Alkalinity and pH - I am confused but happy

[jereece]

I've had a 27' AG pool for 7 years. I have followed advice on Pool Forum for about 5 years. However I am more confused about pH and Alkalinity than ever.

For the past 7 years, I have tried to keep my alkalinity around 70 - 100. I have to periodically add some baking soda, but have been able to maintain it within reason. I typically have had to adjust my pH once a week or so as it gets higher each addition of bleach.

Last year I got too busy at work and accidentally let my alkalinity drop to 40. At the same time I noticed that I was not having to adjust my pH as often. So I did some bench tests with my water and it showed that the lower the alkalinity the LESS pH swing. This is exactly opposite what I have always thought to be true.

This spring I replaced my liner. I refilled my pool and decided I would try running with out adding any baking soda. My source water is very soft and has very little natural alkalinity or calcium. I added CYA to about 25, added bleach to maintain 3 to 6 ppm. After all the CYA dissolved (~3 days), my final pH was about 7.4.

I add about 3/4 gallon of bleach each night to maintin my chlorine levels. My pH is still at 7.4 after a month!

From a chemical standpoint, not having to worry about alkalinity yet having very stable pH is wonderful. My water is as clear as ever.

My questions are,

1) Is something going on with my water that makes it different than most others since running higher alkalinity is normal for most people?

2) Does running with low alkalinity cause me any problems such as increased rates of corrosion?

I appreciate your insight.

Jim

[dawndenise]

I've been in the same boat as you...my pool's TA was always dropping below my targeted 80. If I tried to increase it, my pH went up. Then I'd add acid which dropped my TA - an ongoing cycle that I detailed here: http://www.poolforum.com/pf2/showthread.php?t=6998

Chemgeek gave me some insight and, after some experimentation, it appears my pool likes to live at TA of 50-60 and a pH of around 7.7-7.8. So, it appears I'm in line to increase my CH to keep things in balance.

Hopefully, Chemgeek or some of our other chemistry gurus will give you some specific advice for your pool.

[chem geek]

Jim,

There are many over-simplifications or direct falsehoods the pool industry has with regards to pool water chemistry. One is that higher TA makes pH more stable. That is only half-true. Higher TA that comes from carbonates, which is the largest component in TA (another is a fraction of the CYA level and even less is the Borates level) is a source of rising pH itself. The reason is that pools are intentionally over-carbonated, just like a beverage, since they have more carbon dioxide dissolved in the water than would be naturally found from exposure to air. This over-saturation is done when you add baking soda or Alkalinity Up upon pool startup though even fill water has some TA.

The reason for having this over-saturation of carbonates is twofold. First, it does buffer pH FROM OTHER SOURCES of pH change, but it has the side effect of being a source of rising pH in its own right. Second, the carbonates portion of TA is necessary in plaster/gunite pools to prevent corrosion (technically dissolving) of such surfaces.

So a higher TA makes sense when using an acidic source of chlorine such as Trichlor (or even Dichlor which is technically acidic when one takes into account chlorine usage). It does not make as much sense to have high TA when using a pH neutral source of chlorine such as bleach or chlorinating liquid. I say "pH neutral" because though the initial addition of bleach or chlorinating liquid makes the pH rise, the usage of the chlorine (breakdown from sunlight, oxidation of organics, killing algae or bacteria, etc.) is acidic so the net result is very nearly pH neutral.

So it has been the recommendation on this forum for a while to lower the TA level if you are using bleach or chlorinating liquid and find a tendency for the pH to rise. The pH rises more quickly at higher TA and lower pH and with more aeration. This chart shows the relative rate of carbon dioxide outgassing at various pH and TA levels. This is a relative rate since the actual rate depends on the amount of aeration. These relative numbers would correspond to the amount of acid you needed to add each week. This chart shows the relative rise in pH and shows the fact that the lower TA is a smaller source of pH rise, but the lower TA also buffers the pH less so these two factors somewhat cancel each other out, but there is still a small net improvement at lower TA levels.

