Salt chemistry and conservation of mass

[Testani]

Hi all,

I have read several threads on this site about the reason that we have a large need to add HCl to saltwater pools. The threads focus on co2 off gassing as the source of proton loss and often dismisses the poster when suggesting alternative mechanisms. Although proton loss through co2 is clearly plausible, but if this is the only aspect of the equation than we would have either 1) buildup of molar levels of chloride ions without a cation (impossible, pool would literally have lightning bolts coming out of it) 2) accumulation of another cation-chloride pair. Since many people are regularly adding gallons of high concentration HCL, it seems unlikely that this level of buildup of an alternative chloride salt is the answer to the mass/charge balance equation (i.e., would be an issue over time we would all be aware of i would think).

Thus it would appear that the only solution to this would be that there is a simultaneous chloride loss to maintain charge neutrality and net conservation of chloride mass. I have been having some difficulty figuring out where the chloride is going based on available information if its not off gassing of chlorine gas (which I believe has been dismissed as non-plausible in prior posts on this site). To me this is the easiest way to balance the equations and get progressive loss of protons and chloride (which is necessary to explain how we can be adding gallons of HCl without #1 or #2 happening).

Looking forward to the chemistry people's thoughts.

Jeff

 

[duraleigh]

TFP is a teaching forum. We focus on practical everyday methods of managing pool water.......methods the pool owner can use to provide sanitary, crystal clear water.

I am leaving this post up for now, but I will caution that if this descends into an argumentative philosophical discourse then TFP is not the place for that.

 

[jgofnj]

I read this a few times and still have NO CLUE what is being discussed. My question WHY? and WHO CARES?

 

[mgtfp]

Protons don't get lost. In all involved chemical reactions, charges remain balanced.

The involved equilibrium reactions in CO₂ outgassing are:

(1) CO₂(aq) + H₂O <--> H₂CO₃
(2) H₂CO₃ <--> HCO₃^- + H⁺
(3) HCO₃^- <--> CO₃^2- + H⁺

When CO₂(aq) outgasses, equilibrium (1) gets disturbed and reaction (1) runs a bit faster to the left. That messes with equilibrium (2) and reaction (2) runs a bit faster to the left, resulting in a few H⁺ reacting with HCO₃^- to form H₂CO₃. That messes with equilibrium (3) and reaction (3) runs a bit faster to the left, resulting in a few H⁺ reacting with CO₃^2- to form HCO₃^-. This happens until a new equilibrium is restored. In the process, a few protons have reacted with HCO₃^- and CO₃^2-, and have therefore effectively been removed from the water, driving pH up. But the protons didn't get lost, they reacted with negative ions, charges remain balanced.

 

[Testani]

I don't disagree with the above chemistry. I use this routinely in my lab to get rid of NaOh post reaction (bubble co2 in a ph 14 solution you get bicarb eventually, with the corresponding lower ph) but what about the chloride (I am not a chemist but do run a large biomedical lab at Yale)? You add HCl to the pool (proton and chloride anion). The proton combined with bicarb in the pool to make co2 which leaves the pool. This is essentially consuming a proton right (turns it into water, which has no charge)? Thus you have remaining chloride that needs to be dealt with. This is not a one time thing, people are putting moles of HCl (and thus chloride) into their pool each summer. Somehow that chloride either needs to build up (balanced by something that has not been specified, oxidation product?) or leave the pool (e.g., as chlorine gas).

I doubt that the pool accumulates pounds of some organic chloride salt (or whatever is balancing the chloride). Seems like if this was a thing we would know about it and know how to deal with it.

In the end, protons are effectively leaving the pool but we don't' know what is happening to the chloride.

The question is not where does the proton go, but where does the chloride go?

BTW, this is more than just an existential question to argue about. To fix the "leak" of protons out of the pool through off gassing is challenging, but if we know where the chloride is going mitigation could be considered. For example, if its off gassing of chlorine gas theoretically we could shift that to the right by running the ph higher and thus less chlorine loss.

