CO2 vs. Muriatic Acid

[apwilson78]

Helpful experts,

I'm fairly new to pool chemistry but am working on a business school project for some uses of CO2 in non traditional aspects and have been reading on its use as Carbonic acid to lower pH in pools to maximize chlorine effectiveness. For anyone that has used CO2 or has contemplated making the switch, could you help answer some of the following:

1. Besides all of the safety highlights of using CO2 vs. HCl, what are the pros and cons of the two? I've read about alkalinity issues with CO2 (at least it doesn't lower alkalinity). Also, does using HCl force pool owners to replace equipment (pumps, pipes) more often than a non corrosive CO2?
2. Are there any economic benefits of using CO2 vs. HCl? I know there are liquid CO2 suppliers who rent tanks and then come fill them up once a week or a couple times a month based on the size of the pool, but the control and injection equipment for CO2 may be more expensive
3. If I were looking at converting a pool using "X" gallons of 30% muriatic acid solution per day, how could I convert this to a number of pounds of CO2? Just to look at the cost/benefit analysis
4. Are you seeing any trends/shifts in the number of commercial pools moving towards CO2 for safety reasons?

Any help would be appreciated, thank you.
[email protected]

 

[257WbyMag]

Welcome to TFP!

This should be an interesting discussion. This forum is really for the homeowner/operator but we do have some commercial folks here using ORP controllers and the like. Let's see what they say.

 

[chem geek]

Welcome to TFP!

I will answer some of the technical questions while others with experience can answer the practical ones.

As you have pointed out, HCl will lower both pH and TA while CO₂ will only lower pH with no change in TA. So in theory, HCl makes more sense to use if the pH rise is from an added base such as the excess lye in bleach or chlorinating liquid while CO₂ makes more sense to use if the pH rise is from carbon dioxide outgassing. However, if their is carbon dioxide outgassing, that can be lessened by lowering the TA level.

Now in practice, use of CO₂ normally results in a rise in TA over time because at least some of the pH rise is due to the excess lye in bleach and chlorinating liquid. Also, in saltwater chlorine generator pools, some chlorine gas may not dissolve and instead outgasses which behaves the same way as excess lye.

As for safety or corrosion, there is no issue on pumps or pipes if the HCl is injected downstream of equipment as it should be. PVC tolerates HCl but obviously pump seals and copper heat exchangers do not, but HCl injection can easily be added after such equipment. In residential pools, it is typically added slowly over a return flow and for safety light brushing is done in the area to ensure thorough mixing.

One pound of CO₂ is equivalent to 32.8 fluid ounces (4.1 cups) of full-strength Muriatic Acid (31.45% Hydrochloric Acid) in terms of lowering pH. Likewise, 1 gallon of Muriatic Acid is equivalent to 3.90 pounds CO₂. Note that these are for incremental changes, but for large changes in pH, the TA will move with Muriatic Acid so these equivalent calculations will no longer work.

 

[apwilson78]

Thank you chem geek for the helpful response! Quick question: how did you get to the equivalency of CO₂ to muriatic acid? I want to make sure I understand the cost/benefit tradeoff. Roughly 4 lbs of carbon dioxide has equivalent power to change the pH as one gallon of full strength muriatic acid, and one gallon of muriatic acid is roughly 9.6lbs, correct (I'm just using this for bulk pricing)?

Just doing a quick google which is always dangerous, I found that HCL 31% solution costs around $300 per ton (2,000lbs). So 2,000lbs of solution divided by 9.6 lbs per gallon yields 208 gallons of solution per ton, correct? So the cost per gallon is roughly $1.44.

CO₂ in bulk costs roughly $0.30 per lb, so $1.20 total for the 4 lbs.

Both will also have the additional cost of renting the holding tanks.

I'm going to have to justify this math as part of the substitution process of HCl to CO₂, so just want to be as buttoned up as possible when people grill me. If there are any helpful websites you know of, I would really like those as well.

Also, I looked into taking a 2 day class to become a certified pool operator, just so I understand all the lingo. Are these a waste of time or are they pretty helpful to a novice like me?

Thank you again.

 

[Richard320]

Thank you chem geek for the helpful response! Quick question: how did you get to the equivalency of CO₂ to muriatic acid? I want to make sure I understand the cost/benefit tradeoff. Roughly 4 lbs of carbon dioxide has equivalent power to change the pH as one gallon of full strength muriatic acid, and one gallon of muriatic acid is roughly 9.6lbs, correct (I'm just using this for bulk pricing)?

Just doing a quick google which is always dangerous, I found that HCL 31% solution costs around $300 per ton (2,000lbs). So 2,000lbs of solution divided by 9.6 lbs per gallon yields 208 gallons of solution per ton, correct? So the cost per gallon is roughly $1.44.

CO₂ in bulk costs roughly $0.30 per lb, so $1.20 total for the 4 lbs.

Both will also have the additional cost of renting the holding tanks.

I'm going to have to justify this math as part of the substitution process of HCl to CO₂, so just want to be as buttoned up as possible when people grill me. If there are any helpful websites you know of, I would really like those as well.

