Actual Cl concentration in bleach and shock?

[Jo Jesty]

I think there's a lot of misleading info about chlorine concentrations, particularly about sodium hypochlorite--i.e. bleach, or shock. Please correct me if I'm wrong. Clorox bleach is about 8.5% and pool shock is 12.5%, but that's hypochlorite, and chlorine is only 47% of sodium hypochlorite by weight. So when you quote ppm chlorine, is that pure Cl by weight, or hypochlorite equivalents? It obviously makes a very large cost-arithmetic difference when my gallon of shock is really 6% chlorine, not 12.5%.

Dichlor arithmetic (the commonly available sodium salt, dihydrate) is a bit easier since it's a dry solid, and it is 28% chlorine by weight. So 1 lb in 10K gallons (80K pounds of water) should max out at 3.5 ppm chlorine if completely broken down to free chlorine in the water. The bad news is that about 50% of dichlor is of course CYA stabilizer, which is why continual use of dichlor or trichlor can drive up CYA levels much too high.

 

[chem geek]

chlorine is only 47% of sodium hypochlorite by weight.

What you wrote is incorrect because you are forgetting that chlorine measurements are in ppm Cl₂ units with a molecular weight of 70.906 g/mole. Sodium hypochlorite has a molecular weight of 74.442 g/mole. So the weight conversion from sodium hypochlorite to weight % available chlorine is multiplying by a factor of 70.906/74.442 = 0.9525, but as I show below you should be looking at the Volume % Available Chlorine aka Trade % because you usually pay for chlorinating liquid and bleach by volume, not by weight.

Clorox concentrated bleach is 8.25% sodium hypochlorite by weight. Chlorinating liquid varies with some being 10% and some being 12.5% and these are usually Trade % which is % Available Chlorine by volume.

It is easiest to dose knowing Trade % since one gallon of 12.5% Trade in 10,000 gallons results in 12.5 ppm FC. Perhaps the following formulas and table will help you compare more effectively where the Trade % is the best to use for price comparisons.

Trade % = Weight % Available Chlorine * Specific Gravity (density) = Weight % Sodium Hypochlorite * (70.906/74.442) * Specific Gravity (density)

Product ......................... Specific Gravity ......... Weight % ................ Weight % ...... Trade % (Volume %
........................................... (density) ...... Sodium Hypochlorite . Available Chlorine . Available Chlorine)
5.25% Bleach ....................... 1.07 g/ml ................. 5.25% ....................... 5.00% ................... 5.35%
6.00% Bleach ....................... 1.08 g/ml ................. 6.00% ....................... 5.71% ................... 6.17%
6.15% Bleach ....................... 1.08 g/ml ................. 6.15% ....................... 5.86% ................... 6.33%
8.25% Bleach ....................... 1.10 g/ml ................. 8.25% ....................... 7.86% ................... 8.64%
10% Chlorinating Liquid ......... 1.14 g/ml ................. 9.21% ....................... 8.77% .................. 10.00%
12.5% Chlorinating Liquid ...... 1.16 g/ml ................ 11.31% ..................... 10.78% .................. 12.50%
15.0% Chlorinating Liquid ...... 1.20 g/ml ................ 13.12% ..................... 12.50% .................. 15.00%

I wouldn't call this a very large difference using 8.25% instead of 8.64% -- a roughly 5% error -- given typical tests being at least that amount of error (up to 10%).

Now if you wanted to compare across different types of chlorine sources, then you could use the Weight % Available Chlorine but for pricing chlorinating liquid or bleach you have to convert the price per volume to a price per weight. Though not dated in terms of prices, see Cost Comparison of Chlorine Sources.

 

[Jo Jesty]

Goodness, that was a quick response: thank you again TFP Expert (after my original problems, last year, solved with your advice).

HOWEVER, I don't find your explanation works at the level of the simple molecular formula and the atomic weights (AW): NaOCl has only one Cl atom, not two, and the Cl (ca 35.5 AW) is indeed 47% by weight of NaOCl. But, reading fairly clearly between your lines, I understand that "trade" Cl, which is what everyone uses and relies on is not actual Cl, AND ALSO that "Free Chlorine" measurements are similarly out of whack, being apparently based on counting NaOCl as being 95% free chlorine, which it is simply not. But as long as this ~ 50% error is built into all FC calculations, that's fine. Perhaps my "3" ppm FC (Taylor's pink reagent, or the common yellow stuff) is always referenced to the trade %, while it has always been about a true 1.5 all along? OK with me.

Any analytical (non-pool) chemists around these parts who can take 1 ml of my 12.5% and assay the real chlorine content? I am predicting it will be close to 60 mg, while the trade will say it's 125 mg. No bets, though!

