Carbon Dioxide To Clean Cell?

[Jason8031]

My neighbor told me that I could use Carbon Dioxide to clean the scale from my salt cell by filling the cell with water and bubbling the Carbon Dioxide through the water. He said it is less corrosive to my cell than using muriatic acid. Does anyone know about how to do this procedure, or if it works?

He also told me that I could use a chemical called EDTA. Has anyone used these methods instead of the acid?

My neighbor does not have a pool, but he seems to know a little bit about chemistry.

[chem geek]

My neighbor told me that I could use Carbon Dioxide to clean the scale from my salt cell by filling the cell with water and bubbling the Carbon Dioxide through the water. He said it is less corrosive to my cell than using muriatic acid. Does anyone know about how to do this procedure, or if it works?

He also told me that I could use a chemical called EDTA. Has anyone used these methods instead of the acid?

My neighbor does not have a pool, but he seems to know a little bit about chemistry.

It is true that bubbling carbon dioxide will lower the pH, but I wouldn't say that it's less corrosive than muriatic acid. It completely depends on how much muriatic acid you add. Yes, if you add too much Muriatic Acid so that it uses up the TA and the pH crashes (usually below a pH of 4.5), then that will be more harsh, but it's fully under your control. The main advantage with using carbon dioxide is that it won't lower the TA so the pH won't crash lower and instead will drop in a fairly steady manner being self-limiting as excess carbon dioxide will outgas.

EDTA is a metal sequestrant (not as good as HEDP), but I've never heard of using this for cleaning. Perhaps he is thinking it would help dissolve calcium carbonate scale since some of the calcium can get taken up by the EDTA, but usually acid alone (i.e. low pH) is all that is needed. If he can tell you more about this, it would be helpful for us to know.

[txpoolguy]

CO2 should not lower pH much past 6.5, in which case you'd spend a month and a million bucks buying enough CO2 to clean scale. You should be using a mild acid solution to clean the cell (25-35%) and repeat, if necessary rather than leaving it in a strong solution for a long time.

[Jason8031]

I think that the idea is that the calcium carbonate would be converted to highly soluble calcium hydrogen carbonate like in this equation: CaCO3 + H2O + CO2 <> Ca(HCO3)2 which would have less of a chance to react with the metals in the cell than using acid. If most of the CO2 reacted with the calcium carbonate rather than with the water, then the pH would not drop as much as when using acid.

For example: If half of the carbon dioxide reacted with the calcium carbonate to form calcium bicarbonate and the other half of the carbon dioxide lowers the pH forming an acid, then one third of the acid would be neutralized by the bicarbonate, one third would dissolve more of the remaining calcium carbonate and one third would be available to react with the titanium alloy.

In addition, I think that the high chloride levels provided by using muriatic acid would exacerbate the corrosion of metal in an acid solution.

[chem geek]

I think that the idea is that the calcium carbonate would be converted to highly soluble calcium hydrogen carbonate like in this equation: CaCO3 + H2O + CO2 <> Ca(HCO3)2 which would have less of a chance to react with the metals in the cell than using acid. If most of the CO2 reacted with the calcium carbonate rather than with the water, then the pH would not drop as much as when using acid.

For example: If half of the carbon dioxide reacted with the calcium carbonate to form calcium bicarbonate and the other half of the carbon dioxide lowers the pH forming an acid, then one third of the acid would be neutralized by the bicarbonate, one third would dissolve more of the remaining calcium carbonate and one third would be available to react with the titanium alloy.

In addition, I think that the high chloride levels provided by using muriatic acid would exacerbate the corrosion of metal in an acid solution.

Well, that simply doesn't happen as calcium bicarbonate doesn't exist to any great extent (except possibly as a weak neutral ion pairing). It's mostly only calcium ions and bicarbonate ions in solution. There is a calcium bicarbonate ion pair, CaHCO₃⁺, but it's at a far lower concentration than calcium or bicarbonate separately.

