Does Alkalinity Cause pH To Increase Faster?

[jereece]

I recently read some articles on PoolForum about pH increasing over time as you add bleach each day so you have to lower it using muriatic acid. For years I have just accepted this as a fact from the bleach having a high pH. Decreasing the pH creates the conditions favorable for Alkalinity to decrease and so mine does. But recently, this post raised some questions about whether or not the bulk of the pH increase is actually due to the bleach. I used to be a lab chemist years ago, so I decided to run some tests to see if I could find anything unusual. I don't have a lab, labware or highly accurate measuring instruments like I once did, so my results are "ball park".

I used 3/4 gallon bleach jugs that were cleaned, rinsed multiple times and allowed to sit dry for 3 days. In bottle #1 I collected straight tap water. My water is provided by the city and the source is a local lake. So it is chlorinated and soft (tastes good too!). In bottle #2 I collected straight pool water. Bottle #3 was half pool water and half tap water.

For the water in each bottle I took initial readings. Then I added 3 drops of 6% bleach, mixed and took readings again after 10 minutes. Each drop of bleach should add approximately 1ppm chlorine. For those interested, there are 20 drops in 1ml and there are 3785 ml in a gallon. So 3785 X 0.75 X 20 = 56,775 drops. 6% bleach is 60,000 ppm. So if you do the concentration conversion C1V1=C2V2, you come up with each drop adding 1.06 ppm chlorine (basic emergency plan training).

After the 10 minute readings, I cut the top off of each bottle so that the water depth was about 5 inches and set them outside on my deck table. The air temperature was about 95 and there was a good 7 hours of direct sunlight, so the water in each bottle got to about 110F. I mixed and analyzed again after 4 hours and again after 24 hours. I basically wanted to see what the increase in pH was at various alkalinity.

To minimize this post length, here's a link to my results in an Excel spreadsheet.

My conclusion is that pH in my pool increased more at higher alkalinity and it takes more acid to reduce the pH. In the first test (3 drops/ppm), the pH of Tap Water (Alk 20) increased from 7.5 to 7.6 after 24 hours. However pool water with 70ppm Alkalinity increased from 7.4 to 7.7. In the second test (20 drops/ppm) a few days later, the pH of Tap Water (Alk 20) increased from 7.5 to 7.6 after 24 hours. However pool water with a 60 ppm Alkalinity went from 7.4 to 7.8.

This surprised me as I thought higher alkalinity would keep the pH from increasing so fast. I have also noticed that as my alkalinity has been decreasing over the past month that I have in fact not had to adjust the pH as often. At an alkalinity of 120, I was having to adjust the pH twice a week. Now that my alkalinity is down to 60, I am only adjusting once a week. And it takes half the acid to bring the pH down.

So, am I dreaming here or what? Is this possible? I thought higher alkalinity slowed pH change but my tests seem to indicate that it increases it. I intend to let my alkalinity decrease over time to see if I notice any problems.

Any feedback, comments or things I have not considered are welcome. I am just trying to learn more.

Thanks,
Jim

[chem geek]

[Ben, you can move this entire thread to The China Shop if you beleive it to be getting too technical -- if you do, you can just add this to the existing Carbonate Alkalinity thread since it is related.]

Jim,

Your results are consistent with the hypothesis that carbon dioxide is outgassing at a faster rate when the alkalinity is higher. This makes pefect sense since water with such excess carbonate alkalinity is similar to soda water (except that soda water has a huge excess of carbonate compared to pool water). When exposed to the air, the excess carbonate in the water leaves the water as carbon dioxide and when this happens, this raises the pH (the alkalinity stays the same for technical reasons I won't get into here -- see the Carbonate Alkalinity thread for more info). The more CO2 outgassing that occurs, the more acid you need to add to restore the pH.

The effect of alkalinity buffering pH so that the pH will not move as much when acid or base is added is true for a closed system. If you did your same experiment but added acid or base (pure base, such as lye, not sodium carbonate which also increases carbonate alkalinity) to your samples, then you would find that the pH moved less with the samples that had higher alkalinity. In addition, the total amount of base or acid that you would need to add to restore the pH would be the same for all samples (assuming you added the same amount of acid or base to each sample initially). So higher alkalinity (pH buffering) reduces the pH swing, but does not change actual acid/base demand.

One way to look at this apparent paradox is to think of the higher alkalinity in an open system exposed to air as causing base to be added to your system and that the higher alkalinity increases the rate at which this occurs and this outweighs the pH buffering effect. Technically, it is carbon dioxide that is leaving your water, but this is similar to adding base to your water because the carbon dioxide that leaves is taking away H2CO3 (H2O + CO2) which is an acid (removing acid is equivalent to adding base).