 

[chem geek]

You are missing the point that the FC measurement itself is in ppm Cl₂ units and that chlorine gas is defined as 100% Available Chlorine even though only ONE of its two chlorine atoms results in chlorine in the pool. This is why Trichlor can be around 90% Available Chlorine (99% purity of 91.5% Available Chlorine) where it only has half that or 45% chlorine atoms by weight in that molecule. The same is true for sodium hypochlorite where the chlorine atom is only 35.453/74.442 = 47.6% but in terms of the DEFINITION of Weight % Available Chlorine you have to multiply this by a factor of 2 to get 95.2% that is the Weight % Available Chlorine of pure sodium hypochlorite (that doesn't exist, but for the purposes of this discussion).

I know that this somewhat arbitrary definition of Weight % Available Chlorine being 100% for molecular chlorine in spite of only one of its two chlorine producing hypochlorous acid in water is confusing, but that's how the industry defined it. You are stuck on the Weight % Available Chlorine being the weight of chlorine atoms when it is not. It is the weight of chlorine gas equivalent added to the pool and as shown below only HALF its weight becomes chlorine in hypochlorous acid in the pool.

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

That is, half of chlorine gas with water becomes hypochlorous acid while the other half becomes hydrochloric acid. Or put another way, 100% Available Chlorine as represented by chlorine gas has 200% chlorine atoms in it (i.e. 2 chlorine atoms per 1 chlorine that becomes hypochlorous acid). The industry decided to standardize its weight percent around chlorine gas and unfortunately only half of that weight is the type of chlorine that we care about.

But as long as this ~ 50% error is built into all FC calculations, that's fine. Perhaps my "3" ppm FC (Taylor's pink reagent, or the common yellow stuff) is always referenced to the trade %, while it has always been about a true 1.5 all along? OK with me.

Any analytical (non-pool) chemists around these parts who can take 1 ml of my 12.5% and assay the real chlorine content? I am predicting it will be close to 60 mg, while the trade will say it's 125 mg. No bets, though!

You are correct that the FC test is standardized/defined to return FC in ppm Cl₂ units which means converting the molar concentration of hypochlorous acid into chlorine gas by multiplying moles/liter by ((70.906 g/mole Cl₂)/(74.442 g/mole NaOCl)) * (1000 mg/g) to get milligrams/liter of Cl₂ equivalent. This works because after dissolving in water only one of the Cl in Cl₂ is chlorine at the +1 oxidation state (in HOCl) while the other is at the -1 oxidation state (in chloride, Cl^-).

1 ml of your 12.5% that I assume to be Trade % means that it contains 0.125 g of Cl₂ equivalent so 125 mg. This is because

(Weight % Available Chlorine) * (Weight of Sample) = (Weight of Cl₂ equivalent)
((12.5 Volume % Available Chlorine)/Density) * ((volume of 1 ml)*Density) = (12.5 Volume % Available Chlorine) * (volume of 1 ml)

So you see that the density cancels out (yes, I know this is confusing, but it's the way it is by definition of what Volume % Available Chlorine means). If you were to add this to 1 liter of water, then this would be 125 mg/L or ppm FC.

Now the question is what do you mean by "the real chlorine content"? If you want that measured in a weight then you need to DEFINE the equivalent molecule you want the measurement to be referenced against. It's 125 milligrams of Cl₂ gas in terms of its effect in the water where, again, only half of that chlorine gas weight becomes the chlorine of interest in the water (hypochlorous acid). In terms of sodium hypochlorite, it's 125*(74.442/70.906) = 131 grams while in terms of hypochlorous acid it's 125*(52.46/70.906) = 92.5 grams while in terms of chlorine atom it's 125*(35.453/70.906) = 62.5 grams (again, not counting chlorine atoms that are part of chloride salt since that is also present in chlorinating liquid and bleach).

Salt Test

By the way, ALL of the analytical tests we deal with regarding concentration (ppm) have this same issue where you need to know the molecule you are referencing against. For example, the Taylor K-1766 salt test analytically measures chloride ion because it titrates using silver nitrate to precipitate silver chloride until all the chloride is precipitated at which point the excess silver reacts with the chromate indicator dye to form the red/brown silver chromate. In spite of analytically measuring chloride ion, the result is reported in ppm sodium chloride regardless of whether there is any sodium present and it is instead all potassium or something else.

Total Alkalinity (TA) Test

Another test that is even more confusing than the salt test because it has a factor of 2 that will drive one nuts is the Total Alkalinity (TA) test. This analytically measures all ions that can accept a hydrogen ion down to a pH of around 4.5 which is the transition point in the test. Bicarbonate ion gets counted once because it can accept one hydrogen ion, carbonate ion gets counted twice, cyanurate ion gets counted once (though doubly and triply deprotonated cyanurate ions would get counted more but they are too small in concentration to worry about), borate ion will be counted once (though most boron will be in boric acid and not counted). This test reports in ppm calcium carbonate (CaCO₃) and as I just noted carbonate counts twice towards TA compared to bicarbonate. What this means is that if you were to add a known weight of sodium bicarbonate to the water and then thought "well, I'll just multiply by the molecular weight of calcium carbonate of 100.0869 g/mole and divide by the molecular weight of sodium bicarbonate of 84.0066 g/mole" you would be off by a factor of 2. The TA will be half this calculated amount because it would take half as much calcium carbonate on a mole per mole (or molecule per molecule) basis as it would take sodium bicarbonate to produce that TA.

