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Key Takeaways:

  • After accounting for chlorine usage/consumption which is acidic, the use of hypochlorite sources of chlorine is close to pH neutral (slightly alkaline).  This includes bleach, chlorinating liquid, Cal-Hypo, lithium hypochlorite and saltwater chlorine generator (SWG) systems.
  • The use of Dichlor is net acidic.  The use of Trichlor is even more acidic.  The use of chlorine gas is the most acidic.
  • For hypochlorite sources of chlorine (including SWG systems), the primary source of pH rise comes from Total Alkalinity (TA) that is too high (even 80 ppm can be too high).


Hypochlorite sources of chlorine include chlorinating liquid, bleach, Cal-Hypo, lithium hypochlorite, and saltwater chlorine generators.  Though the addition of this chlorine raises the pH, the consumption/usage of chlorine (from any source) is an acidic process that lowers the pH.  This is described in detail in this post.


The net result from the addition and usage/consumption of hypochlorite sources of chlorine is pH neutral, at least from the chlorine itself.  There is a fairly small net pH rise from the excess lye (sodium hydroxide) in chlorinating liquid and bleach and from similar chemicals in Cal-Hypo (calcium hydroxide, calcium carbonate) and in lithium hypochlorite (lithium hydroxide, lithium carbonate).  The amount of these chemicals varies by product.  6% Clorox Regular unscented bleach has the lowest amount of "excess lye" at around 0.06% resulting in a product pH of around 11.9.  High quality 12.5% chlorinating liquid has around 0.25% excess lye with a product pH of around 12.5, but many other chlorinating liquids have higher pH with a common value for 12.5% chlorinating liquid of 0.50% excess lye for a pH of 12.8.


Most pools can attain reasonable pH stability when using chlorinating liquid or bleach if the Total Alkalinity (TA) level is kept lower since it is the outgassing of carbon dioxide from higher TA levels that is the primary cause of rising pH unless total chlorine usage is very high using chlorinating liquid with substantial excess lye.

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Replies to This Discussion

Comment by David Rockwell 1 hour ago

I am startled by your comment that chlorine usage/consumption is acidic. I have been going to "advanced" chemistry classes for over 20 years, and am well of what happens when you add each type of chlorine. But I have never been taught what happens after.

My question is, is it just the consumption of chlorine that is low pH, how about ozone?

While hypochlorite sources of chlorine including that produced from a saltwater chlorine generator increase pH upon addition and have it lowered back down when used/consumed, ozone is essentially pH neutral through both processes.


Ozone auto-degradation to oxygen gas and oxidation of ammonia and probably urea are all pH neutral.  Relevant net equations are as follows:


2O3 ---> 3O2

Ozone ---> Oxygen Gas


3O3 + 2NH3 ---> N2 + 3O2 + 3H2O

Ozone + Ammonia ---> Nitrogen Gas + Oxygen Gas + Water


3O3 + CO(NH2)2 ---> CO2 + N2 + 3O2 + 2H2O

Ozone + Urea ---> Carbon Dioxide + Nitrogen Gas + Oxygen Gas + Water


Basically, creation of ozone doesn't change the pH and the usage or consumption of ozone does not change the pH.  Just as with chlorine, there are some side reactions that can have an effect on pH, but the above are the dominant reactions.


By not disclosing what happens to pH with chlorine when it gets used/consumed (including breakdown from sunlight), this has people think that rising pH must be due to the chlorine when in fact it is due to the Total Alkalinity (TA) being too high.  The result is often using more acid and sodium bicarbonate than necessary.  I believe the CPO course should teach the effect on pH of net chlorine use, not just chlorine addition.

I agree 100% that CPO should teach the net effect of chlorine use, especially since this is the gold standard of water management training. I have also attended many water chemistry couses over the years and I have not heard this discussed, ever. I guarantee that if you asked any pool guy I know the net effect in the pool of dichlor they would say pH neutral, and trichlor is slightly acidic in the pool. That misunderstanding leads to these two chemicals being the most overused & abused substances that go into pools, In my opinion.

Though I agree that many have the misunderstanding thinking that Dichlor is pH neutral and Trichlor is acidic, as you point out the truth is that when accounting for chlorine usage/consumption, Dichlor is acidic while Trichlor is very acidic.  The following compares what happens to pH after adding 10 ppm Free Chlorine (FC) from various chlorine sources and then having that chlorine used/consumed (say from breakdown from sunlight or oxidizing ammonia, urea or other nitrogenous organics).  I assume initial conditions of a pH of 7.5, TA of 100, CYA 30.


12.5% Chlorinating Liquid: Addition pH 7.5 --> 8.02; Net pH --> 7.51; TA 100 --> 100.2 (CarbAlk +0.2)

Cal-Hypo: Addition pH 7.5 --> 8.04; Net pH --> 7.53; TA 100 --> 100.5 (CarbAlk +0.4); CH +7.1

Dichlor: Addition pH 7.5 --> 7.46; Net pH --> 7.22; TA 100 --> 96.5 (CarbAlk -4.7); CYA +9.1

Trichlor: Addition pH 7.5 --> 7.33; Net pH --> 7.13; TA 100 --> 92.9 (CarbAlk -6.7); CYA +6.1

In practice, one keeps the TA higher when using Trichlor pucks/tabs (usually at least 120 ppm) and this keeps the pH more stable since carbon dioxide outgassing raises the pH to counteract the pH lowering from the Trichlor.  The TA still drops over time, however.  Even with Dichlor, one should have the TA be high enough to make the pH stable though this may be only 80-100 ppm in spas.


