand, though, which is why I thought to reduce the time, instead. I'd rather not put a tool to it.

I'll report back with results when I see some.

Thanks very much.

 

[Jaywalker]

Follow-up, as promised, though delayed a little due to need to address mustard algae, now completed.

It does appear that the suggested resolution of letting the TA drift down was effective in restraining my overactive pH. I added sufficient muriatic acid to drop the pH to 7.2 and the corresponding TA to 80. The pH has drifted up to 7.7, but slowly and has stayed there for 4 days now. I believe I'll tweak that just a bit to get the pH down to 7.5 - 7.6, but this indicated that the carbon dioxide outgassing from the higher TA was likely at fault, so thank you all very much. It's nice to have a solution.