Chlorine turns into salt?

[jimbethesda]

So, I was having this discussion about how traditionally chlorinated pools end up with a salt level that approaches that of a “saltwater” pool because when chlorine gets used up, it turns to salt. But since I’m not a chemistry expert (but their son just graduated from college with a chemistry major), I could’t really explain it, and they didn’t believe me.

So, do all forms of chlorine (liquid, dichlor, trichlor, cal-hypo) turn into salt once it is used up? And if so...what is the chemical process?

Thanks!

 

[JoyfulNoise]

Sanitizing chlorine is in a +1 oxidation state (Cl+ as in HOCl). When it causes and oxidation reaction or sanitizes a biological pathogen, the chlorine is reduced to chloride which has a -1 oxidation state (Cl-).

If the kid has a chemistry degree, oxidation/reduction reactions are covered in 1st semester chem-101 ...

 

[jimbethesda]

So when/how does it become NaCl? Or does it?

 

[JamesW]

Chlorine and chloride are both based on the chlorine atom. Chlorine in the active form has 6 electrons in the outer valence electron shell. When chlorine oxidizes something, it takes two electrons and the atom becomes chloride with 8 electrons in the outer valence electron shell.

Salt is sodium chloride or one of the many other “salt” compounds, like calcium chloride etc. When salt is added to water, the sodium and chloride split apart and become individual sodium ions and chloride ions. Salt is only salt when it’s dry. Once it gets in the water, the individual ions float around in the water at random. So, there’s no actual “salt” in the water, just ions.

So, anything that adds sodium ions or chloride ions or calcium ions etc. increases the “salt” level.

Liquid chlorine is sodium hypochlorite. The sodium ions go in right away and the chlorine atoms also go in and eventually become chloride. The liquid also contains extra salt due to the manufacturing process.

All forms of chlorine have chlorine atoms that become chloride ions when they oxidize something.

Chlorine gas has only chlorine atoms. So, only chloride is added, but the process is acidic and you have to add sodium hydroxide, sodium bicarbonate, sodium carbonate etc. to raise the pH. So, there are your sodium ions.

Calcium hypochlorite adds chlorine and calcium.

Lithium hypochlorite adds lithium and chloride.

Trichlor adds cyanuric acid and chlorine atoms, but also require pH increaser.

Sodium dichlor adds sodium and chlorine atoms.

A SWG removes 2 electrons from chloride ions in the cell and turns them into chlorine atoms. Then, the chlorine atoms take two electrons from something and turn back into chloride.

 

[jimbethesda]

Ok, so is it fair to say that pools that use Trichlor and cal-hypo exclusively do not have a steady increase of salt in their pool (just CYA and calcium)? It’s only the bleach/liquid chlorine folks that get a steady increase?

 

[JamesW]

Liquid chlorine has the most salt, but all forms of chlorine increase the salt level.

You can go to PoolMath and, at the bottom, there is a section called “Effects of adding chemicals” where you can choose a chlorine product and it will tell you how much it will raise the FC and salt level.

PoolMath

Discover the calculator behind the perfect pool, PoolMath. Dealing with pools has allowed TFP to become the original authors of the best calculator on the net.

www.troublefreepool.com

Note that hydrochloric acid also adds chloride because it’s just hydrogen ions and chloride ions. The hydrogen ions are the acid.

Baking soda is sodium bicarbonate. So, that adds sodium ions.

Calcium chloride is a type of salt and adds calcium and chloride ions.

So, most products add sodium or chloride to the water and contribute to the “Total Dissolved Solids” (TDS) of the water.

The two main types of salt test are titration and conductivity.

A titration test is specific to chloride ions but the amount of sodium ions will usually balance. So, the test is accurate for sodium chloride.

A conductivity test measures the conductivity of the water, which is affected by all charged ions, but most of the ions will be sodium or chloride. So, the test is accurate for sodium chloride with normal pool chemistry.

 

[ajw22]

Bleach/liquid chlorine has salt added to the liquid during the manufacturing. That salt accumulates in the water if you use it.

Trichlor contain chlorine, cya, and acid.

Cal-hypo contains chlorine and calcium.

Chlorine is naturally a gas and each source needs a liquid or solid chemical to bind the chlorine to.

 

[JoyfulNoise]

I think the kid's parents need to get a refund on all the tuition they just paid....the kiddo with the chemistry degree should have been able to articulate all of what has been said after his sophomore year (sometimes the freshman year is more liberal arts and survey classes than degree program core classes...).

?‍

 

[an1vrsy]

All of the above explanations are informative. When I came to TFP site last year, I had been a puck user. Long story short, I now exclusively use liquid chlorine 12.5% from my local mom & pop pool store.

After being on the site and reading about sodium hypochlorite, I wondered after a year of using liquid chlorine what was my salt level. I ordered the Taylor K-1766 salt test and lo and behold it was 1400 ppm.

So, 1 more year and I guess at 2800 ppm, I'll have a salt pool by only adding liquid chlorine.

 

[ajw22]

So, 1 more year and I guess at 2800 ppm, I'll have a salt pool by only adding liquid chlorine.

Will be time to get a SWG and end jug dumping.

 

[jimbethesda]

Thanks all. It sounds like I was right with the end result (non SWG chlorine pools over time end up with a salt level approaching an SWG pool), but just said it wrong.

The correct way to explain it is that all forms of chlorine typically put into pools contain salt. The chlorine part gets used up, but the salt remains. Correct?