Another way of looking at this is the acid/base equivalency.  Hypochlorite sources of chlorine add a very small amount of excess lye.  1 gallon of 12.5% chlorinating liquid with a pH of 12.5 has the equivalent of 0.3 ounces weight of sodium hydroxide aka lye or 0.8 ounces of sodium carbonate aka pH Up (except that chlorinating liquid has no carbonate).  The net effect on pH with 1 pound of Dichlor is equivalent to 10.4 fluid ounces of full-strength Muriatic Acid (31.45% Hydrochloric Acid) while 1 pound of Trichlor is equivalent to 24.1 fluid ounces of acid.


I don't think that Trichlor is so popular for pools because of the misunderstanding about the pH.  I believe it is popular because it is the only form of chlorine available that is slow-release (not counting Cal-Hypo tabs that tend to fall apart) so is much more convenient.  Dichlor is popular for spas because with the high temperatures and aerating jets the pH tends to rise from carbon dioxide outgassing so having a net acidic chlorine source helps to keep the pH more stable, though it does lower TA over time.  One CAN operate a spa using bleach with reasonable pH stability (after raising the CYA first with either pure CYA or with initial Dichlor use), but one must keep the TA very low, usually 50 ppm or lower, and should use 50 ppm Borates as a supplemental pH buffer.


For 12.5% chlorinating liquid (chlorine bleach), I believe you are using the figure of 0.25% excess caustic for your calculations.  I have called four of the major manufacturers of bleach for the swimming pool industry, and they say that the amount of excess caustic is usually about 0.5% average, sometimes 0.4% and often at least  0.6%.  I know some company's Tech sheet will cite about 0.25%, but in reality, they also tell me that they generally add more than that amount. 

I have gone to the trouble to test and determine the actual amount of excess caustic in a gallon of bleach, and found that it is indeed about 0.5%, average. 

If I am correct about the above, the amount of pH increase would be slightly higher than you calculate, and the alkalinity contribution would be a little higher when using 12.5% bleach. I know you acknowledged above that some other chlorinating liquids (bleach) have a higher pH and TA (from excess caustic), therefore, I think you should also include a higher percentage figure of excess caustic in your calculations listed, because I really believe that most service techs will be using a product that contains a higher amount of excess caustic. 

That's fair.  Using 0.5% excess lye would give the following (changing the first line, but repeating the others for comparison):


12.5% Chlorinating Liquid: Addition pH 7.5 --> 8.05; Net pH --> 7.53; TA 100 --> 100.6 (CarbAlk +0.6)

Cal-Hypo: Addition pH 7.5 --> 8.04; Net pH --> 7.53; TA 100 --> 100.5 (CarbAlk +0.4); CH +7.1

Dichlor: Addition pH 7.5 --> 7.46; Net pH --> 7.22; TA 100 --> 96.5 (CarbAlk -4.7); CYA +9.1

Trichlor: Addition pH 7.5 --> 7.33; Net pH --> 7.13; TA 100 --> 92.9 (CarbAlk -6.7); CYA +6.1


In a pool with high bather load, one could be adding much more than 10 ppm FC per week so the "small" effect on pH and TA can be much more readily seen in high-bather load pools, say those with 10 ppm FC per day.  A pool with 1 bather per 1000 gallons over 8 hours (so, say, 8 bathers 1 hour each, in every 1000 gallons) could have this sort of chlorine demand (i.e. 10 ppm FC per day) or even higher if there are competitive swimmers or urinating kids.  Nevertheless, actual pH rise that is seen is most likely due to carbon dioxide outgassing since having a high bather load usually implies a much higher aeration from splashing/movement/swimming.

Another source to consider is Chlorine Gas. I am always having to try and separate residential from commercial as I am involved with both a lot and gas is still used in many facilities. The effects of course become much more pronounced in a public pool.


As to Ozone many will say it does raise pH, and in the real world it does. This would be from the aeration as the Ozone is injected into the water. Straight air fed systems will use many cu.ft. per minute which is another good reason to use oxygen fed systems, less air volume injected.

Yes in practice the air injection from ozonators aerates the water more so results in more carbon dioxide outgassing that raises the pH (carbon dioxide diffuses into the air bubbles and the surface disruption also increases outgassing).  Of course, having a lower TA level would reduce this effect.  My earlier post on ozone was only considering the chemical effects.


As for chlorine gas, I've added that to the list below for a 10 ppm FC addition and then consumption:


12.5% Chlorinating Liquid: Addition pH 7.5 --> 8.05; Net pH --> 7.53; TA 100 --> 100.6 (CarbAlk +0.6)

Cal-Hypo: Addition pH 7.5 --> 8.04; Net pH --> 7.53; TA 100 --> 100.5 (CarbAlk +0.4); CH +7.1

Dichlor: Addition pH 7.5 --> 7.46; Net pH --> 7.22; TA 100 --> 96.5 (CarbAlk -4.7); CYA +9.1

Trichlor: Addition pH 7.5 --> 7.33; Net pH --> 7.13; TA 100 --> 92.9 (CarbAlk -6.7); CYA +6.1

Chlorine Gas: pH 7.5 --> 7.11; Net pH --> 6.96; TA 100 --> 85.9 (CarbAlk -11.2)


The above calculations are done using my Pool Equations spreadsheet.


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