The way scale dissolves even with carbon dioxide is via the increased acidity which has there be far less carbonate in the water. There is an equilibrium between calcium carbonate solid and calcium ions and carbonate ions in solution:

CaCO₃(s) <---> Ca²⁺ + CO₃^2-

The amount of carbonate ion, CO₃^2-, is significantly lower under acidic conditions so that causes the above equilibrium to shift to the right dissolving the calcium carbonate scale. The carbon dioxide itself has nothing to do with the reaction of dissolving calcium carbonate except its indirect effect in lowering the pH via formation of carbonic acid. The acid can catalyze the above reaction by combining with the carbonate to form carbonic acid and then carbon dioxide and water.

The reactions that occur are as follows when carbon dioxide is dissolved in water that is already near saturation with calcium carbonate:

CO₂(g) ---> CO₂(aq)

CO₂(aq) + H₂O ---> H₂CO₃

H₂CO₃ ---> HCO₃^- + H⁺

H⁺ + CO₃^2- ---> HCO₃^-

CO₂ does stay dissolved in water to some extent, but the relative species of dissolved carbon dioxide, carbonic acid, bicarbonate ion, and carbonate ion is completely dependent on the pH. The net result of dissolving carbon dioxide is a lowering of pH, a large increase in carbonic acid, some increase in bicarbonate ion, and a strong percentage decrease in carbonate ion.

It is true that for some metals such as stainless steel the high chloride levels interfere with the reformation of an passivity layer, but this may not be true for the kinds of metals used in salt cells. If that was a big concern, you could use a different form of acid (see below).

As txpoolguy says, you can completely moderate the effects through dilution and exposure control. People use dilute acid solutions to clean their cells all the time and don't end up ruining their cells. The cells wear out much more from the electrolysis than by acid washing, unless done improperly. If you wanted to use something other than acid to clean the scale, then you can use something like CLR which has a composition described in this MSDS with a pKa of 3.85 compared to full-strength Muriatic Acid with a pKa of -1. The patent for this product describes how it works here.

If your chemistry friend wants to see more details about pool water chemistry, have him look at my spreadsheet here.

Richard

[GM@n]

Richard,

Can you tell me or point me in the direction to this question? I am trying to determine how much C02 would be required over a period of time to lower the pH of a swimming pool. I know it should be a factor of how much water (gal.) and how much the pH needs to be lowered ( i.e. 7.5 to 7). Any help you can provide would be appreciated.

Ken

[chem geek]

Richard,

Can you tell me or point me in the direction to this question? I am trying to determine how much C02 would be required over a period of time to lower the pH of a swimming pool. I know it should be a factor of how much water (gal.) and how much the pH needs to be lowered ( i.e. 7.5 to 7). Any help you can provide would be appreciated.

Ken

Ken,

1 pound of carbon dioxide is identical in its pH lowering effects as 29.6 fluid ounces (about 3-3/4 cups) of full-strength Muriatic Acid (31.45% Hydrochloric Acid). The difference is that the Muriatic Acid will lower the TA by 11.6 ppm while the carbon dioxide won't change the TA. In 10,000 gallons, it would drop the pH from 7.5 to around 7.0 if the TA were near 100 ppm.

Richard

[quantumchromodynamics]

The reactions that occur are as follows when carbon dioxide is dissolved in water that is already near saturation with calcium carbonate:

CO₂(g) ---> CO₂(aq)

CO₂(aq) + H₂O ---> H₂CO₃

H₂CO₃ ---> HCO₃^- + H⁺

H⁺ + CO₃^2- ---> HCO₃^-

CO₂ does stay dissolved in water to some extent, but the relative species of dissolved carbon dioxide, carbonic acid, bicarbonate ion, and carbonate ion is completely dependent on the pH. The net result of dissolving carbon dioxide is a lowering of pH, a large increase in carbonic acid, some increase in bicarbonate ion, and a strong percentage decrease in carbonate ion.

Richard, it seems from your equations that the bicarbonate (alkalinity) will be raised by the addition of the CO₂(g). And, here is a reference that supports the alkalinity increase theory.