Also, if you were to repeat your tests and add a stirrer to some of your samples and/or add a blower to create some wind over some of your samples, you should find that those samples with the higher aeration increased in pH more rapidly and required more acid to be added to restore pH.

Now the question becomes what is the rate of this CO2 outgassing? That becomes very complicated to answer exactly. Though it is true that the outgassing will become more rapid at higher alkalinity and at lower pH and with more aeration, it is very hard to quantify the aeration effect. The alkalinity and pH effects are quantifiable and I have done so with a chart and graph (these are for a CYA of 30 though this doesn't change that much with CYA level).

The CO2 outgas rates at your pH and alkalinity in your initial samples (before bleach was added since that was not a primary factor) were as follows (I assumed that your "Alk" was really "TA" for total alkalinity not adjusted by CYA to get carbonate alkalinity):

pH    TA  CYA   Relative CO2 Outgas Rate
7.5   20    0            1.3
7.4   70   30            7.9
7.5   40   15            3.1

The numbers above are very approximate since your measurements are rough, but it does show that the 50/50 sample should outgas over twice as fast as tap water and that your pure pool sample should outgas over twice as fast as the 50/50 sample. The amount of CO2 outgassing outweighs the pH buffering effect though you did seem to find that the 50/50 sample and the pure pool water were somewhat similar with both pH drift and acid demand (which is unexplained, but the measurements are rough and the aeration is not guaranteed to be consistent). Also, the addition of chlorine is like adding a base while it will act like an acid when it gets used up. So the fact that chlorine levels changed kind of fouls up a lot of the accuracy of the experiment.

What is interesting, and has little to do with the alkalinity, is what happens to your chlorine in each sample. The tap water sample without CYA loses chlorine very quickly. Since your bottles were inside and I assume were not exposed to sunlight, then the chlorine reduction could have been partly due to chlorine outgassing though this rate is quite slow and greatly increases pH (which you did not see). It is more likely that the chlorine broke down through usage (oxidation of organics and breakpoint of ammonia). This doesn't bode well for your tap water! It would appear that it has chlorine demand! To see if this is really true, try adding additional chlorine after 24 hours and see if it "holds" for another 24 hours. If it does (eventually), then your tap water isn't "clean" and has something in it to eat up chlorine (which should eventually stop after adding enough chlorine -- it might have chloramine in it which would show up in the CC test and could "eat up" chlorine to breakpoint). If instead you find that you have to keep adding chlorine to the tap water every day at a much higher rate than your pool water or a 50/50 mix, then the CYA is preventing chlorine loss beyond that predicted and I would be very interested in knowing that.

I found it rather surprising that your pH level returned so quickly after adding the chlorine even with the pool water sample that still measured >5 ppm. Since 3 drops was around 2 ppm, the 20 drops would be around 13 ppm. When the chlorine gets used up, this is an acidic process that pretty much exactly compensates for the "base" effect of the initial added chlorine. It is still possible that some of your pH rise (and acid demand) is due to chlorine outgassing. It would be better to do your experiment using samples that had little or no chlorine in them so you could then just focus on the carbonate alkalinity (the CYA is OK as its effect isn't as huge as the chlorine effect on pH).

Richard

[jereece]

Thanks Richard for your very interesting and educational response. Just a few comments.

1. You said "Since your bottles were inside and I assume were not exposed to sunlight". That is not correct. As I said in my post "I cut the top off of each bottle so that the water depth was about 5 inches and set them outside on my deck table. The air temperature was about 95 and there was a good 7 hours of direct sunlight, so the water in each bottle got to about 110F."

2. You said "The amount of CO2 outgassing outweighs the pH buffering effect...". Does this mean that I am better off running at lower alkalinity? If so, why do most people recommend running 100-120 alkalinity? What alkalinity would you recommend me run at?

3. You said "I found it rather surprising that your pH level returned so quickly after adding the chlorine even with the pool water sample that still measured >5 ppm." I agree and do not really understand this.

4. Is it possible that my very low CH concentration has any affect on all of this?

5. As I have allowed my alkalinity to get lower and lower, I am consistently seeing that I don't have to adjust my pH as often. I am liking running at lower alkalinity. Are there any potential problems with running at low alkalinity and if not why doesn't everyone just run at low alkalinity?

6. You said "The effect of alkalinity buffering pH so that the pH will not move as much when acid or base is added is true for a closed system." Obviously my pool is not a closed system, so I thought you were saying this does not apply. However you go on to say "So higher alkalinity (pH buffering) reduces the pH swing, but does not change actual acid/base demand." But I am not sure that I saw that in my samples as pH increased in most in samples with the most alkalinity. How can this be?

Again I really appreciate the education.

