Low pH and High Alkalinity

Problems relating to pH and total alkalinity.
Increase ph, increase TA. Reduce pH, reduce TA.
pH chemistry advice and techniques for the pool.
chem geek
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Low pH and High Alkalinity

Postby chem geek » Fri 01 Oct, 2010 23:53

There is a procedure on lowering TA is here. Aeration is doing anything that will increase air/water exchange such as turning returns up with the pump on high or running a waterfall, spillover or fountain or splashing. It increases the rate at which carbon dioxide leaves the pool. Pools are intentionally over-carbonated to provide a pH buffer and to protect plaster surfaces. Adding acid to water lowers both pH and TA, but the the outgassing of carbon dioxide raises the pH with no change in TA.


James Watson

Low pH and High Alkalinity

Postby James Watson » Fri 01 Oct, 2010 23:55

I am a little curious about the aeration and how that affects the alkalinity.
anyone care to explain or link me to an explanation?


HCO3- + H2O < > H2CO3 + OH- < > CO2(gas) + H2O + OH-

bicarbonate + water < > carbonic acid + hydroxide < > carbon dioxide + water + hydroxide

On the left, you have bicarbonate, which is a component of total alkalinity. On the right, you end up with carbon dioxide and hydroxide (we will ignore the water, as it cancels out).

The hydroxide increases the pH, and it is also a component of total alkalinity. Since one hydroxide ion contributes the same towards total alkalinity as one bicarbonate ion does, the total alkalinity remains the same.

As the carbon dioxide is off gassed by aeration, more bicarbonate ions combines with more water, and the process continues. The net result is an increase in pH with no change in Total Alkalinity. The carbonate alkalinity is decreased, but it is exactly offset by the increase in hydroxide alkalinity.

You list the Cyanuric Acid as "150/300". What is the level, 150 or 300?
James Watson

Low pH and High Alkalinity

Postby James Watson » Sat 02 Oct, 2010 01:16

Also, note that although the hydroxide is a component of total alkalinity, it does not provide any buffering capacity, whereas the bicarbonate does. Adding acid to water with hydroxide alkalinity produces a direct 1 to 1 pH change, whereas adding acid to water containing bicarbonate alkalinity will result in less pH movement.

Therefore, although aeration does not change the TA, it does reduce the TA's buffering capacity.
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Low pH and High Alkalinity

Postby chem geek » Sat 02 Oct, 2010 02:49

Well, it's a little more complicated than that. Unfortunately, the term "buffer capacity" is used in different ways. TA measures the total buffer capacity against a drop in pH in the sense that it measures the amount of pH buffering available all the way down to when the TA test changes color which is at around a pH of 4.5. 25-1/2 fluid ounces of full-strength Muriatic Acid (31.45% Hydrochloric Acid) lowers the TA 10 ppm. When the pH rises from aeration, the TA isn't changed which means that it takes the same amount of acid to get the pH lowered to 4.5 as it did before the aeration. So in this sense, there is still a lot of buffer capacity. It is true that the hydroxyl ions themselves aren't providing that (in practice), but there is very little change in the amount of bicarbonate ion after the aeration -- basically, what mostly happens is that the amount of carbon dioxide in the water is significantly reduced and the shift in pH shifts a little bicarbonate to carbonate and to carbon dioxide (and carbonic acid, which are in equilibrium) which mostly leaves the pool. Some more details about this are in this post and look especially at this post where going from the data listed at pH 7.0 from adding CO2 to the data listed at pH 7.5 is similar to outgassing CO2. The amount of bicarbonate as well as the carbonate alkalinity hardly changes, but the Total Carbonate (which includes carbon dioxide and carbonic acid) does change -- basically, it is the concentration of CO2(aq) that is mostly changing (in absolute, not percentage, amounts).

Another measure of buffer capacity is the resistance to an incremental change as expressed in millimoles/pH (for 1 liter so really it's millimoles/liter/pH). This measure drops dramatically for the bicarbonate buffer system (technically, the bicarbonate / carbonic acid system) as the pH rises, but it's not really due to hydroxyl being weaker as much as it has to do with the pH moving further away from the pKa of that system (around 6.3). If one has 50 ppm Borates in the water, then the borate buffer system gets stronger as the pH rises since the pH is getting closer to the pKa in this case (around 9.1).
california
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Low pH and High Alkalinity

Postby california » Sat 02 Oct, 2010 11:59

James Watson wrote:
You list the Cyanuric Acid as "150/300". What is the level, 150 or 300?


