Miscellaneous Questions -- Stop Green, Carbonate, CYA

[chem geek]

The following is from a PM I received on another Pool Forum, but I thought the questions and answers were useful in general so I am posting this here with permission.
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Hi Chem Geek,

You come highly recommended from a few people whom I work with, and I had a few questions regarding some pool water chemistry.
I work at a certain pool store down in FL, and I try my very best to educate customers (those who wish to put up with it anyway) when it comes to pool water chemistry. So getting to my questions:

1. We use a product called "stop green", it is not sodium bromide (which I think is mostly known as yellow out or yellow treat). I have searched my MSDS, looked on the internet and I can not find chemical make up of it. It is an ammonia based chemical and is supposed to work at higher PH's. Do you know anything about this chemical, and if you do could you explain to me how it works at higher PH's? I always thought that when the ph of pool water was around 7.8 or so, the hypochlorus acid broke down into the hypochlorite ion and hydrogen, which significantly reduces the oxidation effect of your bleach, so what doesn't make sense to me is how this works at high PH's. What I have always had very good success using sodium bromide at lower ph's. do you recommend one over the other?

2. I was thumbing through the forums and I found a good post about the differences between bicarb and just regular sodium carbonate.
I think I understand for the most part that bicarb raises your TA more than the PH, and sodium carbonate raises both PH and TA a decent amount. Can you explain to me as best you can what happens on a molecular level with these two? From what I understand PH is affected by the concentration of hydronium ions in the water? To me it would make sense that when the bicarb is dissolved in water the carbonates will affect the TA, sodium is just sodium, what does that extra hydrogen do? Does it not create more hydronium? Does it even break off of the carbonate ion?

3. I know CYA is a pretty touchy subject, and I am a strong believer in the 10% rule, which is keeping your FC 10% of your CYA. My question comes in the Trichlor- Tablets. There is a very big misconception in my specific area about Trichlor tablets, a lot of people "use" them as their main source of oxidation, this results in unGodly high stabilizer levels. One of the reasons that this misconception could be stemmed from is the way that our company advertises them, which is as "Chlorine Tablets". I know that the chemical break down of Trichlorisocyanuric acid is messy, but could you kind of break it down for me the best you can? I guess my question in specific is that they advertise 99% Available Chlorine in these things, how much hypchlorus acid do you get out of these vs actual CYA?

Thank you, and I look forward to hearing from you.

 

[chem geek]

My response was the following:

Stop Green sounds like it is actually ammonia itself or something very similar to ammonia. When you add ammonia to a pool with chlorine, you form monochloramine which is a decent algaecide and it works fine at higher pH. I suspect that is what it is. With sodium bromide, chlorine converts bromide to bromine which is also OK at killing algae. Both of these products simply work around the problem of having too much Cyanuric Acid (CYA) in the pool since that makes chlorine less effective. Neither monochloramine nor bromine combine with CYA so are at their full-strength in fighting algae. However, neither of these products is necessary if one manages the CYA level (i.e. lowers it since it's likely too high) and uses a sufficiently high Free Chlorine (FC) level. They are extra cost and have side effects (the monochloramine takes more chlorine to get rid of while the bromine will increase chlorine usage in an outdoor pool as bromine breaks down in sunlight -- eventually the bromine will outgas away).

When you add sodium bicarbonate to the water, it mostly does the following:

NaHCO₃ ---> Na⁺ + HCO₃^-
Sodium Bicarbonate ---> Sodium Ion + Bicarbonate Ion

so it mostly just adds bicarbonate to the water and that increases the Total Alkalinity (TA). The main reason the pH rises is that when you add this, the TA gets very high near the surface causing the following reaction to occur:

HCO₃^- ---> CO₂ + OH^-
Bicarbonate Ion ---> Carbon Dioxide + Hydroxyl Ion

so that carbon dioxide outgasses and hydroxyl ion is leftover which, by definition, has the pH rise.

When sodium carbonate is added to the water, it mostly does the following:

Na₂CO₃ + H₂O ---> 2Na⁺ + HCO₃^- + OH^-
Sodium Carbonate + Water ---> Sodium Ion + Bicarbonate Ion + Hydroxyl Ion

Or the carbonate takes a hydrogen away from water to form bicarbonate leaving hydroxyl. So the TA increases from both the bicarbonate and the hydoxyl ions (since both count towards TA) while the pH rises due to the hydroxyl ion.

Trichlor is around 90% Available Chlorine, not 99%, but you need to understand that "Available Chlorine" is relative to the amount of hypochlorous acid produced from chlorine gas -- that is, it is NOT a weight percentage of chlorine atoms in the Trichlor molecule. You can look at the Trichlor molecule here, Cyanuric Acid (CYA) here, and hypochlorous acid here. The molecular weight of Trichlor is 232.4103 g/mole, CYA is 129.075 g/mole and hypochlorous acid is 52.4603 g/mole. So on this basis one could say that Trichlor is 129.075/232.4103 = 55.5% CYA with the rest being chlorine, but as I mentioned, chlorine is measured in ppm that is weight relative to chlorine gas which has a molecular weight of 70.906 g/mole. Since only one of the two chlorine atoms in chlorine gas becomes hypochlorous acid (see the following reaction)

Cl₂(g) + H₂O ---> HOCl + H⁺ + Cl^-
Chlorine Gas + Water ---> Hypochlorous Acid + Hydrogen Ion + Chloride Ion

and each of the three chlorine atoms in Trichlor becomes hypochlorous acid as follows

CY-Cl₃ + 3H₂O ---> CY-H₃ + 3HOCl
Trichlor + Water ---> Cyanuric Acid + Hypochlorous Acid

so 3*70.906/232.4103 = 91.5% Available Chlorine. It's usually quoted as 90% Available Chlorine because Trichlor is around 98% pure.

From the above equation you can see that there are 3 hypochlorous acid for 1 cyanuric acid so using the convention of chlorine measured as ppm of chlorine gas we have Trichlor that adds 1 ppm chlorine also adding 129.075/(3*70.906) = 0.607 ppm Cyanuric Acid (CYA). In fact, the following are chemical rules that are independent of concentration of product and of pool water volume:

For every 10 ppm Free Chlorine (FC) added by Trichlor, it also increases Cyanuric Acid (CYA) by 6 ppm.
For every 10 ppm FC added by Dichlor, it also increases CYA by 9 ppm.
For every 10 ppm FC added by Cal-Hypo, it also increases Calcium Hardness (CH) by 7 ppm.

It doesn't take very long for the CYA to build up. Even at a very low 1 ppm FC per day chlorine usage, after 6 months this is 6*30*0.607 = 109 ppm CYA so over 100 ppm CYA in just 6 months. If the chlorine usage were 2 ppm FC per day, then it would take only 3 months to get over 100 ppm CYA. Of course, this ignored dilution from backwashing, splash-out, rain overflow, etc. but such dilution is not that high unless the pool is quite small.

Richard

 

[Water_man]

A couple of remarks:
1. Sodium Bromide is marketed as "Pink Treat", not "Yellow Out".

2. "Yellow Out" by Coral Seas is used at high pH along with shocking levels of FC to treat difficult cases of algae and it's made of a disodium salt of ethylenediaminetetraacetic acid, dehydrate,Disodium sulfate (from MSDS.)

3. As discussed here, and elsewhere by chem geek and waterbear relating to my case of fighting "Pink Algae", using these "exotic" chemicals can and should be prevented by maintaining the proper level of FC according to the FC/CYA chart. I can attest based on my personal experience.