Jim

[medvampire]

Jim
If I may add to this discussion. I guess this is over simplifying but the way I look at ALK and pH they are tied together like 2 balls with a rubber strap between them. If you have a high ALK the pH will slowly want to rise to equalize to the ALK. If you have a low ALK the pH will fall to equalize as well. The pool is trying to reach a point where you have equilibrium between ALK and pH. When the pool reaches this point the pH will not move as much and we have achieved the best buffering capacity for our pool. pH and ALK are not the same but are linked. Remember pools are like people and each one is different so in my pool I have found that if I keep my ALK around 80 to 90 I do not use as much acid. If I let my ALK rise above 110 or so I have to adjust more.
Click for larger pic.
Steve

[chem geek]

Jim
If I may add to this discussion. I guess this is over simplifying but the way I look at ALK and pH they are tied together like 2 balls with a rubber strap between them. If you have a high ALK the pH will slowly want to rise to equalize to the ALK. If you have a low ALK the pH will fall to equalize as well. The pool is trying to reach a point where you have equilibrium between ALK and pH. When the pool reaches this point the pH will not move as much and we have achieved the best buffering capacity for our pool.

Steve,

What you say is true, but the equilibrium point is far outside where most people run their pools. At a TA of 80, you have to have your pH at almost 8.4 before there is balance between dissolved CO2 in the water and in the air. Alternatively, at a pH of 7.5, you would have to have a TA of 14 to achieve this same balance.

Fortunately, the rate of carbon dioxide moving out of the pool is rather slow so you can be quite a bit out of equilibrium and not notice the pH rise (i.e. it is small and dwarfed by other factors that change the pH of your pool water). I'd love to know whether the kinetic rates for outgassing are based directly on the concentration of dissolved vs. external CO2 or if there is some power relationship. Henry's Law and other equilibrium laws don't give the kinetics and what we are interested in is how fast, not the fact that it's out of equilibrium.

The good news is that this discussion has got people thinking about lower alkalinity levels since many many people seem to be fighting rising pH. Some have had success by lowering alkalinity (a lot -- to 60-70, for example), but some have not (though I don't think they tried lowering that far). One example of success at 80 with a pH of 7.5 is described in this thread. If only everyone had the same success story.

Richard

[medvampire]

Richard
Before we get too far into this subject and get mod slapped or at best moved to the china shop. I do tend to oversimplify my answers here in the general forums. I do agree with you on the equilibrium point is out of normal pool conditions by the models. The equilibrium point would be affected by other reactions going on in the pool we can’t predict. This is might be one of those situations where the math just doesn’t meet the real world and we have to adjust and measure the result. I do intend to lower my ALK a bit more to around 70 and monitor my acid usage.
Steve

[chem geek]

My responses to your questions are in bold below.

Thanks Richard for your very interesting and educational response. Just a few comments.

1. You said "Since your bottles were inside and I assume were not exposed to sunlight". That is not correct. As I said in my post "I cut the top off of each bottle so that the water depth was about 5 inches and set them outside on my deck table. The air temperature was about 95 and there was a good 7 hours of direct sunlight, so the water in each bottle got to about 110F."
It could help if I could read! Sorry to miss that the first time. Well, sunlight will certainly burn through the chlorine and that explains why the levels dropped, even with CYA from the pool. Thanks for clearing that up so I don't have yet another mystery to solve!

2. You said "The amount of CO2 outgassing outweighs the pH buffering effect...". Does this mean that I am better off running at lower alkalinity? If so, why do most people recommend running 100-120 alkalinity? What alkalinity would you recommend me run at?
This is an excellent question and I, too, have been questioning why the standard recommendations for alkalinity are as high as they are. I can see that when using Tri-Chlor tablets, you pretty much have to have higher alkalinity to help slow down the rather rapid decrease in pH from those tablets, but when using liquid chlorine or bleach, I don't see the need as much. When people are seeing a rise in pH due to outgassing of carbon dioxide, then I would suggest running at a lower TA and a normal pH (7.4 or so) and avoid low pH. However, there are some people who report rising pH and regular adding of acid with little or no change in alkalinity over time so that implies other sources of "base" entering into the pool in which case lowering your TA won't help and could even hurt a bit (by allowing larger pH swings). So the jury is still out on exactly what to do and I hesitate to give a catch-all recommendation.

I would say that it doesn't hurt to "try" a low 80 TA and normal 7.4-7.5 pH or even in more extreme situations try a 60-70 TA. If one finds that they are no longer fighting pH rise (and adding acid regularly), then they've solved a problem. If this doesn't help, and especially if it makes things worse, then they can always go back up to more "normal" TA levels. By the way, the recommended range from NSPI is 80-120, not 100-120. In fact, the 100-120 is when you use Tri-Chlor. When you use liquid chlorine or bleach, the recommendation is 80-100.