 

[SoDel]

I’m not a chemist but an observation — maybe not as much HCl is added to pools as you may think. This year I’ll go through maybe four gallons. My NaCl runs over 3,000 ppm. I drain more than 1,500 gallons of that solution annually and it’s replaced with either tap water or rain. So the chloride from the HCl addition is like more of a rounding error than a significant addition. I can’t add much as far as where it goes, but my (WA) guess is some form of salt which is basically at an immaterial amount, or some chloramine which is disassociated by UV and off-gassed.

 

[ajw22]

I doubt that the pool accumulates pounds of some organic chloride salt (or whatever is balancing the chloride). Seems like if this was a thing we would know about it and know how to deal with it.

Speaking in laymans language - accumulation of salt in pools due to the salt added from chlorine and muriatic acid is a thing.

Salt testing is not normally done on pools that do not have a SWG so many pool owners do not see the level of salt accumulating in their pools. The salt level is often seen when someone is installing a SWG and takes our advice to test the water salinity before adding any salt. They are often surprised to find 2,000 ppm or more of salt is already in their pool water. This usually happens in areas that do not drain and close their pools.

 

[duraleigh]

theoretically we could shift that to the right by running the ph higher and thus less chlorine loss.

I have NO IDEA of the validity of the science but, AGAIN, this forum is not for theoretical ideas that some (few?) find interesting. It is for teaching a procedure that works and is validated by thousands of real world experiences

Other theories of pool care belong in the "Other Theories of Pool Care" forum........Oops! we don't have one

 

[JamesW]

The pH rise from CO2 offgassing only goes so far before it stops because the CO2 in the pool will reach equilibrium with the CO2 in the atmosphere.

So, in cases where the pH continuously rises, there has to be a different source of pH rise, which is usually a hydroxide, carbonate or bicarbonate, which usually comes with a positive counter ion like sodium or calcium.

H⁺ + HCO3^- --> CO₂ + H₂O

Na⁺ + OH^- + H⁺ + Cl^- --> H2O + Na⁺ + Cl^-

Na⁺ + HCO3^- + H⁺ + Cl^- --> H⁺ + HCO3^- + Na⁺ + Cl^-

Ca²⁺ + CO3^2- + 2H⁺ + 2Cl^- --> H⁺ + HCO3^- + Ca²⁺ + 2Cl^-

 

[JamesW]

The acid (a positively charged hydrogen ion/proton) reacts with the bicarbonate, which came in with its own positive counter ion (usually sodium), so the equation balances on charge.

H⁺ + Cl^- + NaHCO₃ --> CO₂ + H₂O+ Na⁺ + Cl^-

 

[JamesW]

In the below reaction, you have a balance of charge because you lose one negatively charged ion (bicarbonate) and you gain one negatively charged ion (chloride).

H⁺ + Cl^- + HCO₃^- --> CO₂ + H₂O + Cl^-

 

[Testani]

Hi James,

So agree entirely:

1) H+ + Cl- + NaHCO3 --> CO2 + H2O+ Na+ + Cl-

This regenerates the salt that was lost when chlorine gas was generated. However, feed that same Na and Cl molecule back into the salt generator and you get chlorine gas and sodium hydroxide. This sodium hydroxide combines with co2 to make sodium bicarb, and you are back on the left side of your equation where you add acid.

So you have the same sodium molecule that you used in #1 (recycled), but you are adding a new chloride exogenously (muriatic acid). Repeat this cycle thousands of times and you start to see how if you are using the same Na but bringing in new chloride you start to imbalance the equation.

So the chloride that accompanied that NaCl in your equation got turned back into chlorine gas. It either oxidized something and created a cation that will build up with the chloride (usually anions created by oxidation right, sulfate, nitrate, etc, would have to think about what cation it could be) or some chlorine left the pool (which chlorine gas would be a good candidate).

Thoughts?

 

[JamesW]

Anode 4Cl -> 2Cl2.

Cathode 4H2O -> 2H2 + 4OH.

2Cl2 +2H2O -> 3H + HOCl + OCl + 2Cl.

