Why Does my pH Rise So Quickly?

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B

BikerBill

0
Jul 6, 2012
26
West Newbury, MA
Background information: 22,500 gallon gunnite/plaster pool with pebble finish, 5 years old, DE filter, Whisper Flow pump, Nature2 Pro G mineral sanitizer, in-floor cleaning system, no water features, light bather load, sunshine on pool from 9 AM to sunset, located in northeastern Massachusetts.

I use liquid bleach, baking soda and muriatic acid in the pool. The water is crystal clear and feels great. But I don’t understand why I need to add so much muriatic acid to keep the pH in range. I’ve kept track of my chemical usage for the last 5 or 6 weeks. TA is usually 80-90, calcium hardness 230 per my Taylor test kit. I don’t what the CYA is. One pool place has told me 20 – 50 over the summer. Another guy says 78. Take your pick.

I add about 36 ozs of 8.3% bleach every day. So for the past 37 days that works out to 1332 ozs. During that same time I’ve added 495 ozs of 31.45% muriatic acid. So for every 10 ozs of bleach I need to add 3.7 ozs of acid. I know bleach has a very high pH so it will drive the pH up. But during that same time rain enters the pool and our rain is usually acidic.

Is it expected or normal I need to add so much muriatic acid? If this is not normal, what do I need to do get back to a normal regime?
 
I would raise your TA a bit higher to say 120 and see if that helps buffer your PH rise. We don't recommend the use of those mineral sanitizers btw. Search the forum and you'll see why.
 
Unless your pool is covered, do not increase the TA level as that will only make the pH rise worse. TA not only buffers pH, but is a SOURCE of rising pH in its own right as it increases the rate of carbon dioxide outgassing.

You should not try to lower your pH too much. If it likes to settle in closer to 7.8, let it do that. Do not try and lower it below 7.5. This chart shows how a higher TA and a lower pH will have more carbon dioxide outgassing so a faster pH rise (higher numbers mean the pool is more over-carbonated).

What brand of bleach are you using? Perhaps it has a lot of excess lye in it.

Since you have a Taylor kit, why don't you know the CYA level? If it's not a Taylor K-2006 kit, you need to get that or the TFTestkits TF-100 which is a better value (see Test Kits Compared).
 
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Not a lot of covered pools here in New England--this one is open to the elements.

I have tried to keep the pH in the 7.2 - 7.6 range. But it quickly rises to the high end of that range and then keeps on going up. It doesn't seem to "settle" anywhere. I will try not lowering the pH below 7.5 as you suggest. As I recall, carbon dioxide dissolves in water to form carbonic acid, a weak acid. So if I have less CO2 outgassing does that mean the pool retains more carbonic acid which then lowers the pH?

The bleach I use is from a local supermarket chain called Market Basket. Their bleach is freshness dated and is made in Canada according to the label.

I do have the Taylor K-2006 test kit. I tried measuring the CYA level once and thought it was kind of Mickey Mouse (not to denigrate my favorite cartoon rodent). I can try using it again.

Can you offer any guidance on whether my muriatic acid consumption is normal, moderately high, very high, off-the-charts, etc.?
It's my understanding some commercial/public pools use sodium hypochlorite as a sanitizer. And right next to the sodium hypochlorite tank is a muriatic acid tank. Are there some rules of thumb for muriatic acid consumption in such applications?
 
You are correct that carbon dioxide dissolved in water forms carbonic acid so what happens is that carbon dioxide outgasses into the air and carbonic acid then forms more carbon dioxide to take its place and bicarbonate ion forms more carbonic acid. Essentially the outgassing of carbon dioxide is the same as removing carbonic acid from the pool. Removing an acid raises the pH just as adding an acid lowers the pH. The TA actually doesn't change because the acid is a weak one and the acid and carbonate parts exactly counteract each other with respect to TA.

So if you can reduce the amount of carbon dioxide outgassing, say by lowering the TA level, reducing aeration sources, and setting a higher pH target, then you reduce the rate of pH rise as well as the amount of acid you need to add. If you don't lower the pH to 7.2, you may find that though the pH still rises from 7.5 to 7.8 you will use a lot less acid. It takes 62% as much acid to go from 7.8 to 7.5 as it does to go from 7.5 to 7.2.

As for your consumption, 36 fluid ounces of 8.25% bleach in 22,500 gallons would be only 1 ppm FC per day. That seems low given that you said the pool is exposed to sunlight all day. More normal consumption is around 2 ppm FC per day. Maybe your CYA level is high and your FC/CYA ratio low. The 13.4 fluid ounces of acid per day is a lot for the amount of bleach you are adding -- very high, I would say.

You say you also use baking soda so that implies that your TA is dropping over time. That is a clear sign of carbon dioxide outgassing because it raises the pH with no change in TA and then the acid you add lowers both pH and TA -- the net change being a lowering of TA. The acid you add lowers the TA by 2.3 ppm per day. If the pH were rising only from carbon dioxide outgassing, then you should be seeing the TA drop by 10 ppm every 4-5 days and you would need to add around 11.5 ounces weight of baking soda every day on average (how much have you been adding?). If the pH were rising only from excess lye in the bleach, there would not be a change in TA as the acid and the lye would counteract each other.

As for rules-of-thumb, it really varies by pool and the amount of aeration in the pool, splash activity, temperature, etc. If you lower your TA to 70 ppm and don't go below 7.5 in pH I'd expect you to use less than half the acid you are currently using and perhaps less than that.
 
