Reduce Total Alkalinity Levels

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
Pool Industry Leader
Pool Industry Leader
Posts: 2382
Joined: Thu 21 Jun, 2007 21:27
Location: San Rafael, California

Reduce Total Alkalinity Levels

Postby chem geek » Tue 07 Dec, 2010 20:22

Though it's unusual to be using a hypochlorite source of chlorine and have the TA be at 150 ppm and have the pH be stable, if that's the way it is for this pool, then leave it as is. As noted above, your saturation index is OK so nothing to fret about.

I suspect that you must be using enough MPS that this is keeping the pH down. MPS is acidic, though not as strong as using Trichlor, for example. In terms of the net effect on pH and TA, one pound of non-chlorine shock (43% MPS) is equivalent to 6 fluid ounces of full-strength Muriatic Acid (31.45% Hydrochloric Acid).

Unless you have a lot of evaporation and refill, the use of MPS should have the TA slowly drop over time.


somaholiday

Reduce Total Alkalinity Levels

Postby somaholiday » Wed 08 Dec, 2010 15:29

Cool. Always wondered why my TA stays somewhat under control even while using Co2 for PH control. Regular use of MPS.

You guys are awesome, thanks a lot.
chem geek
Pool Industry Leader
Pool Industry Leader
Posts: 2382
Joined: Thu 21 Jun, 2007 21:27
Location: San Rafael, California

Reduce Total Alkalinity Levels

Postby chem geek » Thu 09 Dec, 2010 02:51

If you were to lower the TA level, you should find that you will need far less carbon dioxide to keep the pH down. You had said your pH was stable, but really it isn't because you are adding carbon dioxide to keep it down. If you want to continue to use MPS, which is acidic, then just lower the TA until you find that you no longer need to add carbon dioxide and instead have stable pH just from the chlorine and MPS.

Adding carbon dioxide does not change the TA level. Yes, it adds carbonates to the water, but it also lowers the pH and these two effects exactly cancel each other out. Essentially, adding carbon dioxide is like adding carbonic acid (which is carbon dioxide plus water) and the increase in bicarbonate (and some carbonate) that increases TA equals the increase in hydrogen ion that decreases TA (and decreases pH).

There is a net rise in TA over time when using hypochlorite sources of chlorine along with carbon dioxide for pH control, but that is mostly due to your chlorinating liquid since that has some excess lye in it. There may also be some increase from evaporation and refill with water which has TA (this also increases CH over time as well, unless you've got water dilution from splash-out, backwashing or rain overflow).
somaholiday

Reduce Total Alkalinity Levels

Postby somaholiday » Fri 10 Dec, 2010 09:56

Awesome. I was under the wrong assumption for c02 and I'm a little ashamed I didnt put 2 and 2 together. As for a stable pH, I have a relatively high bather load (swim teams, aerobics, parties, etc.) which would make a stable pH without automated c02 control impossible, right? I mean within 10 minutes of our largest swim team getting in that c02 is kicking on. I hope I'm not overlooking something else, but I'm all for making my job easier and my budget better looking.

I don't mean to get too too personal, but some of this knowledge and advice is very impressive. Do we have industry professionals here? Chemists? Or just years of experience? This advice is second to none.
chem geek
Pool Industry Leader
Pool Industry Leader
Posts: 2382
Joined: Thu 21 Jun, 2007 21:27
Location: San Rafael, California

Reduce Total Alkalinity Levels

Postby chem geek » Fri 10 Dec, 2010 15:32

No need for any embarrassment. Understanding how CO2 works isn't obvious and many in the industry get this wrong as well. People who don't know chemistry well just think "carbon dioxide adds carbonates to the pool therefore it must increase TA" and don't consider the pH effect nor the difference in adding sodium bicarbonate (a carbonate salt) which increases TA vs. effectively adding carbonic acid which does not (while a weak acid such as carbonic acid does not decrease TA, a strong acid such as Muriatic Acid does).

Anyway, what happens with the bather load is that the splashing and kicking aerates the water and that drives carbon dioxide out of the water and into the air faster and this causes the pH to rise with no change in TA (essentially carbonic acid is removed from the water). The rate at which this occurs depends roughly on the square of the TA level (from experiments made by Wojtowicz) so a lower TA level reduces the rate of pH rise from outgassing. Yes, a lower TA provides less pH buffering, but the outgassing effect outweights the buffering effect. If the TA were at equilibrium with the air, which is very low at around 10 ppm (if there is no CYA in the water and at a pH of 7.5), then there would be no pH rise at all since there would be no carbon dioxide outgassing. However, one needs some TA to provide at least some pH buffering and to provide a source of carbonate ions which along with calcium ions protect plaster surfaces.

Basically, with your higher TA, you are just see-sawing between faster carbon dioxide outgassing during bather load that you are compensating by adding carbon dioxide back into the pool. The higher the TA, the faster this outgassing occurs requiring even more CO2 and likewise the lower the TA the slower this occurs requiring less CO2.

In a residential pool, one can use 50 ppm Borates to provide an alternative source of pH buffering thereby letting one have a much lower TA level (and to provide other benefits such as inhibiting algae growth), though with plaster pools one must have a higher CH to compensate. And as I noted earlier, the rate of outgassing is higher at lower pH so one can set a higher pH target to minimize this effect. This chart shows how much the water is out-of-equilibrium with the air in terms of the amount of dissolved carbon dioxide in the water compared to its equilibrium amount. This chart is roughly linear with respect to TA, but the rate of outgassing seems to be related to the square of the TA -- Wojtowicz postulates that this is due to facilitated ion transport.