HOCl + OCl + uv light -> O2 + H + 2Cl.

Following the process, we can see that there are 4H and 4OH created, which nets out to ph neutral.

The chlorine gas generated is very acidic and creates 3 hydrogen ions for every 4 hydroxide ions created.

As the hypochlorous acid is broken down by uv, 1 more hydrogen ion is created for a net neutral result.

Assuming that chlorine gain and loss are equal, there's no ph rise.

Anode 4Cl^- -> 2Cl₂.

Cathode 4H₂O -> 2H₂ + 4OH^-.

2Cl₂ +2H₂O -> 3H⁺ + HOCl + OCl^- + 2Cl^-

HOCl + OCl^- + uv light -> O₂ + H⁺ + 2Cl^-.

The net result is:

2H₂ + O₂ + 4H2O + 4Cl^-

The only thing that you lose is water in the form of hydrogen and oxygen gas, which is basically the same as the electrolysis of water.

The chloride ion is merely converted from chloride to chlorine and back to chloride.

 

[Testani]

I don't disagree with your cation accounting.

But how to we explain constant addition of chloride? We are adding pounds of it each year. Where is it going in your equations above?

 

[JamesW]

When you add acid, the chloride does build up and it increases the “salt” level.

The reason you need to add acid is because there is something basic being added that raises the pH and the TA.

So, it depends on what is causing the pH and TA to rise.

For example, new plaster has calcium hydroxide (Ca(OH)2) and calcium carbonate CaCO3 that can dissolve into the water.

This brings a balanced charge.

Adding acid to counteract the pH and TA rise is charge neutral.

Ca(OH)2 + 2H⁺ + 2Cl^- --> Ca²⁺ + 2H₂O + 2Cl^-

If pH rise is only from carbon dioxide offgassing, the addition of acid is neutral as far as charge is concerned because you lose a negative ion (bicarbonate) for every negative chloride ion.

The process of oxidation is just chlorine atoms taking two electrons from something and becoming chloride.

Chlorine has 6 electrons in the outer shell and that gives the chlorine atom a +1 charge. In hypochlorous acid you have chlorine +1, oxygen -2 and hydrogen +1 for a net charge of 0. Hypochlorite has a chlorine +1 and oxygen -2 for a net charge of -1.

Chlorine atoms need 8 electrons in the outer shell to have a full shell. So, they take two from something like oxygen or nitrogen to fill their outer shell. This creates chloride with a charge of -1. Electrons each have a charge of -1, so as atoms gain electrons, their charge drops by 1 per gained electron.

A SWG plate has a positive charge that removes an electron from a chloride ion to make it into a chlorine atom with a charge of 0. Two chlorine atoms with a charge of 0 combine to make chlorine gas (Cl2), which dissolves into the water. When chlorine gas dissolves one atom ends up with a charge of -1 (chloride) and the other ends up with a charge of +1 and combines with oxide to form hypochlorite (OH-) or hypochlorous acid (HOCl).

With the outer shell full, the atom has a -1 charge.

In hypochlorite, oxygen has a -2 oxidation state. When the oxygen atom has a full outer shell of 8 electrons, it has a -2 oxidation state. This is its preferred state because it is the most stable state.

When UV light hits a hypochlorite ion, two electrons get knocked loose from the oxygen atom and the chlorine takes them. Oxygen doesn't really want to let them go but chlorine has a strong ability to take electrons.

This causes the oxygen to go from -2 (oxide) to an oxidation state of 0, which combines with another oxygen atom to form oxygen gas. Oxygen gas wants to get back the two electrons to get a full outer shell and it will oxidize something else if it gets a chance.

When oxidizing a nitrogen compound, like ammonia, chlorine again takes 2 electrons from the nitrogen, which changes it from -3 to 0. It takes 3 chlorine atoms (In HOCl or OCL-) or 3 chlorine gas (Cl2) molecules to oxidize 2 nitrogen atoms in ammonia to nitrogen gas.

Ammonia is a chemical compound where the nitrogen is in the -3 oxidation state and there are 3 or 4 hydrogen ions in the +1 state.
Because they are opposite charges, they combine together in an ionic bond.