_____________________________________________________________________________

http://www.alisonosinski.com/pooltips/18.htm

"Unfortunately, carbon dioxide when added to pool water also forms bicarbonates, and causes a rise in total alkalinity." - Alison Osinski Ph.D.

______________________________________________________________________________

Also, one commercial pool operator that I talk to tells me that when they use carbon dioxide, their alkalinity rises.

Is the increase basically minor compared to the decrease in pH and therefore it can be ignored?

What are the ideal conditions to use carbon dioxide, and when should it be avoided?

[chem geek]

Richard, it seems from your equations that the bicarbonate (alkalinity) will be raised by the addition of the CO₂(g). And, here is a reference that supports the alkalinity increase theory.

_____________________________________________________________________________

http://www.alisonosinski.com/pooltips/18.htm

"Unfortunately, carbon dioxide when added to pool water also forms bicarbonates, and causes a rise in total alkalinity." - Alison Osinski Ph.D.

______________________________________________________________________________

Also, one commercial pool operator that I talk to tells me that when they use carbon dioxide, their alkalinity rises.

Is the increase basically minor compared to the decrease in pH and therefore it can be ignored?

What are the ideal conditions to use carbon dioxide, and when should it be avoided?

Alkalinity is not simply bicarbonate. Hydroxyl ion also counts towards alkalinity and hydrogen ion counts negatively (takes away from) alkalinity. So when carbon dioxide is added to the water, there is no change in alkalinity. It is the exact opposite reaction as occurs when carbon dioxide outgasses from the water and in that case the pH rises with no change in alkalinity. Basically, whenever there is a change in bicarbonate ion from addition or subtraction of carbon dioxide, there is a change in hydroxyl ion (or hydrogen ion) corresponding with it that exactly cancels it out. The net result is the following:

CO₂(aq) + H₂O ---> HCO₃^- + H⁺

or equivalently

CO₂(aq) + OH^- ---> HCO₃^-

where it is more clear that a hydroxyl ion is changed into a bicarbonate ion. Both of these contribute to alkalinity equally as both accept a proton (down to a pH of 4.5 which is what the Total Alkalinity test measures). One can also look at this from the point of view of preservation of charge balance where both species contribute to alkalinity. This is distinct from adding sodium bicarbonate or sodium carbonate where half of the charge, the sodium part, does not contribute to alkalinity at all (neither positively or negatively).

Alison is wrong since she is not accounting for hydroxyl/hydrogen ions and their contribution to measured alkalinity. All such alkalinity definitions include the sum of all species that accept protons, such as [HCO₃^-] and count twice [CO₃^2-], but also include the difference in concentration of hydroxyl ion and hydrogen ion, i.e. [OH^-]-[H⁺] though technically it's the activity of hydrogen ion since the TA test indicator is measuring hydrogen ion activity (but this is a very minor effect). It's not carbon dioxide going to bicarbonate that causes a rise in TA, but rather any other sources of rising pH (other than carbon dioxide outgassing) that are the reason carbon dioxide is being added to lower pH in the first place.

Now I have heard the same thing about TA rising (slowly over time) when carbon dioxide is used, but it seems to be a slow effect and isn't anywhere near what would be predicted if one considered only bicarbonate ion. So I think it's probably similar to what I see in my own pool where there is a slow rise in TA over time that is due to evaporation and refill of water since the TA in fill water will get added to the pool water while evaporation does not remove alkalinity. If one is comparing adding a regular acid instead of carbon dioxide, then in that case the acid lowers both pH and TA so one does not notice the contribution of evaporation and refill in partially offsetting the drop in TA.

Also, carbon dioxide is being added because the pH is rising so if that's due to carbon dioxide outgassing then a lower TA level would help and such addition and removal of carbon dioxide would not change the TA, but part of the reason for the pH rise may be from pure base being added and that would increase the TA along with the pH. One example of this would be plaster curing which introduces calcium hydroxide into the water. Another example would be a saltwater chlorine generator (SWG) if the chlorine gas that was generated was not fully dissolved and some outgassed (since that prevents the acidic process of chlorine getting used up or consumed into chloride). These latter examples would increase TA over time.

Richard