As for your question of side effects, a lower TA is more corrosive to plaster/gunite so those with such pools would need to either increase the Calcium Hardness level or the target pH or both to compensate. Since a higher pH also results in less of a tendency to rise further, most people lower the TA level and target a somewhat higher pH, such as 7.7 instead of 7.5.

As for how low to go, I wouldn't go below 50 ppm because you do want some pH buffering in the pool and also may need some carbonates if you have plaster/gunite. If you had much lower TA, then a relatively small amount of acid (say, from Trichlor or Muriatic Acid) could make the pH move more than expected and if the pH got low then that could corrode metal as well as plaster/gunite. Since you have a vinyl liner and don't need the calcium nor the carbonates to prevent corrosion, the only risk you are taking is if some acid or base is added to your pool. Even relatively soft water will have some TA in it and the CYA itself contributes to TA, but what you are seeing just confirms the theory that we've been saying for a while.

Pools with SWG systems (which isn't the case with your pool) have a particularly hard time with rising pH because the hydrogen gas bubbles generated in the SWG cell essentially pull the carbon dioxide out of the pool through aeration and that causes the pH to rise quickly. Lower TA helps such pools, but so does the use of Borates since they act as an additional pH buffer and are also an algaecide that let SWG users lower their SWG output which reduces the outgassing and therefore the tendency for the pH to rise.

So how low is your currently very low TA? With your CYA, it's got to be at least 8 or so and even soft water usually has a good amount of TA (technically, soft vs. hard water is about total hardness and is not directly related to TA though sometimes they track together in natural waters) so I suspect you are measuring at least 30 ppm or so. If you let me know your current TA (you already gave CYA and pH) and your pool water volume, then I can give you an idea of how much the pH would move with various amounts of acid or base.

Richard

[jereece]

I really appreciate the education. I when ahead and ran a complete set of analysis and I have a question about the pH. I used a Taylor K-2500 kit.

Free Cl2 = 5
pH = ~ 7.1 (Taylor) 7.37 (Laboratory Probe)
CYA = 30
TA = 40
CA = 20

I was a little surprised that TA was 40. I thought it would be lower.

As for my pH, my Taylor kit shows a color somewhere between 7.0 and 7.2. However I took a sample into work and had one of our chemistry tech run the pH using a calibrated probe. They got 7.37. I suppose I need to raise the pH a little as my primary concern is corrosion of the walls should I have a leak. As I said in my original post, I replaced the liner this spring and to my surprise found a good bit of surface rust on the walls. I removed the rust, treated it with rust converter, then painted the entire wall with aluminum Rustolium paint and then coated the lower 1/3rd (where most of the rust was) with rubberized undercoating.

I originally thought my pH would come up naturally using bleach, but with such low alkalinity I may need to bring it up another way.

Questions

1) Should I go ahead and manually bring the pH up to some higher value?

2) If so, what pH should I target considering my low alkalinity?

3) What should I use to get the pH up?

Again, I really appreciate the help.

Jim

[chem geek]

My responses in bold in your quoted post below.

I really appreciate the education. I when ahead and ran a complete set of analysis and I have a question about the pH. I used a Taylor K-2500 kit.
I think you mean the K-2005 test kit. This uses a DPD chlorine test which only lets you measure chlorine up to 5 ppm. A more accurate test is found in the K-2006 test kit and is called a FAS-DPD test. It can measure up to 50 ppm and has a resolution of 0.2 ppm or 0.5 ppm depending on sample size and it accurately measures both Free Chlorine (FC) and Combined Chlorine (CC). You can just get the FAS-DPD test separately as the K-1515-A from Taylor here.

Free Cl2 = 5
pH = ~ 7.1 (Taylor) 7.37 (Laboratory Probe)
CYA = 30
TA = 40
CA = 20

I was a little surprised that TA was 40. I thought it would be lower.
Fill water usually has some TA in it as well as some Calcium Hardness (CH). The TA of 40 is not at all unusual, though the CH of 20 does mean the water is "slightly hard" since the total hardness is probably around 30 ppm. Soft water is < 17 ppm total hardness, slightly hard is 17-60, moderately hard is 60-120, hard is 120-180 and very hard is > 180.

