Does Alkalinity Cause pH To Increase Faster?

[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