Visually on the test strip it looked between the 2 values. I am going to try the dilution test, 4 parts tap water to 1 part pool water. I'll let ya know what that reads.
With a 4 to 1 dilution the alkalinity still reads above the 180 comparison on the test strip. Multipling that by 5 and well, That's a high alkalinity. The cyanuric read a respectable 30-50. Multipling that and I lose my respect and have 150 to 250 ppm.
I tested again with a sample of straight pool water (harvested a foot under the surface) and the CyA and alkalinity was off the chart. Funny thing, there seems to be more free Cl in the tap water than in the pool

I have never backwashed or drained any significant part of the pool in 4 1/2 years, just topped up with local water. For the most part I have just used tablets, shock packets and the occasional liquid for a quick boost the Cl. This past year, I have gone through a half dozen bottles of algeacide. Prior years, I used maybe one or two.

From what I have been reading, I think I need to dump some pool water so I can dilute my alkalinity and cyanuric level. But how much do I need to dump? Is the water unsafe for the lawn?
Allen G Myerson

Low pH and High Alkalinity

Postby Allen G Myerson » Sat 02 Oct, 2010 20:27

You can't do the dilution test with total alkalinity because tap water contains total alkalinity. Test your tap water to see what the total alkalinity is.

Dilution works with the cyanuric acid because tap water does not contain cyanuric acid.

What kind of pool do you have, vinyl, fiberglass or concrete?
What is your ground water level?
california
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Low pH and High Alkalinity

Postby california » Sat 02 Oct, 2010 23:11

Allen G Myerson wrote:You can't do the dilution test with total alkalinity because tap water contains total alkalinity. Test your tap water to see what the total alkalinity is.

Dilution works with the cyanuric acid because tap water does not contain cyanuric acid.

What kind of pool do you have, vinyl, fiberglass or concrete?
What is your ground water level?


Darn!
I was thinking i could because I imagined I would dump 80% of the pool water and add tap water.

The pool is concrete with a plaster surface.

Sorry, but I don't understand what you mean by ground water level.

Getting back to a previous question about aeration, I was imagining there is a limit to the amount of OH̄ I can get from the process. I have a built in jaccuzzi so I was running that ramjet loud motor to aerate the water. Routing the pool return into the jaccuzzi, then letting it spill into the pool was probably a 100 times more effective in aerating the water.
I know its all mute because I really need to dump the water. Still, I am very curious, and really appreciate you guys sharing your knowledge.
Allen G Myerson

Low pH and High Alkalinity

Postby Allen G Myerson » Sun 03 Oct, 2010 00:08

The ground water level means, if you started digging a hole, how deep would it be when you hit water? Are there any natural or man-made bodies of water near your property? Anything such as a lake, river, stream etc.? If yes, what is the water level in relation to the water level in your pool?

If the ground water level is higher than the water in the pool when you drain, then the pool is at risk for floating out of the ground. There should be a check valve in the deep end called a hydrostatic relief valve that should open if the water level in the pool is lowered to less than the ground water level. However, you don't want to rely on it, and if it opens, there is a risk of it getting stuck open due to debris getting in the valve and that would cause a leak.

You should not drain your pool water level to lower than the ground water level.
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Low pH and High Alkalinity

Postby chem geek » Sun 03 Oct, 2010 11:39

california,

It sounds like you are using test strips. I would not trust the CYA reading from those strips and even the TA reading may be suspect depending on the strip. Though it does seem that you will likely need to replace a lot of your water to lower the CYA level, please get a good test kit, either the Taylor K-2006 or the TFTestkits TF-100 where the latter has more volume of reagents you use the most so is comparably priced per test. You can compare these test kits here .

Once you get your good test kit, you should test your fill water as well, mostly to see its TA and CH levels.

Richard
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Low pH and High Alkalinity

Postby california » Sun 03 Oct, 2010 16:14

True. Been using test strips. You might not trust the test strips alone but with the previous information I have supplied you might be less adverse to denying that I might not expect the CyA level to be high. The strips might be off but from the way I've been chlorinating it would seem likely that the CyA is infact excessive.I will get a more advanced test kit. Probably order one this week during a break from watching the water level drop in the pool. I don't really want to miss a minute of that.

Ground water
I live in the suburban utopia so when I dump the pool it will be to the gutter/sewer (I think the deck drain goes to the curb) or the lawn if that alkalinity won't kill it. I don't know how high you can go with the alkalinity, low ph and the lawn so if anyone has a guess, ... I won't hold it against you if you are wrong. It couldn't make my lawn look much worse than it does. :lol:
I belive our ground water level is very low here since we've been in a draught for ever. The city use to be pretty smug because it supplied the residents with 85% well water. Now they are importing a much larger percentage because they've sucked up the local water. Well, I'm about to give back to my community. Ok, its effluent, but at least its something.

The pool actually looks pretty sparkly clean now that I've sucked up all the dead algea, scrubbed the walls.
I'm still going to dump the water. 87% dump was required per the pool calculator. Sounds right.

I was hoping I could fix a backwash hose to the inlet to my filter and run that to the drain, or gutter. Is that the idea for draining a pool?

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