How much muriatic acid will I need each week with a SWCG? Let's do real maths here.

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Nowaker

Nowaker

0
Jun 14, 2017
12
San Antonio, Texas
Water Balance for SWG pools guide here doesn't really say how much muriatic acid will be needed weekly or monthly. PoolMath calculator doesn't yet have anything related to FC generation in SWCGs, or muriatic acid usage to fix the pH when SWCGs are running.

Here's my attempt to bring some maths to SWCG topic.

Since I have a 22,000 gal pool, I need 167 grams of chlorine gas to bring FC 0 to FC 2. If daily FC consumption is 2 (this varies by person), daily production needs to be 2 to maintain the level. This is already in PoolMath calculator.

Every single day, 2 FC is generated at the expense of slightly increasing the pH. How much muriatic acid EXACTLY do I need to compensate the pH? That is, add this very amount of muriatic acid that the generator "took away".

To make my 0 PPM FC pool a 2 PPM FC pool I need a daily production of 167 grams of FC. Molar mass of Cl2 is 70.9060 g/mol, so 167 grams of Cl2 is 2.3552309 moles. Weekly Cl2 production = 16.486616 moles. Looking at `2NaCl + 2H2O → electrolysis → 2NaOH + H2 + Cl2` and the reverse `2NaOH + 2HCl → chemical reaction → 2NaCl + 2H2O`, we know we need to supply 2 moles of hydrochloric acid (muriatic acid) for each one mole of Cl2 generated to neutralize 2 OH- ions (NaOH becomes Na+ and OH- in solution). Therefore we need 16.486616 x 2 = 32.973232 moles of hydrochloric acid to compensate it. Molar mass of HCl is 36.46094 g/mol. That is 32.973232 mol * 36.46094 g/mol = 1202.235 g of pure HCl a week. Typical muriatic acid bottle in store is 31.45% strong - the rest is water. Therefore, we need around 1202.235/0.3145 = 3822.6868 grams of store muriatic acid a week, or 134.841308728 ounces by weight, or around the same by volume (*), to compensate our weekly FC producution. 1 gallon, more or less.

(*) This is a simplification. Water density is 1 g / cm3 while pure HCl is 1.2 g / cm3. To be 100% correct, we need to take this into account.

Does it make sense? Can someone validate I didn't make a mistake? jblizzle?

It could be a nice addition to PoolMath calculator.

Thanks.
 
The production of chlorine by an SWG is exactly pH neutral. While it is true that the pH inside the cell increases from the production of chlorine, the pH of the water outside the cell DECREASES because most of the relevant chlorine oxidation reactions are acidic and reduce pH.

The pH rise seen from the use of an SWG is entirely dominated by the outgassing of CO2 from the aeration of the water inside the cell and outside in the bulk pool water. That pH rise caused by that aeration inside the cell is small with respect to the overall pH rise from the general outgassing of CO2 from the pool (aeration from surface effects).

There are TFP members that have SWGs and never have to add acid to their pools - their fill water has low TA and pH.
 
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Your intuition was correct though - inside the cell, the pH can climb very high because the source of the pH rise (the productions of chlorine) is decoupled spatially from the pH fall (oxidation and sanitation reactions). This is one reason why borates can be very helpful to SWG owners - the borates act as a strong pH buffer when the pH is rising. Borates can cut the pH rise inside the cell by half. So, with two identical cells running, one with borated pool water and one without, the potential for calcium scaling inside the cell is much greater in the non-borated pool water.
 
JoyfulNoise said:
Your intuition was correct though - inside the cell, the pH can climb very high because the source of the pH rise (the productions of chlorine) is decoupled spatially from the pH fall (oxidation and sanitation reactions).
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They're decoupled spatially, that's correct, but sooner or later, OH- ions end up in the pool. They won't stay in the cell because of water flow and diffusion. The reaction I used is straight from Wikipedia so I didn't "invent" it. myself. Salt water chlorination - Wikipedia

I have no idea what kind of reactions happen in the pool - and it's probably hard to say because it's an open system. That being said - in a closed environment (like a bucket of water kept in my room for days) - SWCG will increase pH as it runs.

Honestly, I still feel like I'm not yet seeing the big picture - if the system gets more and more OH- ions, where do H3O+ ions come from to balance it and keep pH unchanged? You mention oxidation and sanitation - can you share actual reactions? If they generate H3O+, then it would make sense.

I also thought about chlorine loss due to UV light. What I was able to find first is UV light makes chlorine gas react with hydrogen gas and form HCl. homework - Why can UV light initiate a reaction between hydrogen and chlorine gas? - Chemistry Stack Exchange This would obviously lower the pH if in water. But we don't have hydrogen gas in the pool, it exits the pool milliseconds after it was created in the chlorine generator. Then I found what I was really looking for - what happens to FC in water when it's hit by UV. Guess what - it generates HCl (muriatic acid) and oxygen (released as gas). HCl forms H3O+ and Cl-. That means any FC lost to UV decreases the pH which is a good thing in a saltwater pool. (Source)

By the way - I highly recommend reading the above source. They also cover the topic of cyanuric acid and chlorine effectiveness in relation to pH.

