All chlorine products have a labeled percentage on them. Here's what they mean.
The percentage listed on a chlorine product label is the concentration of the ingredient shown, relative to the total amount of the packaged product.
Clear as mud? Let's unpack what it means.
Weight vs. Available Chlorine vs. Trade %
There are three ways to list chlorine.
- Weight %
- Available chlorine %
- Trade %
To know the weight and available chlorine percentages, we need to know the molar weights of elemental chlorine (Cl), molecular chlorine (chlorine gas, Cl2⇡), and the chlorine products themselves. These molecular weights are known, and publicly available online. Here's a helpful reference chart:
Weight %
Weight % (or "by weight") means the percentage of the actual compound relative to the entire mass of the product in the container. Take chlorine for example. If the product says 99% Trichloro-s-triazinetrione, then 1% of it is other ingredients (like binding agents and whatever else). Let's expand on this further.
99% Trichloro-s-triazinetrione in a 50-pound bucket means that 49.5 pounds of that 50 pound bucket will be trichlor.
But that doesn't tell us much, because we don't know what % of trichlor is chlorine yet. To find that out, we need to know the molar weights of both trichlor and of chlorine. Looking at the reference chart above,
Trichlor (C3Cl3N3O3) is 232.41 g.mol.
Elemental chlorine (Cl) is 35.453 g/mol.Trichlor has three chlorine atoms attached to it, so we triple that:
(35.453 x 3) = 106.359
Then we divide the total chlorine weight (106.359) into the total molar weight of trichlor (232.41) to find out the amount of chlorine in trichlor (by weight).
106.359 ÷ 232.41 = 0.458 = 45.8% chlorine in trichlor (by weight).
Available chlorine %
Available chlorine goes a step further, utilizing the molecular weight (molar weight) of molecular chlorine (Cl2), not elemental chlorine.
Since we know the molar weight of elemental chlorine (35.453 g/mol), we can double it to know the molar weight of molecular chlorine:
Cl2 is 70.91 g/mol.
Next, we identify the molar weight of a chlorine like calcium hypochlorite.
Cl(OCl)2 is 142.98 g/mol.
Notice that calcium hypochlorite has two chlorine atoms on it, so we not only double Cl to Cl2, but we double Cl2 because there are two of them in cal hypo.
(70.91 x 2) = 141.82
...or put another way,
(35.453 x 2) x 2 = 141.82
Then, we divide the total chlorine weight into the total molar weight of calcium hypochlorite to get the available chlorine percentage:
(70.91 x 2) ÷ 142.98 = 0.992 = 99.2% available chlorine in PURE cal hypo
Why PURE cal hypo? Because we haven't yet multiplied by the labeled weight percentage. Cal hypo comes in two different strengths, 65-68% and 70-73%. The larger of the two numbers is the weight %, and the smaller is the available chlorine %, and it's conservative due to EPA labeling requirements. So let's multiply our available chlorine percentage by the labeled weight percent to know what's in the product:
(0.992 x 0.68) = 0.674 = 67.4% available chlorine in 68% cal hypo (rounded down to 65% per EPA labeling rules).
(0.992 x 0.73) = 0.724 = 72.4% available chlorine in 73% cal hypo (rounded down to 70% per EPA labeling rules).
Trade %
Trade % is for liquids that are already dissolved. It's a simplified method of labeling a product so users know how much free chlorine they are getting relative to a certain volume of water. For liquid chlorine, for instance, 10% trade will put 10 ppm in 10,000 gallons of water. This is because a percentage is to be divided by 100, and if you divide by 100 and then into 10,000 gallons, that's one millionth. One millionth is another way of saying parts-per-million (ppm).
1 gallon of (X trade %) sodium hypochlorite added to 10,000 gallons of water results in (X ppm) of free chlorine.
1 gallon of (10 %) sodium hypochlorite added to 10,000 gallons of water results in (10 ppm) of free chlorine.
The same math can be used if it's 12.5% trade, in which one gallon puts 12.5 ppm in 10,000 gallons.