Alkalinity and pH - I am confused but happy

[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

[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