Alkalinity and pH - I am confused but happy

[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

[chem geek]

Just FYI. I added a few lines to each chart (this one for outgas rates and this one for pH rise rates). You may need to option-click on the links to reload the revised version.

[jereece]

Richard, I guess I am still confused...but still happy. Here's my observations.

Referencing your chart, over the last few years I ran TA at ~100 and tried to keep pH around at 7.6. Your chart shows this as 0.67 relative value. During that time, I had to add acid weekly to keep the pH in check. This spring however, I ran TA at 40 and pH at 7.1 (because I thought it would come up naturally but didn't). This has a relative value of 1.04 on your chart. So if I understand your chart correct, my pH should have risen faster this spring than in the past few years, however this spring my pH did not change at all for 6 weeks and then it only changed when I added borax.

Another observation is that you calculated that I needed about 4.5 cups of borax, but I added 5 initially because I have more volume due to a concave pool bottom. Initially, this did bring the pH up to ~7.5, but within a week it was back down to ~7.2. I added borax weekly until now it appears my pH has stabilized at 7.6. But it took 1.75 boxes of borax. The boxes are the standard home size weighing about 4 pounds each. It does not list the volume. So for some reason it took much more borax to get my pH up. My alkalinity is now up to 60.

As I said however, I am happy. I don't worry about alkalinity and my pH stays constant. I just maintain chlorine by adding bleach and everything is fine.

As always, your insight is very interesting.

Thanks,
Jim

[chem geek]

Apparently, you had some sort of source of acid in your pool since that's the only way the pH could have gone down (from 7.5 to 7.2). At the lower TA, any source of acid or base will make the pH change more so by lowering the TA you reduced the carbon dioxide outgassing, but also made your pool more susceptible to whatever is going on with it in terms of acid/base addition. So I really don't have any definite explanation, especially for why the pH went down or stayed low for so long. I can only guess it's the "noise" from acid/base external sources. At the lower TA, you see this up/down swing more, but I can't explain the source of such swings.

Just so you know, the tables show the TA, but are at a specific CYA level of 30 ppm. It isn't really the TA, but rather the carbonate alkalinity, sometimes called "adjusted TA", that is important, but so people wouldn't have to calculate that I just used the real TA in the table with a specific 30 ppm CYA level. Your pool, though, sounded like it was near 30 ppm CYA (I think you said 25 ppm).

As for acid sources for your pool, the using up of chlorine is acidic so one needs to compare pH at the same FC level. If one shocks the pool with hypochlorite, for example, then the pH goes up at first when the FC goes up and then comes back down as the FC drops. If you use other sources of chlorine such as Trichlor or Dichlor, then these are acidic. If you use a non-chlorine shock, then that is acidic. The addition of CYA directly is acidic. Rain can be acidic, though you need quite a lot of it to more the pH (but at lower TA it's easier to move). Certain organic matter falling into the pool may be acidic.

Last, but not least, is that the rise in pH is very much a function of aeration so raindrops aerate the pool so can make the pH go up -- so whether rain causes pH to go down from acid rain or up from aeration depends on the rain's pH. Aeration also varies with wind, if it chops up the water. And, of course, can be from more swimming, splashing, waterfalls, spillovers, fountains, etc.

The bottom line, though, is that lower TA has your pool not have the rising pH and regular acid addition problems and that's what is really important.

Richard

[jereece]

Thanks Richard.

Just as a follow-up, my swimmer load has always been just a couple family members, I don't use any kind of fountains, it has not rained in a month, I have not used Trichlor or Dichlor in years. I always take my readings in the late afternoon (shade). I do get a few leaves every now and then, but nothing abnormal.

Is it possible that the decrease in pH after I added borax was due to my new liner? As I stated in my original post, I put a new liner in this spring filled, with fresh water and added CYA to 25ppm. I know the CYA caused the initial pH drop, but after I added the borax could the liner (being plastic) have some kind of ion exchange going on that would cause the pH to decrease until a balance was reached? I used to work in a chemical treatment plant and know that ion exchange resin is made of plastic and even earth can have some ion exchange properties. I was just wondering if something from the liner could have caused this until it reached equilibrium.

Thanks again for your insight.

Jim

[chem geek]

I really don't know, but many chemicals exposed to new environments (i.e. water) will leach out especially initially before reaching more of an equilibrium. We know, of course, that plaster works that way, but that's a specific chemical process of curing. With vinyl, I'm sure it's not so dramatic, but something like that could be happening.

I would say that if many people with new vinyl pools find initial pH dropping or not rising as quickly and then later find it stable or rising faster, then you very well could be right. Otherwise, this is just one more mystery without an explanation.

Richard