When the nitrogen loses 3 electrons, its oxidation state goes to 0 and it can no longer combine with hydrogen to form ammonia.

Nitrogen in the 0 oxidation state can combine with another nitrogen atom in the 0 oxidation state because they share electrons in a covalent bond.

This is how ammonia is oxidized into nitrogen gas. The nitrogen goes into the atmosphere and the ammonia is gone. It leaves the hydrogen ions behind, so it is acidic.

CCs are not created by chlorine oxidizing something.

CCs are only created if chlorine "combines" with something.

For example, ammonia is NH3 or NH4. Chlorine can either oxidize the nitrogen or it can replace a hydrogen ion and combine with the ammonia molecule.

Chlorine can replace 1, 2 or 3 hydrogen ions to form NH2Cl (monochloramine), NHCl2 (dichloramine) or NCl3 (trichloramine aka nitrogen trichloride).

Nitrogen trichloride is a gas and is what you smell when a pool has a "chlorine" smell. It's also the toxic gas generated if you mix bleach and ammonia. It's not chlorine gas, but it is a chlorinated gas that's highly toxic and even explosive if it is concentrated enough.

3Cl2 + 2NH4+ --> N2 + 6Cl- + 8H+

Combined chlorine only happens if chlorine combines, not when it oxidizes.

When chlorine reacts with algae, it's usually just an oxidation reaction and no combination occurs.

Plants, such as algae, create chemicals like glucose from carbon dioxide and water. Using light, the carbon in carbon dioxide oxidizes the oxide into oxygen.

6CO2 + 6H2O --> C6H12O6 + 6O2

Carbon dioxide + water--> glucose + oxygen.

This is how plants make oxygen and store energy in sugars. Most of the bulk of plants is carbon from carbon dioxide. Most of the weight of a tree comes from the air.

The carbon in carbon dioxide is in the +4 state.

The carbon in glucose is 4 at 0, 1 at -1 and 1 at +1. The carbon is "reduced", which is the opposite of oxidized. Reduced just means that the oxidation state is lower or reduced because the atom gained electrons which are negatively charged.

When chlorine oxidizes the carbon in glucose back to a +4 oxidation state, it reverts back to carbon dioxide.

This is a similar process to an animal using oxygen to burn sugars to release energy and exhale carbon dioxide.

So, chlorine reacting with algae is mostly an oxidation reaction and not a combination. Algae is mostly converted back into carbon dioxide and water.

Chlorine can combine with carbon compounds, such as methane (CH4) by replacing the hydrogen ions.

Carbon in methane is in the -4 oxidation state.

Chlorine killing algae usually does not create any CCs.

Chlorine lost to UV does not create CCs.

CCs are mostly nitrogen compounds. UV helps break down CCs the same way that it causes hypochlorite to be lost, by knocking the electrons loose from the nitrogen so that the chlorine can more easily take them.

Nitrogen compounds are mostly introduced by bather waste such as urine or sweat.

The only really big source of ammonia is if bacteria eat the cyanuric acid and break it down into ammonia.

So, basically, the process of chlorine oxidizing something is just the chlorine atom taking 2 electrons and becoming chloride. The process of becoming chlorine again is just removing 2 electrons to activate the atom again.

Most of chemistry is atoms gaining or losing electrons and then combining or dissociating based on the oxidation states.

If you want to know the oxidation states of the atoms in a chemical compound, go to wolframalpha and enter the chemical and oxidation states. For example, hypochlorite oxidation states or ammonia oxidation states etc.

 

[JamesW]

When adding acid, the hydrogen can react with something like carbonate, bicarbonate or hydroxide and the reaction will charge balance.

3H⁺ + 3Cl^- + OH^- + CO₃^2- + HCO₃^- --> 2H₂O + HCO₃^- + CO₂ + 3Cl^-

As you can see, the chloride can and will build up if you continuously add acid.

This can be measured with a K-1766 salt test kit.