(I had to shorten the link because a word "assessment" was shortened to its first three letters and I couldn't submit the post...)

JoyfulNoise said:
This is one reason why borates can be very helpful to SWG owners - the borates act as a strong pH buffer when the pH is rising. Borates can cut the pH rise inside the cell by half. So, with two identical cells running, one with borated pool water and one without, the potential for calcium scaling inside the cell is much greater in the non-borated pool water.
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Chlorine generation produces OH- ions, while H+ are going away (because gas hydrogen leaves. This website describes why borates "balance" the water: Borate Chemistry| Pool Spa News | Efflorescence, Water, Technically Speaking. ". When you add base (hydroxyl group or OH-), some of the undissociated tri-coordinated borate becomes dissociated and tetra-coordinated, absorbing the base. Thus, the perfect balance is maintained." That of course happens when there's enough undissociated tri-coordinated borates! The more OH-, the less undissociated tri-coordinated borates. So while I can see borates as a way to prevent the increase of pH, it won't prevent it from increasing forever. H3O+ needs to be added somehow to "free" the "used" borates by OH-.
 
I'm happy to read your links but perhaps you'd like to do more reading on water pool chemistry yourself -

Pool Water Chemistry (pay particular attention to Post #3)
Chloramines and FC/CYA
Certified Pool Operator (CPO) training -- What is not taught

Most of what you referenced was worked out by Richard Falk and Ben Powell long ago. The effects of cyanuric acid on chlorine have been known since the early 70's but weren't paid much attention to by the swimming pool industry.

As for the reduction of chlorine by UV, HCl is not created. The largest contributor to UV photolysis of chlorine in water is the conversion of hypochlorite anion (OCl-) to chloride (Cl-) and oxygen gas. No acid generation occurs.

If you wish to read about borates, see these links by Richard as well -

Borates - Why and How
Are Borates Safe to Use?
 
elwood58 said:
While our new pool is consuming MA like crazy during the finish cure, our previous pool consistently used 12 oz every other day, or 180 oz per month. This was while trying to maintain a pH of 7.5. the Pool was 14,100 Gallons.
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Trying to maintain a pH of 7.4 is unrealistic in a lot of situations. There is simply too much carbonate alkalinity (bicarbonate) in the water and the outgassing rate of CO2 is much higher at lower pH making the rate of rise faster. The rate of rise is not linear - at low pH (7.0-7.5), the pH rise is much faster than at higher pH (7.6-8.0) and so one will use a lot of acid trying to keep the pH below 7.5. Lowering TA to 50-60ppm will help a lot but the better way to maintain pH is to target a pH no lower than 7.6 and then let the pH naturally rise to 7.8. Once the pH goes above 7.8, only add only enough acid to get the pH back to 7.6.

With 50ppm borates in my pool water and lower TA (around 60ppm), my pH will hang at 7.7 for a very long time (7-10 days). That keeps my acid addition frequency low enough to not be a nuisance.
 

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Anode 4Cl -> 2Cl2.
Cathode 4H2O -> 2H2 + 4OH.
2Cl2 +2H2O -> 3H + HOCl + OCl.
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.
 
shoot-straight said:
keep the info coming- my 1 year old SWG plaster pool pH creep up is terrible. it seems to use about 1/2 gallon a week to keep it in check.
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santacruzpool said:
This season I am adding about 16oz of the "Green" low fume MA every 7 days.
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Only to give perspective, this is really not that much. Many of us in the South and Southwest would be extremely grateful for such low acid demand. My makeup TA is 130-140 minimum, and pH in the high 8 range. Indeed, some people have it much worse. Be thankful of your small demand, and be glad you don't need to buy acid 16-20 gallons a whack to make it less painful.
 
Nowaker, IMHO the variables vary too much to pre calculate when your daily testing will tell the tale from week to week. Eg kids splashing vigorously? PH will rise. Hard rain? It will rise. Using a solar cover? It won't rise. TA decreasing from prior acid additions? It will rise less quickly.

In this case, your proof's in the pool, if you'll excuse the pun ;)

However, this caught my attention:
Since I have a 22,000 gal pool, I need 167 grams of chlorine gas to bring FC 0 to FC 2. If daily FC consumption is 2 (this varies by person), daily production needs to be 2 to maintain the level. This is already in PoolMath calculator.
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Maybe you were just using that as an easy example, but id rather be sure: Please review proper TFP SWG reccs and ratios here: [fc/cya][/FC/cya]. That's the single most important calculation to make ;)
 

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