 

[ajw22]

Salt accumulation in pool water is mosta byproduct of using liquid chlorine

 

[JamesW]

Chlorine and chloride are almost exactly the same thing. The only difference is the number of electrons in the outer shell. Chlorine has 6 and chloride has 8. So, chlorine is chloride with 2 missing electrons. Chlorine gets “used up” when it takes two electrons from something and becomes chloride.

For every 100 ppm fc of chlorine added, here’s how much salt you get by product:

Chlorine gas...................................82 ppm.
Trichlor..............................................82 ppm.
Dichlor...............................................82 ppm.
Calcium hypochlorite................101 ppm.
Lithium hypochlorite.................158 ppm.
Sodium hypochlorite 12.5%....164 ppm.

 

[cfherrman]

Nearly everything you add to a pool has salt in it

 

[Jimbarstow]

A lot of mixed up not-quite-chemistry here. Just to keep things in perspective, the salt (NaCl) concentration in a SWG pool is around 3300 ppm which is in the form of chloride ions, a negatively charged form the chlorine atom. You're going to have to add a lot of bleach to reach that level. For each ppm of chlorine you add via bleach (NaOCl), you will eventually get 1 ppm of chloride (NaCl), more or less. Keep in mind that every time it rains, it will reduce the concentration of NaCl that has been building up since the water will become diluted.

I tried to find out if salt were intentionally added to bleach for some reason and it doesn't appear to be. It is probably there because bleach degrades over time and the primary mechanism is decomposition into oxygen and salt:

2NaOCl --> O2 + 2NaCl

Note that if you keep your pool acidic, you may have degradation to chlorine gas instead.

The way that bleach works is that the form of chlorine in bleach (non-charged) is incredibly reactive and rip apart almost anything it interacts with by stripping off a single electron. The end result is that the chlorine (uncharged) is converted to chloride (negative charge).

For me, with a SWG pool, I completely ignore the chloride added when I use bleach. (I actually use it quite often since it is more efficient to add chlorine after an afternoon of kids via bleach than cranking up the SWG.)

Specifically, in answer to this:

The correct way to explain it is that all forms of chlorine typically put into pools contain salt. The chlorine part gets used up, but the salt remains. Correct?

No, although there may be salt in the bleach (see above) which can contribute to the chloride level in the pool, what happens is that the chlorine in the bleach does its job, and in return gets converted to chloride (the form of chlorine in salt.)

If you have a SWG, the chloride (charged) gets recycled back to chlorine (uncharged) with the help of electricity. This is how bleach is actually made.

This is pretty basic chemistry. Let me guess, the son was pre-med.

 

[JamesW]

There is extra salt in the liquid chlorine due to the production process. 1 ppm chlorine added by sodium hypochlorite adds 1.64 ppm salt.

The chlorine in chlorine gas is uncharged but, the chlorine in hypochlorous acid or hypochlorite has a +1 charge.

Assuming that you add 3 ppm liquid chlorine per day, it would take 203 days to add 1,000 ppm salt to the pool.

For normal pool pH, there will be no appreciable amount chlorine gas in the water.

For every mole of chlorine added by sodium hypochlorite, you get 2 moles of salt NaCl.

1 mole of chlorine is 70.9 grams. 2 moles of salt NaCl is 116.9 grams. 116.9 ÷ 70.9 = 1.64.

 

[JoyfulNoise]

Actually, one has to do an analysis of formal charge on the various chlorine species -

For chlorine gas, Cl2 (Cl—Cl), the overall diatomic molecule is charge neutral but one chlorine atom has a formal charge of +1 and one atom has a formal charge of -1. When chlorine gas is added to water, you get 1 mole of chloride ions (Cl-) right away and 1 mole of hypochlorite (Cl+) ion. However the Cl+ ion is highly unstable and reactive. So it will split a water molecule to form the oxyanion, OCl- (whether in the form of hypochlorous acid or hypochlorite) which is much more stable in aqueous solution. In the case of the oxyanion OCl- the chlorine atom now has a formal charge of zero and the oxygen acquires a formal charge of -1. Eventually the oxyanion reacts with (oxidizes) something, and the chlorine atom attains a formal charge of -1 (chloride) which is highly stable in aqueous solutions. So, 1 mole of chlorine gas eventually yields 2 moles of chloride ion.

In both cases, the chlorine atom with a +1 or zero formal charge is highly unstable and is the source of chlorine’s disinfecting and oxidizing power.

 

[cfherrman]

My fill water is 200 ppm salt (+/- 200 ppm)
My water is on its 3rd year
I have added one 40 lb bag of salt (382 ppm)
Only bleach for clorine
My salt is 800 ppm (+/- 200 ppm)

It does take awhile for it to build up

 

[JamesW]

The chlorine atoms in chlorine gas have a formal charge of zero.

The formal charge of chlorine in hypochlorous acid or hypochlorite is zero, but the oxidation state is +1, which is the relevant number because the chlorine atom takes two electrons to become chloride.

For individual ions, you typically would not refer to formal charge. It's just the charge because it's not bonded to any other atoms.

 

[JoyfulNoise]

Who would have ever guessed that a pool forum would contain a discussion of formal charge versus oxidation number ... next thing you know we’ll be fighting over whether we should define acids & bases using Lewis or Bronsted-Lowry formalisms....I’m much more of a B/L kind of guy....