3. You said "I found it rather surprising that your pH level returned so quickly after adding the chlorine even with the pool water sample that still measured >5 ppm." I agree and do not really understand this.
Well, now that I know you had things exposed to sunlight, there is an explanation for this. When the chlorine gets broken down by sunlight, this is an acidic process so the pH will come back down to where it was, at least when you get to the original chlorine levels. That is probably what happened in your case. When you have very high chlorine levels exposed to sunlight, you will burn through a lot of chlorine quickly, even with CYA, so your pH will drop fairly rapidly.

4. Is it possible that my very low CH concentration has any affect on all of this?
Unlikely since the CH only figures into water balance for plaster/grout preservation and doesn't affect (much) the chlorine and CYA equations. There is a minor effect from TDS but it is very, very small, and even smaller than usual when CYA is present.

5. As I have allowed my alkalinity to get lower and lower, I am consistently seeing that I don't have to adjust my pH as often. I am liking running at lower alkalinity. Are there any potential problems with running at low alkalinity and if not why doesn't everyone just run at low alkalinity?
There really aren't any problems until you get to very low alkalinity at which point you will eventually see that adding small amounts of chemicals to your pool (chlorine or anything else) cause the pH to jump more wildly than you might like. Though you might not get much of a long-term drift, the initial jump in pH when you add chlorine and it's subsequent drop might cause a swing in pH that is too large during the day for swimmers. I wouldn't worry about this until you get to really low aklainity below 50 or so. Just experiment to get to a point you are happy with. If enough people find this is worthwhile, I may create some more graphs showing the amount of pH buffering at different alkalinity levels.

People with plaster/grout pools should probably raise their CH somewhat when they lower their alkalinity or they should raise their pH even more to 7.6-7.7 or so. This is also an area of controversy since it seems that you have to be WAY out of water balance before corrosion or scaling become a problem.

6. You said "The effect of alkalinity buffering pH so that the pH will not move as much when acid or base is added is true for a closed system." Obviously my pool is not a closed system, so I thought you were saying this does not apply. However you go on to say "So higher alkalinity (pH buffering) reduces the pH swing, but does not change actual acid/base demand." But I am not sure that I saw that in my samples as pH increased in most in samples with the most alkalinity. How can this be?
I'm sorry to have confused you. The pool is somewhat of a hybrid open and closed system. It can be treated as a closed system for some things where the equilibria are reached rather quickly and where changes to the system are slow. For other things, or over a longer period of time, it must be treated as an open system and some things may be out of equilibrium. The carbon dioxide dissolved in the pool is one such out-of-equilibrium situation which means it is extremely difficult to predict exactly what will happen and how quickly. Some factors can be calculated since the "rates" are a function of concentration, but without knowing the precise rate reactions (are they linear, or quadratic, etc.) and without knowing the physical factors and formulas, it is nearly impossible to determine absolute rates. Besides, the rate changes over time as splashing, wind, etc. change. Nevertheless, I have put together graphs showing the relative rates for factors that ARE known, but they need to be taken with a grain of salt and are only "accurate" if everything else is held constant.

The pH rise that was higher in the samples with higher alkalinity was due to the outgassing of carbon dioxide which is because your bottles were open to the air. The change showed up much more over a period of a day or more. So in this sense, your pool is open to the air as well and is therefore over days an open system and would also show a rise in pH over time (except the pool's depth and volume to surface area ratio, exposure to wind, splashing, etc. may be different than your bottles).

The greater buffering effect of alkalinity does not stop the pH from rising -- it only slows it down. The outgassing effect is more powerful and therefore wins out. For example, between an alkalinity of 75 and 150, the outgassing rate of carbon dioxide (at pH 7.5) is 2.3 times faster (all else equal). However, the pH buffering effect of the alkalinity is 2.0 so the carbon dioxide wins out in this case. Since the 2.3 vs. 2.0 doesn't seem like much of a difference, it may very well be that the carbon dioxide outgas rate may vary by the square of the concentration instead of being linear (as I assumed) so perhaps the outgas rate increases by over 4 times. I don't have clear answers about this, but regardless of the actual amount of pH rise, what is very clear is that at an alkalinity of 75 you will be adding half the acid (or less) than you have to add at an alkalinity of 150. So, independent of exactly how much the pH rises, the lower alkalinity definitely has you add less acid on a regular basis. At some point, you will get your alkalinity down and pH up to a balance where the amount of acid you need to add will be small and dwarfed by the other changes that happen in your pool and cause pH to go up and down (rain is acidic, bather sweat is perhaps not neutral, liquid chlorine has extra base in it, etc.).

Again I really appreciate the education.
Thanks, and I'm learning too.

Jim