So you can try to see what happens if you target a lower TA of 70 or 80 ppm and also target a pH of 7.6 or 7.7. Then see if this significantly reduces the amount of CO2 that you need to inject to maintain the pH. If it does and you want to keep the lower TA level, then use The Pool Calculator to calculate the saturation index and add calcium chloride (or use Cal-Hypo for a while) to increase the Calcium Hardness (CH) to get the index closer to zero to protect plaster surfaces.

As for my background, I do not work in the pool/spa industry nor in the chemical industry. I am just a pool homeowner who developed a strong interest in pool water chemistry (I majored in physics & chemistry in college). If you want to learn a lot more that isn't taught in the industry, then look at Certified Pool Operator (CPO) training -- What is not taught .

Richard
Johnny Tikinut

Reduce Total Alkalinity Levels

Postby Johnny Tikinut » Fri 10 Dec, 2010 17:16

Hey Chem Geek,

Your advice is awesome. I recently drained my pool here in southern California due to high cyanuric acid. After completely draining and refilling the pool my PH was beautiful at 7.5, however my alkaline was about 175-180 (the test "pinked up" at 170 & went red at 180). The guys at my local pool store and much of the online advice suggested no more than 2 cups of Muriatic acid for 10,000 gallons. They also generally advised to wait 3 days to add more acid. After nine days of struggle I managed to drop the alkaline a whopping 10 PPM. Then I found your advice about adding acid and bottoming out the PH, then bringing the PH back up (chemical free) through aeration. In 5 days I have brought my PH down 45 PPM. I am now at the top of the recommended 80 - 120 range squarely at 120, and plan to be at my goal (100) by tomorrow night.

On another site I found some very interesting designs for aerators using the pool returns. I had tried the compressor method but found it to be too loud for me and my neighbors. The return in my pool was the same size as the one in the aerator example 1 1/2 inch (perhaps that is standard). I picked up a threaded male to female PVC piece that screws into the return. From there you would use a short piece of PVC - 3 or 4 inch - just enough to fit on a 45 degree elbow. From there you would use a piece of PVC long enough to get you a little under a foot above water level. On the sample I worked from the author then had a 90 degree elbow so the water would shoot back with force at the water surface. In my case I didn't feel my pressure was strong enough to warrant the elbow, so my water shoots up and the falls a bit harder on the surface. Either way you finish the design off with a 1 1/2 inch PVC cap that you must drill holes through. The internet version drilled 4 holes at 7/16, but I opted to go 8 holes at 7/32. When I made a second one for the other return I wanted to try to increase pressure so I only drilled 5 holes at 7/32. Either design made a nice aeration effect, seemingly without disrupting flow.

Using these two aerators I found that I could add two cups of acid in the morning and two again in the evening without going below 7.2 PH. My apologies up front if you have already addressed this aerator method, and I won't give a site name for what may be a competing website; however for those of you who require pictures to work from I'm sure a little Google creativity will get you there.

Thank you so much Chem Geek. I think it is great that someone living halfway across the state from me would take the time to post information that is valuable, yet free, for so many of us to benefit from.

Thanks,

Johnny
Johnny Tikinut

Reduce Total Alkalinity Levels

Postby Johnny Tikinut » Fri 10 Dec, 2010 18:15

In my previous post I said "In 5 days I have brought my PH down 45 PPM." I meant to say that I brought my alkaline down 45 ppm.
chem geek
Pool Industry Leader
Pool Industry Leader
Posts: 2382
Joined: Thu 21 Jun, 2007 21:27
Location: San Rafael, California

Reduce Total Alkalinity Levels

Postby chem geek » Fri 10 Dec, 2010 19:54

This site does not ban you for linking to other sites if the information is useful and relevant. The Lowering Your Alkalinity post at TFP has links to 3 different styles of home-made aerators plus one that brings in air like a spa. I'm glad the method, based on known chemistry/physics, worked for you.
Allen G Myerson

Reduce Total Alkalinity Levels

Postby Allen G Myerson » Fri 10 Dec, 2010 22:12

Johnny Tikinut wrote:I recently drained my pool here in southern California due to high cyanuric acid. After completely draining and refilling the pool my PH was beautiful at 7.5, however my alkaline was about 175-180 (the test "pinked up" at 170 & went red at 180).

There is a service in Southern California that uses commercial reverse osmosis filtration to lower unwanted dissolved solids, such as cyanuric acid. It can be a good option in many cases.
Johnny Tikinut

Reduce Total Alkalinity Levels

Postby Johnny Tikinut » Sat 11 Dec, 2010 20:29

Thanks again to the contributors here. Richard - after reading your reply I realized that I probably got the link to the aerator's from here in the first place. I just followed your latest link "Lowering your alkalinity" on TFP. I'm curious what the theory is about not lowering your alkaline just to reach a target number. I'm pretty sure that's exactly what I just did. I brought my alkaline down from about 180 to 100 because the paperwork with my test kit showed the ideal range to be 80 - 120, so I thought that I would go up the middle. So now I am curious as to why this is not the right thing to do, can you clarify?

Return to “pH & Total Alkalinity”

Who is online at the Pool Help Forum

Users browsing this forum: No registered users and 3 guests