Also, I looked into taking a 2 day class to become a certified pool operator, just so I understand all the lingo. Are these a waste of time or are they pretty helpful to a novice like me?

Thank you again.

Heh heh heh certified-pool-operator-cpo-training-what-is-not-taught-t18432.html

What's wrong with just tossing a chunk of dry ice into the skimmer basket?

 

[bobodaclown]

What's wrong with just tossing a chunk of dry ice into the skimmer basket?

Been there done that! Halloween party, fog on the water. It was pretty cool. MUCH more expensive that muriatic acid at lowering PH.

 

[chem geek]

how did you get to the equivalency of CO₂ to muriatic acid? I want to make sure I understand the cost/benefit tradeoff. Roughly 4 lbs of carbon dioxide has equivalent power to change the pH as one gallon of full strength muriatic acid, and one gallon of muriatic acid is roughly 9.6lbs, correct (I'm just using this for bulk pricing)?

Just doing a quick google which is always dangerous, I found that HCL 31% solution costs around $300 per ton (2,000lbs). So 2,000lbs of solution divided by 9.6 lbs per gallon yields 208 gallons of solution per ton, correct? So the cost per gallon is roughly $1.44.
:
Also, I looked into taking a 2 day class to become a certified pool operator, just so I understand all the lingo. Are these a waste of time or are they pretty helpful to a novice like me?

I used my Pool Equations spreadsheet using negative numbers for carbon dioxide (since the spreadsheet is set up for outgassing rather than addition) and used small incremental amounts so as not to move the TA very much and I used a starting pH of 7.5. Nevertheless, there is another way to calculate it as follows.

CO₂ + H₂O ---> H₂CO₃
Carbon Dioxide + Water ---> Carbonic Acid
H₂CO₃ <--->>> HCO₃^- + H⁺
Carbonic Acid <--->>> Bicarbonate Ion + Hydrogen Ion
HCO₃^- <<<---> CO₃^2- + H⁺
Bicarbonate Ion <<<---> Carbonate Ion + Hydrogen Ion

The problem is that carbonic acid is a weak acid so while at pool pH most of it becomes bicarbonate ion, this is pH dependent so the amount of pH movement from carbon dioxide will depend on the starting and ending pH. Using molecular weights and concentration one gets one gallon of full-strength Muriatic Acid is the same as (3.7845 liters HCl) * (31.45% / 100%) * (1.156 g/ml HCl) * (1000 ml/liter) * (44.01 g/mole CO₂) / (36.46 g/mole HCl) / (453.592 g/pound) = 3.66 pounds CO₂. It takes somewhat more carbon dioxide than this because it is a weak acid and increases the amount of carbonates in the water. The following shows what happens at pH 7.5 starting with 100 ppm TA with no CYA and ignoring ion pairs (with calcium ions).

[H₂CO₃] 120.88 µM/L (this includes CO₂(aq) and in fact there is 650 times more carbon dioxide than carbonic acid)
[HCO₃^-] 1989.4 µM/L
[CO₃^2-] 4.2350 µM/L
TOTAL 2114.5 µM/L

After adding 0.1 pounds CO₂ per 10,000 gallons (27.2 µM/L) we then get:

[H₂CO₃] 147.27 µM/L (this includes CO₂(aq) and in fact there is 650 times more carbon dioxide than carbonic acid)
[HCO₃^-] 1991.0 µM/L
[CO₃^2-] 3.4816 µM/L
TOTAL 2141.8 µM/L

You can see that most of the added carbon dioxide remains as aqueous carbon dioxide in the water. That's what pH buffering does -- it resists changes to pH. When you add Muriatic Acid a similar buffering happens as shown below, though for this example with 3.25 fluid ounces per 10,000 gallons the TA drops by 1.3 ppm and you can see that there is a shift from bicarbonate ion to aqueous carbon dioxide.

[H₂CO₃] 145.40 µM/L (this includes CO₂(aq) and in fact there is 650 times more carbon dioxide than carbonic acid)
[HCO₃^-] 1965.6 µM/L
[CO₃^2-] 3.4373 µM/L
TOTAL 2114.4 µM/L

One gallon of Muriatic Acid is 8.33*1.156 = 9.63 pounds. The cost per gallon of Muriatic Acid in bulk sounds about right since at retail it's around $8-10 per gallon and you'd expect it to be a lot less in high volumes. So you can see that the cost between the two is roughly in the same ball park.

The CPO course is very valuable, but look at the link Richard gave as it will complement/supplement what you learn.

 

[apwilson78]

Thank you chem geek for the big brain answer! Extremely helpful and exactly what I was looking for...and what I couldn't solve myself. Been out of chemistry too long. Look forward to continuing to learn about pool ph Control.

 

[Isaac-1]

One more thing that you should be aware of, from much of the pool information found online people get the impression that all pools use a great deal of acid, this is simply not true, acid use highly depends on the region, fill water and individual pool. We have members here that NEVER need to add acid to their pools, and in fact are often more concerned with raising pH. I am one of the fairly lucky ones with fairly stable pH, that on average does tend to raise slightly over time (a quart of muriatic acid will usually last me 1-2 years). Other members are not so lucky and go through gallons of muriatic acid per week.

Ike