As for my pH, my Taylor kit shows a color somewhere between 7.0 and 7.2. However I took a sample into work and had one of our chemistry tech run the pH using a calibrated probe. They got 7.37. I suppose I need to raise the pH a little as my primary concern is corrosion of the walls should I have a leak. As I said in my original post, I replaced the liner this spring and to my surprise found a good bit of surface rust on the walls. I removed the rust, treated it with rust converter, then painted the entire wall with aluminum Rustolium paint and then coated the lower 1/3rd (where most of the rust was) with rubberized undercoating.
The pH will change over time, usually rising, if your sample got shaken or aerated on the way to the lab. The outgassing of carbon dioxide causes the pH to rise. To verify the accuracy of the Taylor test, you should take the kit with you to the lab and test them side by side on the same sample water. If that is what you did, then one or both tests are a bit inaccurate.

I originally thought my pH would come up naturally using bleach, but with such low alkalinity I may need to bring it up another way.
Bleach and chlorinating liquid initially make the pH go up upon addition, but the usage of chlorine is an acidic process that balances the initial pH rise so the net result is no change in pH (pretty much). Usually, a pool rises in pH due to the over-carbonation. So what you are seeing is normal.

Questions

1) Should I go ahead and manually bring the pH up to some higher value?
Yes you should. If you want to do so without changing the TA, then you can aerate, though at your low TA level this might not be very successful. You can add 20 Mule Team Borax to raise the pH. Don't forget that it will take much less acid or base to move the pH at your TA of 40.

If I assume your 27' is the above-ground pool diameter and that it has a 48" (4') depth, then that's 3.141*(27/2)^2 * 4 = 2290 cubic feet which is 17,130 gallons so let's call it 17,000 gallons. To raise the pH from 7.1 to 7.5 would take 39 ounces weight (about 4-1/2 cups) of Borax and the TA would only rise by 4.5 ppm. To raise the pH from 7.37 to 7.5 would take only 9.5 ounces weight (about 1 cup) of Borax.

2) If so, what pH should I target considering my low alkalinity?
You use the same pH target at the lower TA, namely 7.5 so that it is easiest on the eyes while not being acidic for the vinyl.

3) What should I use to get the pH up?
Either aeration or 20 Mule Team Borax as indicated above.

Again, I really appreciate the help.
That's what we're here for! And you are a great experimenter and help confirm theories that are doubted by some (mostly in the pool industry).

Jim

[CarlD]

Richard,
I'd like to stickie that chart but first we need to know what the individual cells are. You say they are acid demand--in what units? In other words, while the X and Y axes are clear, the UNITS and what is being measured at each coordinate aren't.
Carl

[chem geek]

Carl,

The two charts have a relative scale so the absolute units aren't meaningful. Nevertheless, the first chart of relative CO2 outgas rates has a base unit of 0.0 meaning no outgassing with the normal equilibrium amount of CO2 in the water as there should be when exposed to air. A unit of 1.0 means twice the amount of CO2 dissolved in the water as would normally be there from equilibrium with the air. So if you add 1 to the numbers in the chart, then that represents the factor of how much more CO2 is in the water than there should be if in balance with the air. The more out of balance, the faster the outgassing -- if the number in a cell is twice the number in another cell, then the outgassing should be twice as fast (all else equal).

The second chart is even harder to fathom since there isn't even a meaningful base unit in this case except that 0.00 means no pH rise at all. It is still true that a cell that is twice the number in another cell does mean that the rate of pH rise will be double, but the chart doesn't tell you how much that is in absolute units (I put in numbers that are roughly in pH rise ranges, but that's just an arbitrary scaling).

So the two charts are really just "tendencies" and the color coding is an attempt at giving rough areas of stability (green), rising pH and requirement of acid addition (red) and intermediate levels in orange, but this is very, very rough since aeration can make even some green numbers not be very stable (in an SWG environment, for example).

Jim,

I'm just wondering why you saw such a pH rise even at relatively normal TA levels. Does your pool have any aeration features such as waterfalls, spillovers, fountains, etc.? I'm guessing there isn't since it's an above ground pool. Or has there been summer rain (raindrops aerate the water when they splash)?