The charges balance in the equations and you do not end up with a charge imbalance.

The question is why does the pH increase?

If the pH increase is strictly from carbon dioxide outgassing, then the TA will not rise with the pH rise.

If the TA rises with the pH, then you must have the introduction of a basic compound like carbonate, bicarbonate, hydroxide, borate (like from adding Borax) etc.

Fill water can have high TA.

Products like calcium hypochlorite and sodium hypochlorite are slightly basic when accounting for the chlorine loss.

They both have extra hydroxides in the formula.

Adding Borax, sodium bicarbonate, sodium carbonate, calcium hydroxide, calcium carbonate etc. all raise the pH.

However, these all come with a positively charged ion to balance the negatively charged ions in the basic compound.

 

[JamesW]

I have read several threads on this site about the reason that we have a large need to add HCl to saltwater pools.

SWGs are mostly pH neutral.

My observation is that SWG pools can go for a long time with no pH rise as long as the TA is low enough (Usually about 60 ppm with a CYA of about 60 ppm) and the pH is kept high enough (usually about 7.8).

This puts the carbon dioxide in the water very close to equilibrium with the carbon dioxide in the air.

People that report using a lot of acid need to look elsewhere for the source of the pH and TA rise.

In some cases, it’s fill water with a high TA that is added to a pool with high rates of evaporation.

Water that evaporates is pure water with no TDS being removed.

In cases where people follow the traditional pool industry 80 to 120 ppm TA rule, that is the source of pH rise especially when they keep adding more baking soda to raise the TA after it drops from adding the acid.

This creates a vicious circle of adding acid and then baking soda and then acid and then baking soda and so on indefinitely.

Keeping the pH on the lower end of the scale near 7.2 to 7.4 increases the amount of carbon dioxide, especially when the TA is too high.

Plaster can contribute to pH rise especially when it is new and when people don’t maintain a high enough CSI.

Some plaster can be weak and dissolve at an accelerated rate due to poor quality in materials and/or workmanship.

 

[mgtfp]

By the way: In a non-SWG Pool, chloride doesn't just accumulate by adding muriatic acid.

The reaction

HOCl + OCl- + uv light -> O2 + H+ + 2Cl-.

doesn't care where the chlorine comes from, the same happens to chlorine added via liquid, dichlor, trichlor or cal-hypo. But it doesn't get recycled by a SWG and accumulates gradually with each addition of chlorine.


And before someone complains that the individual reactions

Anode 4Cl- -> 2Cl2.

Cathode 4H2O -> 2H2 + 4OH-.

are not charge balanced (the sum of the two is balanced anyway): In the first reaction, the SWG sucks the electrons in, and spits them out in the second reaction. (Edit: James had mentioned that anyway in his explanation, missed that on the first read).

 

[mgtfp]

In the end, protons are effectively leaving the pool but we don't' know what is happening to the chloride.

The question is not where does the proton go, but where does the chloride go?

I thought by explaining that the proton (and it's charge) doesn't leave the pool, it's clear that there is no mystery in the chloride staying behind.

The proton attaches to negatively charged carbonates until you end up with neutral H2CO3. This eventually ends up as H2O and CO2. The CO2 will leave the pool if the water is oversaturated with CO2, the water (where the proton ended up with) stays. Until it evaporates or gets backwashed out and replaced with another H2O from your tap or rain.

[EDIT]
To answer your question...
This is essentially consuming a proton right (turns it into water, which has no charge)?
... more directly:

The water ends up neutral because in the process the proton combined with negatively charged carbonates. The net charge of the pool hasn't changed.
[/EDIT]

By the way, there are no free protons in the water, the H+ from adding acid attaches to H20 to form H3O+. But this is in the equations usually left out, would just add more complexity that is unnecessary for the net equations we are interested in.

But I think, James has clarified pretty much everything while I was peacefully sleeping on the other side of the world, and proven once more that "42" is the answer. Now we have to start finding the right question again, but we got close. Great effort, @JamesW.

 

[Griswald]

Gallons of HCl?

I haven't used a gallon yet this year.