As for my chlorine consumption I know it does seem low. A few years ago I was using the 3” trichlor tabs and kept track of my usage then as well. It worked out to be quite close to the 1 ppm I currently am using with bleach. So for whatever reason this pool just doesn’t need that much chlorine. I do notice a change in consumption if we get several cloudy days (less chlorine) or sunny days (more chlorine) in a row. But that’s to be expected. I don’t think my CYA is high. I moved away from the tabs because I didn’t want the CYA going above 70-80.

This part you’re going to love. After posting my questions yesterday I decided I should check the TA and calcium hardness levels. I was embarrassed to discover it had been 27 days since I last tested TA and CH (I keep a log). In those 27 days the TA had dropped from 110 ppm to 50 ppm. My bad. Using information on the baking soda container I calculated I would need to add 20.25 lbs or 324 ozs to bring the TA back to 110 ppm. Normalizing that to per day consumption works out to 12 ozs per day, eerily close to your projected 11.5 ozs per day.

So that would seem to confirm I have a carbon dioxide outgassing problem if I understand your reply. You advise running a lower TA (70) and higher pH (7.8). I will try that. But I’m wondering what other factors affect the CO2 outgassing? Are there nucleation sites or catalysts involved here? Is there something I can add that reduces the CO2 outgassing. What things aggravate it? This is our first pool with a pebble finish and I’m wondering if the pebbles play a role. I don’t know if this is relevant but when we were using the 3” trichlor tabs I remember having the same problem, always needing to add acid to lower the pH. Back then, I used dry acid.

You also mentioned some variables that affect outgassing. Just for the record, we have no water features and very little splash activity so I assume little aeration. Does aeration increase or decrease outgassing? Pool temperature is 80 to 84 degrees, unless the weather drives it higher. We do have a heat pump but use it sparingly.
 
Check your log for acid usage. You should find that in the 27 days the amount of acid you needed to add to get the pH to the same place should have been dropping over time. Is that the case? If so, then your having a lower TA will help, as well as not dropping the pH so much. If you were adding the same amount of acid every day, then it's possible the pH was dropping more later and you didn't notice because you didn't measure it. 13.4 fluid ounces of full-strength Muriatic Acid (31.45% Hydrochloric Acid) in 22,500 gallons with a TA of 110 ppm would drop the pH from 7.6 to only 7.47, not to 7.2. With a TA of 50 ppm, it would drop to 7.38. So I'm unclear as to your 7.6 to 7.2 range -- perhaps you didn't try adding enough acid to go from 7.6 to 7.2. If you were instead adding acid to keep the pH close to 7.2, so going from 7.3 to 7.2, then that would explain a lot.

If your pool needs to get to an even lower [EDIT] TA [END-EDIT] closer to 50 ppm to be more stable, then you can use 50 ppm Borates in the pool for additional pH buffering and you'll need to raise your CH to get the saturation index closer to zero to protect plaster surfaces.

By the way, using your numbers, I get a saturation index of -0.8 when your pH is 7.2, TA 50 ppm, CH 230 ppm and assuming 50 ppm CYA. That is much too low and could eventually harm your pool surface. Even when the TA was 110 ppm, the saturation index was -0.3 to -0.4 but becomes 0 when the pH is 7.6 so is OK. If you end up at a TA of 70 ppm and your pH goes from 7.5 to 7.8, then your current CH 230 is not horrible, but 300 would be a little better (not a big deal, though). If you end up using a TA of 50 ppm with 50 ppm Borates, then you'll need your CH to be substantially higher at least 500 ppm. Why don't you first see if you can make a TA of 70 ppm work as that is a more normal level. I really think the main problem was your trying to maintain a pH below 7.5.
 
Additional Information

OK, so I checked my log for acid usage per your suggestion. You should know that instead of adding small amounts of acid or baking soda every day I tend to add larger amounts every several days. So my data is somewhat “lumpy”. I split the 27 days into three 9 day periods to get some averaging. For the first 9 days (when the TA started at 110 ppm) I added 148 ozs 31.45% muriatic acid. The second nine days I added 117 ozs acid, and the for the last nine days 110 ozs. So yes, the acid consumption dropped as the TA dropped.

As for the pH range of 7.2 – 7.6 let me explain how that worked. I would test the pH and almost invariably find it was higher than 7.6, typically 7.8 to 8.0. So I’d say something inappropriate and then add muriatic acid to bring it to the lower end of that range. I was not overly precise in this exercise. If I checked the pH shortly after acid addition and then a day later, I’d find the pH would change measurably in a day. So if the pH got down to 7.2 it didn’t spend much time there.

In your second paragraph where you say “If your pool needs to get to an even lower pH closer to 50 ppm” I assume meant to say TA closer to 50 ppm. Is that correct?

Borates, huh? I picture some grizzled old miners and 20-mule teams hauling the stuff out of Death Valley in creaking wooden wagons with vultures circling overhead, waiting. I can’t figure out my current chemical system. I think I’m going to leave borates out of these interconnected equations for now.

That saturation index number is concerning I agree. I don’t usually calculate the SI. But several times throughout the season I have the water tested at a pool place and they calculate it. For this season the values have been 0.25, -0.35. 0.10, and 0.20 which seem pretty reasonable. I would say that this pool spends a lot more time with a pH of 7.6 or 7.8 than it does at 7.2 so I think your calculated value of 0 is more appropriate than your value of -0.8.

We have two weeks left of this pool season. I will run a TA of 70 and pH of 7.8 and keep track of muriatic acid usage again. Our pool water temperature has dropped to 70 degrees so I assume that’s going to lower outgassing. But tomorrow a high of 92 is forecast so that’s going to bump the temperature back up. I will repost when I have the data in two weeks.
 
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