Calcium Hardness (CH) and Cyanuric Acid (CYA) Tests

Fortunately the Calcium Hardness (CH) and Cyanuric Acid (CYA) tests don't have any factors, but the CH is still in ppm calcium carbonate and not in the calcium chloride that is normally added to the water. The CYA is most direct in that it is in the same ppm Cyanuric Acid units that is the same as pure CYA that is added to the water.

 

[Jo Jesty]

Ah...

Final and perfect clarity, especially your "that's how the industry defined it." Thank you so much. In roaming around I similarly found that dichlor (which I mentioned is actually 28% chlorine) is stated by "the industry" as being 56% "available chlorine": see the Assoc of Spa & Pool Professionals straight statement at https://www.apsp.org/LinkClick.aspx?fileticket=b20av79uFo4%3D&portalid=0. Just like hypochlorite, it's obviously not. So, for chlorine, divide everything by two to get the actual number of (better) take no notice and use for comparative purposes and pool-chemistry measurements only.

I have never understood total alkalinity myself! It's just pH buffering, and as long as there's some there it doesn't bother me (I have almost no Ca or other metals--perhaps a little Fe--in my Long Island water). I always use carbonate (not bicarb) anyway for pH adjustment: per unit alkalinity, it's much cheaper (but, given its high pH, I do add slowly, and straight through the pump, not the filter).

THANK YOU AGAIN, and a pox on official pool "chemists" for their misleading factors-of-two chlorine folderol!!

A good time to end this thread, I think. Thanks to your help last year the pool is in sparkling shape with nice stable free Cl and pH and very reasonable dosing of chemicals.

JJ

PS In the TFP pool school, I didn't find any mention of the chlorine real vs industry 50% "correction". Perhaps someone might add a para for those like me who remember some chemistry?

 

[chem geek]

Perhaps somewhere in a definition of "% Available Chlorine" it could say that this is relative to chlorine gas that is defined as 100% so it is NOT the percentage of chlorine atoms that become chlorine from a product, but is double that number (since only one of the two chlorine in chlorine gas becomes chlorine in water; the other becoming chloride salt). Unfortunately, this is way beyond what most people care about so would likely confuse, but perhaps it could be simply stated that due to the way Available Chlorine is defined, the actual amount of chlorine atoms by weight in product is half the Available Chlorine amount. In other words, Trichlor is not really 90% by weight chlorine atoms but is 45% weight with the rest being essentially CYA.

This topic only comes up occasionally, perhaps once every 3 or so years so it's not something most people are thinking about.

 

[chyvan]

Nice discussion, but it unfortunately reinforces the pool store clerk's agrument that, "It's not our tabs that are causing your CYA to sky rocket. Just look at how pure our tabs are. Says right here that it's 90% available."

 

[JesseWV]

ChemGeek:

I'm updating some formulas in my spreadsheet's Liquid Chlorine Cost calculator and I'm getting a discrepancy with the calculated 8.41% Trade by Volume shown below in red.

My calculations are coming up with 8.64%.

8.25 * (70.906/74.442) * 1.10 = 8.644

Am I missing something?

Trade % = Weight % Available Chlorine * Specific Gravity (density) = Weight % Sodium Hypochlorite * (70.906/74.442) * Specific Gravity (density)

Product ......................... Specific Gravity ......... Weight % ................ Weight % ...... Trade % (Volume %
........................................... (density) ...... Sodium Hypochlorite . Available Chlorine . Available Chlorine)
5.25% Bleach ....................... 1.07 g/ml ................. 5.25% ....................... 5.00% ................... 5.35%
6.00% Bleach ....................... 1.08 g/ml ................. 6.00% ....................... 5.71% ................... 6.17%
6.15% Bleach ....................... 1.08 g/ml ................. 6.15% ....................... 5.86% ................... 6.33%
8.25% Bleach ....................... 1.10 g/ml ................. 8.25% ....................... 7.86% ................... 8.41%
10% Chlorinating Liquid ......... 1.14 g/ml ................. 9.21% ....................... 8.77% .................. 10.00%
12.5% Chlorinating Liquid ...... 1.16 g/ml ................ 11.31% ..................... 10.78% .................. 12.50%
15.0% Chlorinating Liquid ...... 1.20 g/ml ................ 13.12% ..................... 12.50% .................. 15.00%

 

[chem geek]

That's a mistake in my table, apparently only with that line. It looks like I was using 8% or something close to that instead of 8.25%. I've corrected my post. Thanks for catching that.

 

[JesseWV]

No problem! I'm just glad I'm not going crazy, like my wife says.