Richard

[CarlD]

Richard,

I deal with tables and charts all day, so it doesn't REALLY change or answer my question.

The first chart NEEDS to indicate that the cell is a coefficient for the baseline rate of gassing off, even if that isn't measurable and is only relative.

That baseline point should also be indicated.

Otherwise, as interesting as this table looks, without that information it's very difficult, if not impossible to use.

Trust me on this--we send tables and charts to the FDA for pharma clients ALL the time to demostrate the results of testing of their latest drugs and devices. Clarity of definition is crucial.

Properly labled, I STILL think this table could be a very useful permanent addition. For our purposes, it needs to be clear enough for the layman to comprehend, not just those of us in statistical fields.

[chem geek]

OK, I can add the wording, but need help to make sure it's clear. I already indicate "relative" in the title so if below the first chart I say the following:

"If a number in a cell in the chart is double the amount of a number in another cell then that means that the outgassing of carbon dioxide is twice as fast and, all else equal, that twice as much acid will be needed to compensate for this over any given period of time. A value of 0.0 in a cell means no outgassing while a value of 1.0 means that there is twice as much carbon dioxide in the water as there would be if in equilibrium with the air, a value of 2.0 means there is three times as much as at equilibrium (so is twice as much "out of balance") and outgasses twice as fast as 1.0, etc."

FYI. The offset of 1 comes from the fact that the reaction rate is proportional to the concentration, but there are two reactions going on. There is movement of carbon dioxide from the water into the air and there is movement from the air into the water. The rate of movement from air into the water is fixed since the concentration in the air is constant, but the rate of movement from the water to the air varies as the concentration in the water varies. So if "1" is designated as the fixed rate of movement from air to water, then the net outgassing rate is "x*1 - 1" where "x" is the factor of the concentration of carbon dioxide in the water relative to its equilibrium concentration (which is when "x*1 = 1" or x = 1). So that's how we get "x - 1" numbers in the table and why you need to add 1 to those numbers to know how far the water is out of equilibrium (where "1" would be equilibrium), but if I used "x" instead of "x - 1" numbers in the table, then you wouldn't be able to do a simple ratio between two cells to get relative rates.

and for the second chart:

"If a number in a cell in the chart is double the amount of a number in another cell then that means that the rate of pH rise is twice as fast, all else equal. A value of 0.0 in a cell means no tendency for the pH to rise (because there is no carbon dioxide ougtassing). The absolute scale of numbers in this chart was based on using the relative rate of outgassing carbon dioxide (with 1.0 representing having twice as much dissolved carbon dioxide vs. being at equilibrium with air) and using that as a TA (ppm CaCO3) equivalent quantity of carbon dioxide that is outgassed. This chart is very approximate as it did not use incrementally small values in its computation".

At some point, I will redo the second chart using a much smaller incremental outgas amount and then will scale up the numbers and can remove that last sentence.

I think the description for the first chart sounds OK, but the second one sounds too technical. Should I just leave out the absolute scale basis info and just have the first sentence? Or if you can write something that would be easy to understand, I can add it to the charts and upload an update.

Richard

[jereece]

Richard,

My pool is closer to 19,000 gallons because it has a concave bottom. So I added 5 cups of Borax and allowed my pool to recirc for 24 hours. My pH is now at ~7.5 (Taylor color between 7.4 and 7.6). My alkalinity still shows ~40 (4 drops). So your calcs were dead on the money. Now if my pH and alkalinity will stay there, I will be in heaven. All I will need to do is add bleach each night and that will be it for chemicals.

Also, you said,

I'm just wondering why you saw such a pH rise even at relatively normal TA levels. Does your pool have any aeration features such as waterfalls, spillovers, fountains, etc.? I'm guessing there isn't since it's an above ground pool. Or has there been summer rain (raindrops aerate the water when they splash)?

I do not have any kind of aeration features, however sometimes my wife likes to turn the return port upwards when she is relaxing in the pool. The splashing reminds her of a waterfall.

Thanks again for all your help and education.

Jim